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Wave Glider provides gateway to remote exploration

November 10, 2020

WHOI geochemist Chris German pairs an autonomous surface vehicle (ASV) called a Wave Glider with other vehicles to expand research here and on other Ocean Worlds

Mining ancient dust from the ocean’s loneliest spot

September 24, 2020

Researchers investigate dust from the ocean’s farthest point from land to reconstruct the climactic history of the Southern Hemisphere, and understand how micronutrients have influenced biological productivity in this oceanic desert.

Working from Home: Mallory Ringham

July 2, 2020

WHOI-MIT joint program student outfits her basement to do vital work on a marine carbon sensor

oil spill

A dangerous leak of diesel fuel in the Arctic

June 18, 2020
plastics by the numbers

The many lifetimes of plastics

June 15, 2020

Infographics strive to give us a sense of how long plastic goods will last in the environment. But is this information reliable? The findings of a new study from WHOI may surprise you.

toxins story

Are natural toxins in fish harmful?

May 28, 2020

Marine life has been naturally producing toxic chemicals well before chemical companies were manufacturing PCBs. But are these naturally-produced compounds as harmful as man-made environmental pollutants, and do those pose a human health threat?

Working from Home: Matt Long

May 7, 2020

A marine chemist spends his time at home tinkering on a high-tech buoy in the basement, proving that being homebound doesn’t mean you can’t think big.

Our Radioactive Ocean: Ken Buesseler

April 30, 2020

Ken Buesseler joins the hosts of Future Hindsight Podcast to talk about the safety of the Pacific Ocean, the natural occurrence of radioactivity in our environment, and a citizen scientist project for oceanic testing.

Why Sunlight Matters for Marine Oil Spills

April 30, 2020

A decade of research since the Deepwater Horizon disaster has revealed how sunlight—its importance long understated in oil spill science—substantially alters petroleum floating at the sea surface.

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Finding medical answers in the ocean

March 19, 2020

The test being used to diagnose the novel coronavirus—and other pandemics like AIDS and SARS—was developed with the help of an enzyme isolated from a microbe found in marine hydrothermal vents as well as freshwater hot springs.

News Releases

Climate Change Can Destabilize the Global Soil Carbon Reservoir, New Study Finds

March 23, 2021

The vast reservoir of carbon that is stored in soils probably is more sensitive to destabilization from climate change than has previously been assumed, according to a new study by researchers at WHOI and other institutions.

The study found that the biospheric carbon turnover within river basins is vulnerable to future temperature and precipitation perturbations from a changing climate.

Although many earlier, and fairly localized, studies have hinted at soil organic carbon sensitivity to climate change, the new research sampled 36 rivers from around the globe and provides evidence of sensitivity at a global scale.

“The study results indicate that at the large ecosystem scale of river basins, soil carbon is sensitive to climate variability,” said WHOI researcher Timothy Eglinton, co-lead author of the paper in the Proceedings of the National Academy of Sciences of the United States of America. “This means that changing climate, and particularly increasing temperature and an invigorated hydrological cycle, may have a positive feedback in terms of returning carbon to the atmosphere from previously stabilized pools of carbon in soils.”

The public is generally aware that climate change can potentially destabilize and release permafrost carbon into the atmosphere and exacerbate global warming. But the study shows that this is true for the entire soil carbon reservoir, said WHOI researcher Valier Galy, the other co-lead author of the study.

The soil carbon reservoir is a key component in keeping the atmosphere in check in terms of how much carbon dioxide is in the air. The amount of carbon stored in terrestrial vegetation and soils is three times more than how much the atmosphere holds, and it consumes more than a third of the anthropogenic carbon that is emitted to the atmosphere.

To determine the sensitivity of terrestrial carbon to destabilization from climate change, researchers measured the radiocarbon age of some specific organic compounds from the mouths of a diverse set of rivers. Those rivers—including the Amazon, Ganges, Yangtze, Congo, Danube, and Mississippi—account for a large fraction of the global discharge of water, sediments and carbon from rivers to the oceans.

Terrestrial carbon, however, is not so simple to isolate and measure. That’s because carbon in rivers comes from a variety of sources, including rocks, organic contaminants such as domestic sewage or petroleum that differ widely in their age, and vegetation. To determine what’s happening within the rivers’ watersheds, and to measure radiocarbon from the terrestrial biosphere, researchers focused on two groups of compounds: the waxes of plant leaves that serve a protective function for the plants’ leaf surface and lignin, which is the woody “scaffolding” of land plants.

Taking these measurements showed a relationship between the age of the terrestrial carbon in the rivers and the latitude where the rivers reside, researchers found. That latitudinal relationship prompted researchers to infer that climate must be a key control in the age of the carbon that is exported from the terrestrial biosphere to these rivers, and that temperature and precipitation are primary controls on the age of that carbon.

“Why this study is powerful is because this large number of rivers, the wide coverage, and the wide range of catchment properties give a very clear picture of what’s happening at the global scale,” said Galy.  “You could imagine that by going after lots of rivers, we would have ended up with a very complicated story. However, as we kept adding new river systems to the study, the story was fairly consistent.”

“In many respects, Earth scientists see rivers as being a source signal that is sent to sedimentary records that we can interpret,” said Eglinton. “By going to sedimentary records, we have the opportunity to look at how the terrestrial biosphere has responded to climate variability in the past. In addition, by monitoring rivers in the present day, we can also use them as sentinels in order to assess how these watersheds may be changing.”

New observation network will provide unprecedented, long-term view of life in the ocean twilight zone

February 8, 2021

The ocean twilight zone, a dimly lit region roughly 200–1000 meters (650–3200 feet) below the surface, contains the largest amount of fish biomass on Earth—yet it remains largely unexplored by scientists. A new observation network under development by the Woods Hole Oceanographic Institution (WHOI) seeks to change that. Encompassing 250,000 square kilometers (roughly 155,300 square miles) of the northwest Atlantic Ocean, the network will collect around-the-clock data about the twilight zone over months or even years, offering unprecedented insight into this little-known, yet vitally important region of the sea.

“It will cover a really huge piece of the ocean,” says WHOI ocean ecologist Simon Thorrold, a Principal Investigator for the network. “We’re used to going out for several weeks on a research vessel to study just a small area, and returning there maybe once or twice a year if we’re lucky,” Thorrold says. “Now, we’ll be able to get continuous measurements from a large chunk of the ocean twilight zone over significant periods of time. It’s very exciting.”

The network will give scientists a comprehensive view of the twilight zone, or mesopelagic, using several different technologies including moored buoys equipped with acoustic survey systems; a swarm of optical and geochemical sensors; and new fish-tracking tags that will continuously record the position of major predators such as sharks and tuna. All of these components will connect to the network’s buoys using acoustic signals underwater and an Iridium satellite link at the surface.

The information provided by the network will improve estimates of the density and distribution of fish and invertebrates in the twilight zone, reveal new insights about their interactions and daily migrations to and from the surface—and help fuel new strategies for conservation and policy making. The network will also help researchers better understand how the twilight zone affects carbon cycling and global climate, says WHOI marine radiochemist Ken Buesseler.

“Plankton—tiny plant-like organisms—at the surface remove carbon dioxide from the atmosphere as they grow,” Buesseler says. “When animals from the twilight zone migrate up to feed on those plankton and then return back to deeper waters, they take that carbon with them. The question is, how much does that natural cycle of life and death affect the amount of carbon that is sequestered in the deep ocean? And if humans start removing large numbers of fish from the twilight zone, how could that change?”

Buesseler compares the new observation network to a “field of dreams.”

“You build something like it, and all kinds of researchers will come and use it, because they’ve just never had the opportunity or infrastructure in place for them to be able to do these sorts of observations,” Buesseler says.

Also collaborating on the project are WHOI scientists Andone Lavery and Dana Yoerger, and Melissa Omond from the University of Rhode Island.

Former WHOI President and Director Mark Abbott says it would be challenging to fund this kind of multidisciplinary, large-scale, long-term marine infrastructure through federal sources. Abbott was instrumental in planning the observation network during his tenure. Instead of going through traditional funding channels, he turned to German philanthropist Otto Happel, whose interest in WHOI’s work in the ocean twilight zone led to a generous gift from the Happel Foundation.

“I think what’s really exciting about Otto is his deep appreciation and understanding and curiosity about the science, the engineering, and how this informs ocean policy,” Abbott says. “He’s concerned about all the changes we’re seeing in the marine environment, and he wants to fund work that enables people to make better decisions about the ocean.”

The Happel Foundation’s support will enable WHOI’s ocean twilight zone research team to turn their plans for an observation network into a reality. Work is already underway on sensors and other network components.

“The ocean always has been my passion, in many respects,” Happel says. “I’m thrilled that with a relatively small amount of funding, we can start to answer questions about it that may be vital to changing how we operate and how we live in this world.”

The Woods Hole Oceanographic Institution (WHOI) is a private, non-profit organization on Cape Cod, Massachusetts, dedicated to marine research, engineering, and higher education. Established in 1930, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate an understanding of the ocean’s role in the changing global environment. WHOI’s pioneering discoveries stem from an ideal combination of science and engineering—one that has made it one of the most trusted and technically advanced leaders in basic and applied ocean research and exploration anywhere. WHOI is known for its multidisciplinary approach, superior ship operations, and unparalleled deep-sea robotics capabilities. We play a leading role in ocean observation, and operate the most extensive suite of data-gathering platforms in the world. Top scientists, engineers, and students collaborate on more than 800 concurrent projects worldwide—both above and below the waves—pushing the boundaries of knowledge and possibility. For more information, please visit www.whoi.edu

 

New study takes comprehensive look at marine pollution

December 3, 2020

Paper finds ocean pollution is a complex mix of chemicals and materials, primarily land-based in origin, with far-reaching consequences for environmental and human health, but there are options available for world leaders

 

For centuries, the ocean has been viewed as an inexhaustible receptacle for the byproducts of human activity. Today, marine pollution is widespread and getting worse and, in most countries, poorly controlled with the vast majority of contaminants coming from land-based sources. That’s the conclusion of a new study by an international coalition of scientists taking a hard look at the sources, spread, and impacts of ocean pollution worldwide.

The study is the first comprehensive examination of the impacts of ocean pollution on human health. It was published December 3 in the online edition of the Annals of Global Health and released the same day at the Monaco International Symposium on Human Health & the Ocean in a Changing World, convened in Monaco and online by the Prince Albert II de Monaco Foundation, the Centre Scientifique de Monaco and Boston College.

“This paper is part of a global effort to address questions related to oceans and human health,” said Woods Hole Oceanographic Institution (WHOI) toxicologist and senior scientist John Stegeman who is second author on the paper. “Concern is beginning to bubble up in a way that resembles a pot on the stove. It’s reaching the boiling point where action will follow where it’s so clearly needed.”

Despite the ocean’s size—more than two-thirds of the planet is covered by water—and fundamental importance supporting life on Earth, it is under threat, primarily and paradoxically from human activity. The paper, which draws on 584 peer-reviewed scientific studies and independent reports, examines six major contaminants: plastic waste, oil spills, mercury, manufactured chemicals, pesticides, and nutrients, as well as biological threats including harmful algal blooms and human pathogens.

It finds that ocean chemical pollution is a complex mix of substances, more than 80% of which arises from land-based sources. These contaminants reach the oceans through rivers, surface runoff, atmospheric deposition, and direct discharges and are often heaviest near the coasts and most highly concentrated along the coasts of low- and middle-income countries. Waters most seriously impacted by ocean pollution include the Mediterranean Sea, the Baltic Sea, and Asian rivers. For the many ocean-based ecosystems on which humans rely, these impacts are exacerbated by global climate change. According to the researchers, all of this has led to a worldwide human health impacts that fall disproportionately on vulnerable populations in the Global South, making it a planetary environmental justice problem, as well.

In addition to Stegeman, who is also director of the NSF- and NIH-funded Woods Hole Center for Oceans and Human Health, WHOIbiologists Donald Anderson and Mark Hahn, and chemist Chris Reddy also contributed to the report. Stegeman and the rest of the WHOI team worked on the analysis with researchers from Boston College’s Global Observatory on Pollution and Health, directed by the study’s lead author and Professor of Biology Philip J. Landrigan, MD. Anderson led the report’s section on harmful algal blooms, Hahn contributed to a section on persistent organic pollutants (POPs) with Stegeman, and Reddy led the section on oil spills. The Observatory, which tracks efforts to control pollution and prevent pollution-related diseases that account for 9 million deaths worldwide each year, is a program of the new Schiller Institute for Integrated Science and Society, part of a $300-million investment in the sciences at BC. Altogether, over 40 researchers from institutions across the United States, Europe and Africa were involved in the report.

In an introduction printed in Annals of Global Health, Prince Albert of Monaco points out that their analysis, in addition to providing a global wake-up, serves as a call to mobilize global resolve to curb ocean pollution and to mount even greater scientific efforts to better understand its causes, impacts, and cures.

“The link between ocean pollution and human health has, for a long time, given rise to very few studies,” he says. “Taking into account the effects of ocean pollution—due to plastic, water and industrial waste, chemicals, hydrocarbons, to name a few—on human health should mean that this threat must be permanently included in the international scientific activity.”

The report concludes with a series of urgent recommendations. It calls for eliminating coal combustion, banning all uses of mercury, banning single-use plastics, controlling coastal discharges, and reducing applications of chemical pesticides and fertilizers. It argues that national, regional and international marine pollution control programs must extend to all countries and where necessary supported by the international community. It calls for robust monitoring of all forms of ocean pollution, including satellite monitoring and autonomous drones. It also appeals for the formation of large, new marine protected areas that safeguard critical ecosystems, protect vulnerable fish stocks, and ultimately enhance human health and well-being.

Most urgently, the report calls upon world leaders to recognize the near-existential threats posed by ocean pollution, acknowledge its growing dangers to human and planetary health, and take bold, evidence-based action to stop ocean pollution at its source.

“The key thing to realize about ocean pollution is that, like all forms of pollution, it can be prevented using laws, policies, technology, and enforcement actions that target the most important pollution sources,” said Professor Philip Landrigan, MD, lead author and Director of the Global Observatory on Pollution on Health and of the Global Public Health and the Common Good Program at Boston College. “Many countries have used these tools and have successfully cleaned fouled harbors, rejuvenated estuaries, and restored coral reefs. The results have been increased tourism, restored fisheries, improved human health, and economic growth. These benefits will last for centuries.”

The report is being released in tandem with the Declaration of Monaco: Advancing Human Health & Well-Being by Preventing Ocean Pollution, which was read at the symposium’s closing session. Endorsed by the scientists, physicians and global stakeholders who participated in the symposium in-person and virtually, the declaration summarizes the key findings and conclusions of the Monaco Commission on Human Health and Ocean Pollution. Based on the recognition that all life on Earth depends on the health of the seas, the authors call on leaders and citizens of all nations to “safeguard human health and preserve our Common Home by acting now to end pollution of the ocean.”

“This paper is a clarion call for all of us to pay renewed attention to the ocean that supports life on Earth and to follow the directions laid out by strong science and a committed group of scientists,” said Rick Murray, WHOI Deputy Director and Vice President for research and a member of the conference steering committee. “The ocean has sustained humanity throughout the course of our evolution—it’s time to return the favor and do what is necessary to prevent further, needless damage to our life planetary support system.”

Funding for this work was provided in part by the U.S. Oceans and Human Health Program (NIH grant P01ES028938 and National Science Foundation grant OCE-1840381), the Centre Scientifique de Monaco, the Prince Albert II of Monaco Foundation, the Government of the Principality of Monaco, and Boston College.

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The Woods Hole Oceanographic Institution (WHOI) is a private, non-profit organization on Cape Cod, Massachusetts, dedicated to marine research, engineering, and higher education. Established in 1930, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate an understanding of the ocean’s role in the changing global environment. WHOI’s pioneering discoveries stem from an ideal combination of science and engineering—one that has made it one of the most trusted and technically advanced leaders in basic and applied ocean research and exploration anywhere. WHOI is known for its multidisciplinary approach, superior ship operations, and unparalleled deep-sea robotics capabilities. We play a leading role in ocean observation, and operate the most extensive suite of data-gathering platforms in the world. Top scientists, engineers, and students collaborate on more than 800 concurrent projects worldwide—both above and below the waves—pushing the boundaries of knowledge and possibility. For more information, please visit www.whoi.edu

 

First Detailed Oil Sample Analysis Completed from Mauritius Oil Spill

October 29, 2020

When the Japanese bulk carrier MV Wakashio struck a coral reef off the coast of Mauritius on July 25, 2020, and began leaking fuel oil two weeks later, local residents and the international community sprang into action to protect the pristine habitats that fringe the Indian Ocean nation. But they did so without insight from careful chemical analysis, which is customary after spills to help guide short and long-term response plans and protects the health of people facing such a spill.

Now the first ultra-high-resolution analysis of an oil sample from Mauritius shows that the material is a complex and unusual mix of hydrocarbons—and even though some of the components in it may have already degraded or evaporated, what remains still gives it the ability to persist in the environment. That is the conclusion of marine oil spill experts Chris Reddy and Bob Nelson at the Woods Hole Oceanographic Institution (WHOI), who are collaborating with Professor Kliti Grice, Director of the Western Australian Organic Isotope Geochemistry Centre (WA-OIGC) and Associate Professor Monique Gagnon and Alan Scarlett at Curtin University in Perth, Australia. Both teams analyzed a sample of floating residue collected August 16.  They subjected it to many of the same analytical techniques used to “fingerprint” samples from other large spills around the world.

The two teams use comprehensive two-dimensional gas chromatography (GC×GC), complemented by compound-specific isotope analyses to identify compounds specific to spilled oils. The analyses performed by the two laboratories provide some of the most highly advanced analytical services and scientific support to oil spill responders. They found their results were consistent with what they would expect from a sample of fuel oil, which is a non-standardized mix of petroleum products that ships often use to run their engines. In recent years, calls to improve air quality around ports has driven the industry to adopt new low-sulfur formulations that reduce emissions. At the same time, however, the potential is rising for spills of this type, but the scientific community has yet to see what happens to these new low-sulfur fuels when they enter the environment.

Key Takeaways

  • Marine oil spill experts from Woods Hole Oceanographic Institution recently provided one of the first detailed oil sample analysis from the July 25 Mauritius oil spill.
  • WHOI worked with experts from the Western Australian Organic Isotope Geochemistry Centre (WA-OIGC) and Curtin University in Perth, Australia.
  • Their analysis shows that the material is a complex and unusual mix of hydrocarbons.
  • This type of analysis is the gold standard in oil spills—it’s not only used to fingerprint the oil, but also to positively identify the constituent parts of an oil to figure out how to best fight a spill and guide short and long-term response plans while protecting the health of people facing such a spill.

A comprehensive two-dimensional gas chromatography (GCxGC) of the fuel oil collected in Mauritius after the wreck of the ship Wakashio reveals (A) a relatively low ratio of aromatic hydrocarbons relative to saturated hydrocarbons, likely a result of the de-sulfurization process the fuel underwent; and (B) a high concentration of secohopanes compared to hopanes, which can be used to help “fingerprint” the sample and potentially identify the source of future samples. (Image by Bob Nelson, ©Woods Hole Oceanographic Institution) A comprehensive two-dimensional gas chromatography (GCxGC) of the fuel oil collected in Mauritius after the wreck of the ship Wakashio reveals (A) a relatively low ratio of aromatic hydrocarbons relative to saturated hydrocarbons, likely a result of the de-sulfurization process the fuel underwent; and (B) a high concentration of secohopanes compared to hopanes, which can be used to help “fingerprint” the sample and potentially identify the source of future samples. (Image by Bob Nelson, ©Woods Hole Oceanographic Institution)

“Fuel oils, are arguably the most challenging petroleum products to analyze and investigate following marine-based spills,” said Reddy. “There is no single recipe or set of ingredients, and it gets even more complicated with these new low-sulfur fuel oils that require more steps in their manufacture. We don’t know if this was a low-sulfur material, but it’s unlike anything we’ve seen spilled before—that alone demands a closer look.”

Analysis by WA-OIGC at Curtin and also confirmed by WHOI’s Organic Geochemistry Analysis Lab showed that the sample contained relatively low levels of polycyclic aromatic hydrocarbons (PAHs), which are known carcinogens in humans and animals. Although low, the levels of PAHs might accumulate in certain parts of the marine environment. In addition, Reddy found a relatively high concentration and variety of secohopanes and surprisingly low levels of hopanes, both of which are “biomarkers” that will help connect future samples of oil to the Wakashio ship. These substances were present in minute amounts, but still detectable thanks to the technology available in the collaborating laboratories. Only comparison to a “fresh” sample taken from the ship would allow researchers to determine what has already been lost from the oil as a result of evaporation, dilution, photodegradation, and other effects.

“This was just a first step,” said Grice. “Our limited view of what spilled only reinforces the need for long-term monitoring, access to samples from the ship, and a more in-depth analysis that officials can incorporate into detailed plans to help Mauritius and its environment recover from this.”

The sample was acquired by Associate Professor Monique Gagnon at Curtin University. If members of the public have photos of oil and oiled material or other information, including photos of unoiled beaches and reefs, that might help Reddy and Grice and their research teams better understand the nature of the spill, they are welcome to email oilsamples@whoi.edu.

The Woods Hole Oceanographic Institution (WHOI) is a private, non-profit organization on Cape Cod, Massachusetts, dedicated to marine research, engineering, and higher education. Established in 1930, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate an understanding of the ocean’s role in the changing global environment. WHOI’s pioneering discoveries stem from an ideal combination of science and engineering—one that has made it one of the most trusted and technically advanced leaders in basic and applied ocean research and exploration anywhere. WHOI is known for its multidisciplinary approach, superior ship operations, and unparalleled deep-sea robotics capabilities. We play a leading role in ocean observation, and operate the most extensive suite of data-gathering platforms in the world. Top scientists, engineers, and students collaborate on more than 800 concurrent projects worldwide—both above and below the waves—pushing the boundaries of knowledge and possibility. For more information, please visit www.whoi.edu

 

New multi-institutional grant will support a fleet of robotic floats

October 29, 2020

On October 29, 2020, the National Science Foundation approved a $53 million grant to a consortium of the country’s top ocean-research institutions to build a global network of chemical and biological sensors that will monitor ocean health. Scientists at the Monterey Bay Aquarium Research Institute (MBARI), Woods Hole Oceanographic Institution (WHOI), University of Washington, Scripps Institution of Oceanography, and Princeton University will use this grant to build and deploy 500 robotic ocean-monitoring floats around the globe.

This new network of floats, called the Global Ocean Biogeochemistry Array (GO-BGC Array), will collect observations of ocean chemistry and biology between the surface and a depth of 2,000 meters (6,562 feet). Data streaming from the float array will be made freely available within a day of being collected, and will be used by scores of researchers, educators, and policy makers around the world.

These data will allow scientists to pursue fundamental questions about ocean ecosystems, observe ecosystem health and productivity, and monitor the elemental cycles of carbon, oxygen, and nitrogen in the ocean through all seasons of the year. Such essential data are needed to improve computer models of ocean fisheries and climate, and to monitor and forecast the effects of ocean warming and ocean acidification on sea life.

Although scientists can use Earth-orbiting platforms and research vessels to monitor the ocean, satellites can only monitor near-surface waters, and the small global fleet of open-ocean research ships can only remain at sea for relatively short periods of time. As a result, ocean-health observations only cover a tiny fraction of the ocean at any given time, leaving huge ocean regions unvisited for decades or longer.

A single robotic float costs the same as two days at sea on a research ship. But floats can collect data autonomously for over five years, in all seasons, including during winter storms, when shipboard work is limited.

Funding for the GO-BGC Array is provided through the NSF’s Mid-scale Research Infrastructure-2 Program (MSRI-2). The GO-BGC Array is the National Science Foundation’s contribution to the Biogeochemical-Argo (BGC-Argo) project. It extends biological and chemical observing globally, and builds on two ongoing efforts to monitor the ocean using robotic floats, both of which have been highly successful.

The first of these programs, the Argo array, consists of 3,900 robotic floats that drift through the deep ocean basins, providing information on temperature and salinity in the water column. Since its inception in 1999, Argo data have been used in 4,100 scientific papers. As the first global, subsurface ocean observing system, the Argo array has done an incredible job of measuring the physical properties of our ocean, but Argo floats do not provide information about the ocean’s vital chemical and biological activity.

Starting in 2014, the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program deployed a large array of robotic “biogeochemical” floats, based on the Argo design, but carrying sensors to monitor the chemical and biological properties of the ocean. SOCCOM floats have operated for nearly six years in the remote, stormy, and often ice-covered Southern Ocean—arguably one of the harshest marine environments on Earth. These floats have already provided critical new information about how the Southern Ocean interacts with the Earth’s atmosphere and winter sea ice.

Similar to the SOCCOM floats, the new GO-BGC floats will carry a number of sensors in addition to the core Argo sensors for temperature, depth, and salinity. These include instruments to measure oxygen concentration, pH (ocean acidity), nitrate (an essential nutrient for microscopic algae), sunlight (required for algal growth), chlorophyll (an indicator of algal populations), and particles in the water (including microscopic algae). Over the last few years, researchers have been testing, refining, and calibrating these sensitive instruments as part of the SOCCOM program and other international efforts.

The instruments on these floats will allow researchers to monitor the health of the ocean, including the growth and respiration of phytoplankton (drifting algae and microbes that use sunlight as a source of energy) and the nutrients and light that control these processes. In addition to supporting most of life in the ocean, including commercial fisheries, phytoplankton supply oxygen to and remove carbon dioxide from the ocean and the atmosphere. These microscopic plankton have huge impacts on our climate through their control on carbon dioxide. The new floats will also provide first-hand data on long-term changes in the ocean, including ocean acidification and the expansion of low-oxygen zones.

This five-year effort involves five research institutions. MBARI will coordinate the project, refine the sensors, take the lead in processing data from the floats, and perform outreach for the program. WHOI, the University of Washington, and the Scripps Institution of Oceanography,  will build and deploy floats in collaboration with commercial partners. Researchers at Princeton University will contribute to the array design and project management, and ensure that the data are linked to global computer models of the Earth’s ocean and climate. This program will also have a significant impact on the ocean technology industry, including a number of commercial suppliers of ocean sensors and profiling floats.

A broad public-outreach program, including workshops, web-based curricula, and hands-on activities, will help scientists, teachers, students, and others use these data. In an expansion of the existing SOCCOM Adopt-A-Float program, the floats will be adopted by elementary- to college-level classes. Student activities will be developed through a partnership with the national Marine Advanced Technology Education program. In addition, courses based on GO-BGC technology will be offered through the The Sandbox, a makerspace at the Scripps Institution of Oceanography.

The researchers hope that GO-BGC will inspire other countries to contribute similarly instrumented floats, as part of the new global biogeochemical ARGO effort. Ideally, this expanded network would grow to a sustained array of 1,000 biogeochemical floats uniformly distributed around the world ocean, and spaced about 1,000 kilometers (620 miles) apart.

“Vast swaths of the Atlantic remain unmonitored with respect to ocean health, except for sporadic, infrequent research ship visits and widely spaced moored instruments,” added Susan Wijffels, WHOI senior scientist and co-principal investigator of GO-BCG. “Based on what we know about the Atlantic’s critical role in Earth’s climate and other key planetary systems, we need to fill in those gaps.”

“The GO-BGC Array will provide the scientific community with the unprecedented ability to take the pulse of ocean ecosystems and monitor the health of underlying chemical cycles on a global scale,” said David “Roo” Nicholson, WHOI associate scientist and co-lead of GO-BCG at WHOI. “Such observations are critical to understanding how the ocean will respond to multiple, broad-scale stressors such as warming, acidification, and deoxygenation.”

The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment. For more information, please visit www.whoi.edu.

Key Takeaways

  • A five-year, $53 million grant from the National Science Foundation to a consortium of ocean-research institutions will build a global network of 500 robotic biochemical sensors.
  • The GO-BGC (Global Ocean Biogeochemistry) Array will be led by the Monterey Bay Aquarium Research Institute and include WHOI, the University of Washington, Scripps Institution of Oceanography, and Princeton University.
  • The array will measure critical indicators of ocean health, including temperature, salinity, oxygen concentration, pH, nitrate concentration, chlorophyll, and light levels from the surface to 2,000 meters depth.
  • WHOI’s role will focus on the construction and deployment of floats in the Atlantic Ocean with academic and commercial partners.
SOCCOM float The GO-BGC array will be based on floats used in the SOCCOM (Southern Ocean Carbon and Climate Observations and Modeling) project like the one here being deployed in 2016. (Image courtesy of SOCCOM. SOCCOM is supported by the National Science Foundation under NSF Award PLR-1425989 and OPP-1936222) WHOI engineer Bill Dullea with one of the autonomous profiling floats that WHOI will help deploy in the Atlantic Ocean as part of the GO-BGC Array. (Photo by Ken Kostel, ©Woods Hole Oceanographic Institution) WHOI engineer Bill Dullea with one of the autonomous profiling floats that WHOI will help deploy in the Atlantic Ocean as part of the GO-BGC Array. (Photo by Ken Kostel, ©Woods Hole Oceanographic Institution) Illustration showing the operation of a typical Argo profiling float. (Image by Kim Fulton-Bennett, © 2020 MBARI) Illustration showing the operation of a typical Argo profiling float. (Image by Kim Fulton-Bennett, © 2020 MBARI) Configuration of a GO-BGC autonomous profiling float. (Illustration by Kelly Lance, © 2020 MBARI)

Configuration of a GO-BGC autonomous profiling float. (Illustration by Kelly Lance, ©2020 MBARI)

Additional Information

GO-BGC Array website

Wijffels

Susan Wijffels

Nicholson_David-e1532392918773-320x366

David “Roo” Nicholson

Porites coral

Ocean acidification causing coral ‘osteoporosis’ on iconic reefs

August 27, 2020

Scientists have long suspected that ocean acidification is affecting corals’ ability to build their skeletons, but it has been challenging to isolate its effect from that of simultaneous warming ocean temperatures, which also influence coral growth. New research from the Woods Hole Oceanographic Institution (WHOI) reveals the distinct impact that ocean acidification is having on coral growth on some of the world’s iconic reefs.

In a paper published Aug. 27, 2020, in the journal Geophysical Research Letters, researchers show a significant reduction in the density of coral skeleton along much of the Great Barrier Reef—the world’s largest coral reef system—and also on two reefs in the South China Sea, which they attribute largely to the increasing acidity of the waters surrounding these reefs since 1950.

“This is the first unambiguous detection and attribution of ocean acidification’s impact on coral growth,” says lead author and WHOI scientist Weifu Guo. “Our study presents strong evidence that 20th century ocean acidification, exacerbated by reef biogeochemical processes, had measurable effects on the growth of a keystone reef-building coral species across the Great Barrier Reef and in the South China Sea. These effects will likely accelerate as ocean acidification progresses over the next several decades.”

Roughly a third of global carbon dioxide emissions are absorbed by the ocean, causing an average 0.1 unit decline in seawater pH since the pre-industrial era. This phenomenon, known as ocean acidification, has led to a 20 percent decrease in the concentration of carbonate ions in seawater. Animals that rely on calcium carbonate to create their skeletons, such as corals, are at risk as ocean pH continues to decline. Ocean acidification targets the density of the skeleton, silently whittling away at the coral’s strength, much like osteoporosis weakens bones in humans.

“The corals aren’t able to tell us what they’re feeling, but we can see it in their skeletons,” said Anne Cohen, a WHOI scientist and co-author of the study. “The problem is that corals really need the strength they get from their density, because that’s what keeps reefs from breaking apart.  The compounding effects of temperature, local stressors, and now ocean acidification will be devastating for many reefs.”

Key Takeaways

  • An innovative numerical model developed by researchers at the Woods Hole Oceanographic Institution demonstrates the distinct impact of ocean acidification—separate from ocean warming—on coral growth.
  • The model shows that ocean acidification has caused a 13 percent decline in the skeletal density of Porites corals in the Great Barrier Reef, and a 7 percent decline in the South China Sea since 1950.
  • Pollution and land runoff can exacerbate the effects of ocean acidification, causing corals in local reefs to weaken more quickly than those located farther away from human settlements.
  • A global-scale investigation of coral CT scans could help to target protections for vulnerable reefs.
MIT-WHOI Joint Program student Nathaniel Mollica (left) and WHOI scientist Weifu Guo examine a core extracted from a coral skeleton. Photo by Anne Cohen Lab, ©Woods Hole Oceanographic Institution MIT-WHOI Joint Program student Nathaniel Mollica (left) and WHOI scientist Weifu Guo examine a core extracted from a coral skeleton. Photo by Anne Cohen Lab, ©Woods Hole Oceanographic Institution

In their investigation, Guo and his co-authors examined published data collected from the skeletons of Porites corals—a long-living, dome-shaped species found across the Indo-Pacific— combined with new three-dimensional CT scan images of Porites from reefs in the central Pacific Ocean. Using these skeletal archives, which date back to 1871, 1901, and 1978, respectively, the researchers established the corals’ annual growth and density. They plugged this information, as well as historical temperature and seawater chemistry data from each reef, into a model to predict the corals’ response to constant and changing environmental conditions.

The authors found that ocean acidification caused a significant decline in Porites skeletal density in the Great Barrier Reef (13 percent) and the South China Sea (7 percent), starting around 1950. Conversely, they found no impact of ocean acidification on the same types of corals in the Phoenix Islands and central Pacific, where the protected reefs are not as impacted by pollution, overfishing, runoff from land.

While carbon dioxide emissions are the largest driver of ocean acidification on a global scale, the authors point out that sewage and runoff from land can exacerbate the effect, causing even further reductions of seawater pH on nearby reefs. The authors attribute the declining skeletal density of corals on the Great Barrier Reef and South China Sea to the combined effects of ocean acidification and runoff. Conversely, reefs in marine protected areas of the central Pacific have so far been shielded from these impacts.

“This method really opens a new way to determine the impact of ocean acidification on reefs around the world,” said Guo. “Then we can focus on the reef systems where we can potentially mitigate the local impacts and protect the reef.”

Co-authors of the paper include Rohit Bokade (Northeastern University), Nathaniel Mollica (MIT-WHOI joint program), and Muriel Leung (University of Pennsylvania), as well as Russell Brainard of King Abdullah University of Science and Technology and formerly at the Coral Reef Ecosystem Division of the Pacific Islands Fisheries Science Center.

Funding for this research was provided by the National Science Foundation, the Robertson Foundation, The Tiffany & Co. Foundation, the Atlantic Donor Advised Fund, and WHOI’s Investment in Science Fund.

The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment. For more information, please visit www.whoi.edu.

New paper addresses the mix of contaminants in Fukushima wastewater

August 6, 2020

Nearly 10 years after the Tohoku-oki earthquake and tsunami devastated Japan’s Fukushima Dai-ichi Nuclear Power Plant and triggered an unprecedented release radioactivity into the ocean, radiation levels have fallen to safe levels in all but the waters closest to the shuttered power plant. Today, fish and other seafood caught in waters beyond all but a limited region have been found to be well within Japan’s strict limits for radioactive contamination, but a new hazard exists and is growing every day in the number of storage tanks on land surrounding the power plant that hold contaminated wastewater. An article published August 7 in the journal Science takes a look at some of the many radioactive elements contained in the tanks and suggests that more needs to be done to understand the potential risks of releasing wastewater from the tanks into the ocean.

“We’ve watched over the past nine-plus years as the levels of radioactive cesium have declined in seawater and in marine life in the Pacific,” said Ken Buesseler, a marine chemist at the Woods Hole Oceanographic Institution and author of the new paper. “But there are quite a few radioactive contaminants still in those tanks that we need to think about, some of which that were not seen in large amounts in 2011, but most importantly, they don’t all act the same in the ocean.”

Since 2011, Buesseler has been studying the spread of radiation from Fukushima into and across the Pacific. In June of that year, he mobilized a team of scientists to conduct the first international research cruise to study the early pathways that cesium-134 and -137, two radioactive isotopes of cesium produced in reactors, were taking as they entered the powerful Kuroshio Current off the coast of Japan. He has also built a network of citizen scientists in the U.S. and Canada who have helped monitor the arrival and movement of radioactive material on the Pacific coast of North America.

Now, he is more concerned about the more than 1,000 tanks on the grounds of the power plant filling with ground water and cooling water that have become contaminated through contact with the reactors and their containment buildings. Sophisticated cleaning processes have been able to remove many radioactive isotopes and efforts to divert groundwater flows around the reactors have greatly reduced the amount of contaminated water being collected to less than 200 metric tons per day, but some estimates see the tanks being filled in the near future, leading some Japanese officials to suggest treated water should be released into the ocean to free up space for more wastewater.

One of the radioactive isotopes that remains at the highest levels in the treated water and would be released is tritium, an isotope of hydrogen is almost impossible to remove, as it becomes part of the water molecule itself. However, tritium has a relatively short half-life, which measures the rate of decay of an isotope; is not absorbed as easily by marine life or seafloor sediments, and produces beta particles, which is not as damaging to living tissue as other forms of radiation. Isotopes that remain in the treated wastewater include carbon-14, cobalt-60, and strontium-90. These and the other isotopes that remain, which were only revealed in 2018, all take much longer to decay and have much greater affinities for seafloor sediments and marine organisms like fish, which means they could be potentially hazardous to humans and the environment for much longer and in more complex ways than tritium.

“The current focus on tritium in the wastewater holding tanks ignores the presence other radioactive isotopes in the wastewater,” said Buesseler. “It’s a hard problem, but it’s solvable. The first step is to clean up those additional radioactive contaminants that remain in the tanks, and then make plans based on what remains. Any option that involves ocean releases would need independent groups keeping track of all of the potential contaminants in seawater, the seafloor, and marine life. The health of the ocean—and the livelihoods of countless people—rely on this being done right.”

The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment. For more information, please visit www.whoi.edu.

Key Takeaways

  • Concentrations of radioactive material released into the Pacific from the Fukushima Dai-ichi nuclear power plant and found in the Pacific Ocean or in fish have largely declined to safe levels.
  • A larger problem exists in the form of more than 1,000 tanks full of contaminated, but treated, wastewater stored on the grounds of the power plant and that have been suggested need to be emptied into the ocean to free up space for more untreated water.
  • Current focus on tritium levels overlooks the much larger problem presented by other radioactive isotopes in the treated water, not all of which have been made public.
  • Many of these other isotopes emit more dangerous forms of radiation, and act in more complex ways in the environment, accumulating in seafloor sediments and in marine life, more readily than tritium.

Ken Buesseler

Cafe Thorium

Ocean Topic

Fukushima Radiation

Benjamin Van Mooy

WHOI receives $2.7M from Simons Foundation to study nutrients, microbes that fuel ocean food web

July 23, 2020

The Simons Foundation has awarded Woods Hole Oceanographic Institution (WHOI) scientists Dan Repeta and Benjamin Van Mooy two grants totaling $2.7 million to study key processes that help fuel the health of our ocean and planet.

Repeta’s research will focus on phosphorus, iron and nitrogen, the trinity of nutrients that fuel microbial cycles in the ocean. Van Mooy’s research centers on understanding carbon and energy flow through the microbial food web. His lab uses lipidomics—the comprehensive analysis of a cell or organism’s lipid profile, to learn how ocean microbes harvest, store, and use chemical energy from light. Both research projects are focused on samples and data collected at Station ALOHA—a 6-mile radius circle in the Pacific Ocean north of Hawaii, a hub for oceanographic research projects that is yielding a remarkable collection of observations about Earth’s dynamic oceans and atmosphere.

The Simons Foundation funding will also support research on the inner workings of the mesopelagic zone (aka the twilight zone), a little understood part of the ocean with a major role in sequestering carbon.

Ocean microbes capture solar energy, catalyze biogeochemical transformations of important elements, produce and consume greenhouse gases, and fuel the marine food web. “Microbes sustain all of Earth’s habitats, including its largest biome, the global ocean,” said Marian Carlson, director of life sciences at the Simons Foundation. “It’s critical that we know more about these important processes.”

“We are grateful for the generous support of the Simons Foundation for basic research that is the fundamental underpinning of our knowledge of the ocean, said Richard Murray, WHOI Deputy Director and Vice President for Research. “Understanding elemental ocean processes is the equivalent of understanding the human body’s basic workings. Without this information, we cannot understand, or protect, our ocean’s and planet’s health.”

The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment. For more information, please visit www.whoi.edu.

About the Simons Foundation

The Simons Foundation’s mission is to advance the frontiers of research in mathematics and the basic sciences. We sponsor a range of programs that aim to promote a deeper understanding of our world. For more information, go to www.simonsfoundation.org

WHOI Scientists Make Woods Hole Film Festival Appearance

July 17, 2020

Woods Hole Oceanographic Institution (WHOI) scientists appear in two shorts and a feature film at this year’s Woods Hole Film Festival (WHFF). In addition, scientists will also participate in Q&A sessions connected to three of the festival’s feature-length, ocean-themed entries.

The short films, “Divergent Warmth” and “Beyond the Gulf Stream” are part of a program titled “The Blue Between Us,” offered on-demand from July 25 to August 1 as part of the festival’s virtual program.

In “Divergent Warmth,” producer-director Megan Lubetkin gives viewers a behind-the-scenes look at the synchronized ballet aboard a research vessel during a recent expedition to the East Pacific Rise. Experimental music provides rhythm to imagery of deck operations, launch and recovery of the human-occupied submersible Alvin, and other-worldly views of seafloor hydrothermal vents and lava flows. Interwoven throughout is an evocative reading of Adrienne Rich’s poem, “Diving into the Wreck.”

Dan Fornari, a WHOI emeritus research scholar, acted as associate producer of the 10-minute film. As one of the scientists on the December 2019 expedition, he invited Lubetkin, herself a scientist and the creative exhibits coordinator with the Ocean Exploration Trust, to assist with subsea camera operations and video data management on board. Lubetkin spent her free time shooting additional video, which she edited together while still on the ship to produce a first draft of “Divergent Warmth.”

“I was blown away. It was just fabulous,” Fornari said of his first viewing. “It captures the spirit of going out to sea and being involved in this exploratory effort in the alien realm, where very few people get to go.”

The complex winter currents that collide off the coast of Cape Hatteras are the focus of “Beyond the Gulf Stream,” a short documentary by the Georgia-based production company MADLAWMEDIA. Filmed aboard the WHOI-operated research vessel Neil Armstrong, the 10-minute film features WHOI physical oceanographers Magdalena Andres, Glen Gawarkiewicz, and graduate student Jacob Forsyth as they share their perspectives on the challenges and rewards of doing scientific research at sea, often in difficult conditions.

“I think we have a responsibility to communicate science and the process of doing of science to the public,” said Andres about the film, which was produced in collaboration with WHOI and the Skidaway Institute of Oceanography at the University of Georgia. “It does a really nice job of capturing life at sea in the wintertime.”

As a scientist who uses video to capture data from the ocean depths, Fornari is highly attuned to the impact that visual media can have in capturing the public’s imagination about the ocean.

“These kinds of artistic expressions help open doors to people’s minds.” he said. “That’s crucial for getting the public to understand how critically important the oceans are. Then maybe more students will say, ‘I want to be an ocean scientist when I grow up.’”

In addition to the shorts program itself, WHOI scientists, staff, and students will also participate in “Filmmaker Chats” open to the public and broadcast via Zoom, as well as the WHFF Facebook and YouTube channels. Maddux-Lawrence will take questions about “Beyond the Gulf Stream” on Sunday, July 19, beginning at 9:00 a.m. On Friday, July 31 at 9:00 a.m., Lubetkin will appear with Fornari, as well as Alvin pilot Drew Bewley, MIT-WHOI Joint Program graduate student Lauren Dykman, and Texas A&M graduate student Charlie Holmes II to discuss the making of and science behind “Divergent Warmth.” Recordings of both sessions will also be available for viewing afterward on the festival website.

In addition to the short films, WHOI whale biologist Michael Moore appears in the feature-length documentary “Entangled,” which looks at the intertwined plights of the critically endangered North Atlantic right whale and coastal fishing communities in New England and eastern Canada. After being hunted for centuries, the whales face new challenges in the form of climate change and increased fishing and shipping activity, and Moore has been an outspoken proponent of the need for increased protections to stave off their slide to extinction within the next 20 years.

WHOI scientists will also add their perspective to Q&A sessions following several ocean-themed, feature-length films selected for the festival:

  • Thursday, July 30, at 10:00 p.m.: Research specialist Hauke Kite-Powell will answer questions related to aquaculture and seafood in relation to the film “Fish & Men.
  • Saturday, August 1, from 4:00 to 5:00 p.m.: Marine chemist Chris Reddy will answer questions about microplastics in relation to the film “Microplastics Madness.”
  • Saturday, August 1, from 7:00 to 8:00 p.m.: Marine biologist Simon Thorrold will answer questions about marine protected areas and fishing in connection with the film “Current Sea.”

Key Takeaways

  • Films featuring WHOI scientists will be screened as part of “The Blue Between Us” shorts program at the virtual Woods Hole Film Festival, which may be viewed online by festival passholders and individual ticketholders during the festival, which runs from Saturday, July 25, to Saturday, August 1. Tickets and more information is available here.
  • Whale biologist Michael Moore will appear in the feature-length film “Entangled” about the plight of critically endangered North Atlantic right whales.
  • WHOI scientists will also participate in Q&A sessions associated with several ocean-themed, feature-length festival films.
  • More information is available on the festival website.

What did scientists learn from Deepwater Horizon?

April 20, 2020

Paper reviews major findings, technological advances that could help in next deep-sea spill. 

Ten years ago, a powerful explosion destroyed an oil rig in the Gulf of Mexico, killing 11 workers and injuring 17 others. Over a span of 87 days, the Deepwater Horizon well released an estimated 168 million gallons of oil and 45 million gallons of natural gas into the ocean, making it the largest accidental marine oil spill in history.

Researchers from Woods Hole Oceanographic Institution (WHOI) quickly mobilized to study the unprecedented oil spill, investigating its effects on the seafloor and deep-sea corals and tracking dispersants used to clean up the spill.

In a review paper published in the journal Nature Reviews Earth & Environment, WHOI marine geochemists Elizabeth Kujawinski and Christopher Reddy review what they— and their science colleagues from around the world—have learned from studying the spill over the past decade.

“So many lessons were learned during the Deepwater Horizon disaster that it seemed appropriate and timely to consider those lessons in the context of a review,” says Kujawinski. “We found that much good work had been done on oil weathering and oil degradation by microbes, with significant implications for future research and response activities.”

“At the end of the day, this oil spill was a huge experiment,” adds Reddy. “It shed great light on how nature responds to an uninvited guest. One of the big takeaways is that the oil doesn’t just float and hang around. A huge amount of oil that didn’t evaporate was pummeled by sunlight, changing its chemistry. That’s something that wasn’t seen before, so now we have insight into this process.”

Released for the first time in a deep ocean oil spill, chemical dispersants remain one of the most controversial debates in the aftermath of Deepwater Horizon. Studies offer conflicting conclusions about whether dispersants released in the deep sea reduced the amount of oil that reached the ocean surface, and the results are ambiguous about whether dispersants helped microbes break down the oil at all.

“I think the biggest unknowns still center on the impact of dispersants on oil distribution in seawater and their role in promoting—or inhibiting—microbial degradation of the spilled oil,” says Kujawinski, whose lab was the first to identify the chemical signature of the dispersants, making it possible to track in the marine environment.

Though the authors caution that the lessons learned from the Deepwater Horizon release may not be applicable to all spills, the review highlights advances in oil chemistry, microbiology, and technology that may be useful at other deep-sea drilling sites and shipping lanes in the Arctic. The authors call on the research community to work collaboratively to understand the complex environmental responses at play in cold climates, where the characteristics of oil are significantly different from the Gulf of Mexico.

“Now we have a better sense of what we need to know,” Kujawinski says. “Understanding what these environments look like in their natural state is really critical to understanding the impact of oil spill conditions.”

Additional authors of the review are chemist Ryan P. Rodgers (Florida State University), and microbiologists J. Cameron Thrash (University of Southern California, Los Angeles), David L. Valentine (University of California Santa Barbara), and Helen K. White (Haverford College).

 

Funding for this review was provided by the Gulf of Mexico Research Initiative, the Henry Dreyfus Teacher-Scholar Award, the National Academies of Science, Engineering, and Medicine Gulf Research Program, and the National Science Foundation.

Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment. For more information, please visit www.whoi.edu.

Key Takeaways

  • Some coastal ecosystems around the Gulf of Mexico recovered, but in areas such as deep-sea coral communities, the oil, gas and dispersants combined with other stressors to create long-lasting impacts.
  • Gene analysis tools, used on a wide scale for the first time, provided unprecedented insights into which microbes consumed oil, gas and dispersants in marine ecosystems.
  • Advanced chemical analysis showed for the first time that weathering on the ocean surface, particularly by sunlight and oxygen (photo-oxidation), changed the composition of oil but reduced the effectiveness of dispersants applied to the surface.
  • The spill science community can be most effective by working collaboratively across academia, industry and government in the event of future oil releases in the deep sea and high latitudes.

Oceanus Magazine

Oil in Our Coastal Back Yard

Oil in Our Coastal Back Yard

October 13, 2004

On September 16, 1969, the barge Florida ran aground off Cape Cod, rupturing its hull and spilling 189,000 gallons of No. 2 fuel oil. Winds and waves pushed the oil onto the beaches and marshes of West Falmouth, Massachusetts, carrying with it dead lobsters, scup, and cod.

In the weeks and months after the spill, biologists Howard Sanders and George Hampson from the nearby Woods Hole Oceanographic Institution (WHOI) collected samples of mud and animals from the marsh sediments, particularly from an area known as Wild Harbor. They shared their samples with Max Blumer and Jerry Sass, WHOI geochemists who knew how to analyze oil with one of the field’s newest tools, the gas chromatograph.

Together, they made a discovery that refuted the prevailing wisdom of the day: Oil lurked in the marsh and sub-tidal sediments long after it was no longer visible in the water and on the beaches.

Three decades later, my research group returned to those marshes. Equipped with our own state-of-the-art equipment—a two-dimensional gas chromatograph—we analyzed new sediment samples and made our own discovery. Some of the oil from the Florida spill is still buried in the mud, and its chemical composition has not changed dramatically since the mid-1970s.

The Florida oil spill is perhaps the longest studied in history, and it has fundamentally changed our understanding of what happens to oil in coastal ecosystems. We are still building on this groundbreaking research, seeking knowledge that could help mitigate the environmental impact of future oil spills and the costs of cleaning them up.

Oil spills are awful for the environment, but they provide an excellent opportunity to study how the ocean and its ecosystems respond to extreme events. Most people see a spill and focus only on its toxic effects. But my group also sees it as a huge injection of carbon-based food for microbes in the coastal environment. We ask questions like: How long does it take the oil to decay and be consumed by microbes? How long will oil persist at a particular location and why? Do people need to intervene and assist in cleanups, or can Mother Nature remediate the ecosystem herself?

A history of spills and research

New England relies heavily on barges for transporting fuel to its major ports and cities. For decades, Buzzards Bay has been a major thruway for oil barges, with approximately 2.1 billion gallons of oil traveling through the Cape Cod Canal each year. With so many barges navigating these rocky and narrow waterways, spills due to mechanical or human errors are almost inevitable.

In September 1969, the inevitable happened. When the barge Florida ran aground, it released the largest amount of oil spilled in Buzzards Bay history. “The oil-soaked beaches were littered with dead or dying fish,” wrote Hampson and Sanders in Oceanus at the time. “Fish, crabs, and other invertebrates covered the shores of the Wild Harbor River and large masses of marine worms, forced from their natural habitat in the sediments, lay exposed and decaying in the tidal pools.” (Download a PDF version of the 1969 Oceanus article, located in related files box to right).

Applying the most advanced analytical techniques of his day, Blumer was able to study the chemical composition of the oil from the spill. Oil products such as No. 2 fuel oil are made up of hundreds of individual chemicals that vary in their characteristics, such as volatility, solubility, and toxicity. Blumer was able for the first time to tease out which compounds had evaporated and decayed and which remained in Wild Harbor. He saw some of the oil’s constituent parts, rather than one uniform chemical.

WHOI salt marsh ecologist John Teal and graduate student Kathy Burns also studied Wild Harbor through the mid-1970s, and Teal and chemist John Farrington revisited the old spill in 1989. Each time, remnants of the 1969 oil persisted.

Several other oil spills have occurred in Buzzards Bay since the Florida spill (Table 1). In October 1974, thousands of gallons of No. 2 fuel oil from the barge Bouchard 65 poured into the bay, with the greatest impact in Winsor Cove, just two miles north from Wild Harbor. Building on their experience, WHOI researchers measured and chronicled the 1974 spill for comparison with the 1969 event, as both involved the same type of fuel and neighboring but somewhat different shorelines. We have found that oil at Winsor Cove from the Bouchard 65 spill also continues to persist.

The amount of oil spilled in each case has been rather small compared to some high-profile spills like the Exxon Valdez. But the convergence of these spills, all occurring within 10 miles of Woods Hole, has created a unique natural laboratory for investigations of the short- and long-term fates of oil in the coastal ocean.

Who does the better cleanup job?

The Oil Pollution Act of 1990 requires that parties responsible for an oil spill must attempt to restore the environment to its pre-spill condition. One popular approach is “natural attenuation,” allowing or promoting natural processes to clean up and remove contaminants from affected areas.

Environmental scientists presume that, over time, naturally occurring or artificially transplanted microbes will eat hydrocarbons in the petroleum soup. It is an attractive, feel-good alternative when compared with labor-intensive and costly clean-up schemes, and several studies have shown that natural attenuation can sometimes be as effective as human intervention.

In the first days and weeks after a spill, physical processes churn the oil around in the water, exposing it to air and sunlight, causing some compounds to evaporate or be broken up. Then the “bugs” take over.

Oil spills can deliver a staggering amount of carbon—that is, food for the ecosystem—in a short period of time. A rough calculation from the Florida spill indicates that 50 to 100 grams of carbon were added to each square meter of the impacted area in one day. By comparison, natural photosynthesis by plants yields about 300 grams of carbon per square meter in an entire year.

Petroleum hydrocarbons provide a rich source of high-caloric food for a variety of microbes. In many ways, these microbes match or exceed humans in their chemical skills, using an incredible toolbox of enzymes to break down complex petroleum compounds. But just how do these microbes break down the oil? And why haven’t they eaten all of the oil from the 1969 spill?

Back to the future

Answering these questions requires that we learn a lot more about the chemistry and composition of oil and its natural degradation processes. We started by going back to the site of our predecessors’ work in the salt marshes of Wild Harbor and Winsor Cove.

We collected dozens of sediment cores, particularly from a site in Wild Harbor that was named M-1 (marsh sample 1) by previous WHOI scientists. Like Blumer a generation ago, we brought a powerful scientific tool to bear on the problem. With colleagues Glenn Frysinger and Richard Gaines of the U.S. Coast Guard Academy, we analyzed our sediment samples using a novel technique called comprehensive two-dimensional gas chromatography (GCxGC) in order to observe how the composition of the 30-year old oil had changed while buried in the marsh. It was the first time anyone had used GCxGC to analyze a real-world oil spill.

With traditional one-dimensional gas chromatography (GC, as used by Blumer’s generation of environmental chemists), scientists could identify about 10 percent of the compounds in the oil, a quantum leap for the era. But that process still leaves a haystack of many compounds (such as branched alkanes, cycloalkanes, aromatics) that cannot be identified. On a data plot, it looks like a large hump that we call the unresolvable complex mixture (UCM). Too many of the compounds have similar properties and when analyzed with a simple chromatograph, they merge together, making it impossible to tell one from the other.

With modern GCxGC, we can find needles in that haystack (see bottom of page). We have been able to separate and identify many more compounds and provide a more refined inventory of the petroleum hydrocarbons that persist in the marsh.

We found that the oil at the M-1 site had not weathered significantly since the mid-1970s, and most of the compounds typically found in oil are still present after three decades. As we peered into the previously unresolved mass, for instance, we found that certain types of alkanes remain, despite earlier research that suggested they were completely degraded.

The oil for food program

I doubt many people would have predicted in 1969 that oil from the Florida spill would still be present after three decades. The entire marsh continues to be mildly affected, and there are certain areas along the shoreline where oil is particularly concentrated. Why doesn’t the oil go away?

Our findings from Wild Harbor and Winsor Cove suggest that some marsh sediments might be ideal for preserving partially weathered petroleum. Evidence indicates that oil-consuming bacteria may have stopped eating these hydrocarbons more than 25 years ago. Though a diverse community of microbes exists in the contaminated regions of Wild Harbor, they are not actively consuming the remaining oil.

We have started numerous experiments to figure out this riddle, and knowing what types of chemicals remain can provide essential clues. The contaminated sediments may now lack oxygen required by some microbes to degrade hydrocarbons rapidly. Perhaps the environment is missing a key chemical species—such as sulfate—that bacteria need to consume and change the remaining oil compounds.

Perhaps the chemical bonds and structures of certain oil compounds locked the microbes out, resisting their chemical attacks. Or maybe the microbes prefer to eat more readily available food sources such as plant debris.

A new spill to investigate

On April 27, 2003—just six months after we published our findings on the 1969 oil residues in Wild Harbor—the barge Bouchard 120 struck an underwater ledge while being tugged to a power plant. At least 98,000 gallons of No. 6 fuel oil poured into Buzzards Bay, and within 24 hours, helicopter surveys showed a 12-mile oil slick. Viscous, tarry petroleum washed up on the beaches of one of New England’s richest tourist and shellfishing grounds.

Like our WHOI predecessors, Research Associate Bob Nelson and I went to the beaches to collect samples and observe firsthand the war between industrialized society and Mother Nature. We scooped floating “pancakes” of petroleum, filled bottles with oily blue water, and collected tarred cobbles and sediments.

After a year of analyzing samples, we have been able to determine the original chemical composition of the Bouchard 120 oil and track how it has changed. Our early results show that several groups of compounds were lost to evaporation, water washing, and microbial degradation. The degradation of oil, however, seems to have stalled after the initial breakup in the first six months. Because the responsible party removed nearly all of the oil-impacted rocks at this site, we can no longer collect samples there.

Treasures in the attic

Coastal oil spills are incredibly destructive, with intense short-term consequences and insidious long-term ones. No one wants to witness an oil spill, but they happen. And when they do, we need to take advantage of the opportunity to learn from them.

WHOI is an extraordinary place to do that, thanks to three decades of samples and memories in these labs. As recently as June 2004, Bruce Tripp, a long-time member of the research staff and participant in the earlier oil spill studies, handed me a dusty jar he had recently found in a storeroom. It was a sample of oil collected by WHOI scientists in 1974 from the Bouchard 65 spill, which will be invaluable for our continued work on coastal spills.

—The National Science Foundation, the Petroleum Research Fund, the Environmental Protection Agency, the Office of Naval Research, and the WHOI Coastal Ocean Institute provided funding for this research.

—Science writer Mike Carlowicz and Research Associate Robert Nelson contributed to this article.

Living Large in Microscopic Nooks

Living Large in Microscopic Nooks

August 24, 2004

Between a rock and a hard place is the proverbial worst spot for people to find themselves in. But for certain deep-sea microbes, it’s the place to be. In 2000, to our surprise, we found that microscopic nooks and pits within volcanic seafloor rocks harbor abundant colonies of previously unidentified microbes.

These microbes are different from other microorganisms living in the sunless depths. They do not obtain the energy they need to grow and multiply by metabolizing chemicals dissolved in seawater or in hydrothermal fluids venting from the seafloor. Instead, these newly discovered microbes are living directly off minerals in solid seafloor rocks.

The microbes are oxidizing iron in the rocks, chemically altering the rocks, and harnessing the energy produced by this chemical reaction to live. Their discovery has raised a slew of intriguing questions:

  • Does our planet sustain abundant and ubiquitous populations of these microbes?
  • Do they play a pivotal role in chemically altering Earth’s crust?
  • Were they pioneering life forms on an early Earth, which was largely devoid of oxygen but full of iron?
  • Do they exist on other iron-rich, oxygen-poor planetary bodies such as Mars?

These previously inconspicuous microorganisms may turn out to have starring roles in shaping the evolution of life on Earth and other planets, and shaping the evolution of the planet itself.

So why didn’t we notice them before? Beyond the inherent difficulties and expense of searching for microorganisms at the bottom of the ocean, the answer is that we hadn’t really looked for them before. But now these easy-to-overlook microbes have become hard to ignore.

Pumping iron on the seafloor

More and more, we are learning how life on the Earth and the Earth itself—biology and geology—are intimately intertwined and evolve together. Microbes are ubiquitous catalytic agents, sparking chemical reactions that alter the physical and chemical properties of their surroundings. Beyond our scope of vision, their cumulative metabolic activities play a fundamental role in shaping and regulating our environment. (Our world would be completely different, for example, if microorganisms did not continuously decompose organic matter and transform it back into inorganic material.)

A new field has arisen called geomicrobiology. Scientists are now taking a closer look at many unexplored regions of our planet, and other planets, searching for populations of unknown microbes that may play major roles in cycling chemicals through planetary systems.

In geomicrobiology, the borders between rocks and living things are not so ironclad. Many rocks are, however, and the microbes we found steal electrons from iron atoms in the rock, changing them from ferrous (Fe+2) to ferric (Fe+3). With the energy produced by this chemical reaction, they convert carbon dioxide (from seawater) into organic matter—much the way plants and plankton use solar energy and photosynthesis to accomplish the same.

Microscopic, but mighty
Iron is one of the most abundant and reactive elements in the environment near Earth’s surface, so the discovery of iron-oxidizing microbes raises the potential that massive communities of them may exist on Earth. If so, they could continually extract huge amounts of carbon dioxide from seawater and microscopically exert a huge influence on ocean chemistry over geologic time.

Does this large-scale drawdown of carbon dioxide from seawater help the oceans absorb carbon dioxide, a critical greenhouse gas, from the atmosphere? If so, it would revise our understanding of how carbon cycles through the planetary system—perhaps giving iron-oxidizing microbes an important, previously unknown role in the evolution of Earth’s climate.

In their own way, the rise of microscopic photosynthetic plants caused one of the most devastating, permanent alterations in all of Earth’s history. They changed the chemical composition of the near-surface environment that all life depended on, by simply pumping oxygen into Earth’s atmosphere.

Before then, neither the atmosphere nor the oceans contained much oxygen, but the oceans were filled with iron-rich rocks and tons of dissolved iron. In such an iron-rich, oxygen-poor environment, iron-oxidizing microbes may have been dominant, pioneering life forms—a concept that compels us to reassess our thinking about the evolution of life on early Earth.

The existence of iron-oxidizing microbes also redirects our search for life elsewhere in the universe. Similar microbes could have thrived, or still thrive, in other iron-rich, oxygen-poor locales—such as Mars, with its red, iron-rich soil, or on the volcanic seafloor below the ice-covered ocean of Jupiter’s moon, Europa.

A search for unknown life

These unexpected new lines of inquiry began in 2000 when former WHOI Postdoctoral Scholar Tom McCollom and I, with funding from the Mellon Foundation and the National Science Foundation (NSF), joined a research cruise aboard R/V Atlantis off the Oregon coast.

Since the late 1970s, when hydrothermal vents were discovered, scientists have focused on deep-sea chemosynthetic microbes that derive energy from dissolved hydrogen, hydrogen sulfide, and methane emitted from these sites. Though it is easier for microbes to draw energy from chemicals dissolved in seawater, WHOI biologist Carl Wirsen and others had found evidence of sulfur-oxidizing bacteria that used solid minerals as their only source of energy. (See Is Life Thriving Beneath the Seafloor?)

Enormous amounts of sulfur and sulfides are found in vent chimney rocks, in broken chimney rubble on the seafloor, and in fine-grained mineral particles that precipitate and “rain” out of plumes of hydrothermal fluids spewing out of chimneys. We speculated that this little-recognized but potentially large source of chemical energy may sustain important microbial communities, which, in turn, could play pivotal roles in altering the chemistry of seafloor rocks and the ocean itself.

Our goal in 2000 was to try to identify unknown microbes that live off solid minerals and that might be mediating large-scale geochemical changes on Earth.

The perfect niche for microbes

To explore what might be down there, we used the submersible Alvin to place a variety of microbe-free samples of natural seafloor rock back on the seafloor. Our aim was to see what might “grow” on these “blank slates.”

WHOI geochemist Meg Tivey retrieved our experimental samples for us during an Alvin dive two months later (SeeThe Remarkable Diversity of Seafloor Vents). To our surprise, we found that many of the samples had thick burnt-orange coatings of oxidized iron (or “rust”).

Using a scanning electron microscope, we saw that the surfaces of the samples were scarred with abundant pits and pores less than 20 microns (0.0004 inches) deep and wide. In these tiny pits were large accumulations of corkscrew-shaped stalks made of iron oxide, which created the thick rusty coating.

Here’s what we believe is happening: Iron-oxidizing microbes exploit a niche where the chemistry is just right. At first, oxygen-loving microbes move into the pits. They consume the available oxygen, which is not replenished because seawater does not readily flow into the restricted pit areas.

That creates an ideal situation for the iron microbes, which need low-oxygen conditions. The tiny sheltered coves within seafloor rocks contain just enough oxygen from seawater for the iron microbes to respire, but not an overwhelming amount that would oxidize all the iron—without microbial intervention—before the microbes could use it.

As a byproduct of their iron-oxidizing process, the microbes produce bundles of iron-oxide stalks that resemble a little girl’s braids. These stalk accumulations effectively cap the pits, maintaining the iron microbes’ preferred low-oxygen environment and securing their turf.

FeMO—a microbial observatory

The rapid proliferation and sheer abundance of these iron microbes and the quick chemical transformation of the rocks they lived on were eye-opening. Now we have mobilized research that combines biology, chemistry, and geology to explore many intriguing aspects of these iron microbes.

Among the initial questions are: What kinds of iron-oxidizing microbes are out there? How many are there? How are they making a living?

These species have been notoriously difficult to grow in the laboratory and therefore difficult to learn about. But in our lab Dan Rogers and I, along with WHOI biologist Eric Webb and others, have made strides recently to culture and interrogate these elusive microbes, and we have begun to identify various species of microbes and reveal their biochemical machinery and metabolic capabilities.

Toward this end, we have just established “FeMO”—an Iron (Fe)-oxidizing Microbe Observatory—to study these microbes at a site where they are diverse and prolific. It is located at Loihi, an active, submerged volcano, relatively conveniently located only 25 miles southwest of the big island of Hawaii.

To investigate the potential abundance of iron microbes, WHOI geochemist Wolfgang Bach and I analyzed rock samples retrieved from an assortment of holes drilled by the Ocean Drilling Program into the exposed volcanic rock that spreads out on both sides of the mid-ocean ridge mountain chain encircling the globe. We found that older rocks were depleted of Fe+2 and full of Fe+3—exactly what iron-oxidizing microbes use up and leave behind. The finding suggests that mid-ocean ridge flanks represent millions of square miles of fertile habitat for iron microbes.

Life on early Earth and elsewhere

We have also begun to sequence genomes of these microbes, in a project with Mitch Sogin and Ashita Dhillion at the Marine Biological Laboratory in Woods Hole, funded by the National Aeronautics and Space Administration’s Astrobiology Institutes Program (NAI). These microbes are pioneers that probably lived billions of years ago on Earth and may exist on other planetary bodies. Identifying their genes, the enzymes they produce, and the metabolic pathways these enzymes catalyze will reveal an evolutionary heritage that will help us unravel the emergence and development of life on Earth and guide our search for life elsewhere in the universe.

A key to reconstructing the evolution of life on Earth and other planetary bodies lies in the ability of scientists to read the records, or “biosignatures,” that long-dead microbes leave behind in ancient or extraterrestrial rocks. To do that reliably, scientists must be able to distinguish changes caused by microbial activity from those caused by abiotic oxidizing processes such as rusting.

With this goal, scientists in our group, including Bach, Postdoctoral Scholar Olivier Rouxel, and graduate student Cara Santelli, are advancing a range of new approaches to gain understanding of how microorganisms affect the microtextures, isotopic chemistry, and history of the rocks they interact with.

If we can unravel their story, these long-neglected microbes will reveal a profound tale about the co-evolution of Earth and life.

Mixing Oil and Water

Mixing Oil and Water

June 23, 2004

Drop by drop—that is how most oil enters the oceans. Catastrophic spills make the headlines, but it is the chronic dribble, dribble, dribble of seemingly small inputs that supplies most of the oil polluting the world’s oceans.

In recent decades scientists have made substantial progress in understanding how oil enters the oceans, what happens to it, and how it affects marine organisms and ecosystems. This knowledge has led to regulations, practices, and decisions that have helped us reduce sources of pollution, prevent and respond to spills, clean up contaminated environments, wisely dredge harbors, and locate new petroleum handling facilities.

But tracking the sources, fates, and effects of oil in the marine environment remains a challenge for a number of reasons. For starters, oil is a complicated mixture of hundreds, sometimes thousands, of chemicals. Every source of oil, and even the same general types of oil (crude oils or fuel oils, for example) can have distinctive compositions depending on which oil field or well they came out of and how they were refined.

This varying, complex mixture of chemicals gets spilled or seeps into an already complex chemical chowder of seawater, mud, and marine organisms in the ocean. There, the oil is stirred by currents, tides, and waves, altered by other physical processes, and changed further by chemical reactions and interactions with organisms in the sea.

In the midst of this dynamic situation, scientists seek to pinpoint the impacts of oil on myriad individual species, as well as on entire ecosystems. Here the challenge comes full circle, because we now know that the impact of oil can vary greatly, depending on its distinctive chemical composition. But let’s start at the beginning.

How does oil get into the ocean?

Scientists have known since the 1970s that accidents account for only a small percentage of the oil entering our waters. In fact, accidental spills of all types—from ships, shore facilities, pipelines, and offshore platforms—contributed just 9.8 percent of the oil entering the marine environment on an annual, worldwide basis between 1990 and 1999 (but just 3 to 4 percent in U.S. waters).

That doesn’t mean we should dismiss the importance of spills. Accidents such as the 1989 Valdez incident off Alaska or the 2002 Prestige spill off Spain can have devastating effects on marine life and on people’s ability to use the ocean. The impact of a spill depends on the type of oil, the amount spilled, the ocean and weather conditions, and the dynamics of the area or ecosystem where it takes place.

Progress in prevention—through more stringent laws, rules, and guidelines, and increased vigilance by industry and regulators—has reduced accidental spills, at least in developed countries. For instance, studies of tanker spills have prompted regulations for the steady, ongoing replacement of single-hulled tankers in the world fleet with double-hulled tankers.

But spills are just one small way, albeit dramatic, for oil to mix with our waters. So where is the rest of it coming from?

• Seeps: Between one-third and one-half of the oil in the ocean comes from naturally occurring seeps. These are seafloor springs where oil and natural gas leak and rise buoyantly from oil-laden, sub-seafloor sediments that have been lifted close to the earth’s surface by natural processes.

If oil is natural to the oceans and if it is the biggest source of input, what is the fuss about oil as a pollutant? The answer lies in the locations and rates of oil inputs. Oil seeps are generally old, sometimes ancient, so the marine plants and animals in these ecosystems have had hundreds to thousands of years to adjust and acclimate to the exposure to petroleum chemicals. On the other hand, the production, transportation, and consumption of oil by humans often results in the input of oil to environments and ecosystems that have not experienced significant direct inputs and have not become acclimated.

• Extraction: Accidental and normal operation of oil drilling and production platforms puts some oil into the sea, including oil mixed into the briny waters that escape from the oil reservoirs. But this spillage and waste, and the atmospheric releases from platform equipment, is one of the smallest sources of oil in the sea.

• Transportation: In the 1960s and 1970s, scientists studied tar balls collected along major tanker routes and the beaches downstream. They revealed that large amounts of oil were entering the marine environment as a result of ballast water (bilge) discharges and other aspects of “normal” tanker operations. As a result of that research, international conventions and national laws and regulations have led oil shippers to minimize their discharges, particularly in harbors. Today, in spite of increasing numbers of tankers plying the seas, the amount of oil spewed has stabilized or decreased in many places.

• Consumption: The everyday use of oil in cars, trucks, industrial and manufacturing plants, and other machinery of the modern economy is the most egregious and insidious source of oil pollution. The drippings and emissions from millions of machines accumulate on land and eventually run into our waterways.

From the 1950s through the 1970s, one of the most common sources was the indiscriminate disposal of used automobile crankcase oil down sewer systems. Since that time, scientific findings have prompted regulations and public awareness campaigns that promote the recycling and proper disposal of used oil.

But we still have a problem. Peer underneath your parked car and observe the drip of oil. This happens day and night for millions of automobiles, trucks, and buses, creating a chronic, significant source of oil to the sea. Rainstorms wash these drippings into streams or storm sewers that discharge into harbors and rivers. It is terribly hard to measure these types of inputs.

Fossil fuel hydrocarbons from engine exhaust also accumulate in the atmosphere. Sometimes the soot is deposited into our waters; otherwise it is washed out of the atmosphere and into the oceans by rain or snow.

On a smaller scale, new research has shown that outboard engines of small boats and pleasure craft are a significant source of oil pollution—up to 2.2 percent of all inputs in U.S. waters (worldwide data are not available). Engine manufacturers are responding with new models of engines that release much less oil and gasoline, but it may take some time for these changes to propagate through the boating community.

What happens to oil in the ocean?

Oil chemicals entering the ocean have many fates. Volatile chemicals are lost by evaporation to the atmosphere. Other chemicals are broken up by photochemical reactions (catalyzed by sunlight). Bacteria can degrade certain oil components.

The combination of biological, physical, and chemical processes is usually referred to as weathering. These weathering reactions have different rates depending on the chemical structure of the oil, habitat conditions (such as water temperature or oxygen and nutrient supply), and mixing of the water by wind, waves, and currents. In some spills, oil does not last much beyond weeks to months.

But when oil pours into shallow waters with muddy sediments—such as marshes or lagoons—and conditions allow the oil to become mixed into the mud, it will generally persist for a long time. This is a result of the fundamental chemistry of oil compounds. Since they don’t dissolve in water, oil compounds tend to adhere to particles in the water or get incorporated into biological debris, such as fecal matter or dead organisms. These oiled particles and debris settle from the water column and become part of the sediments on the bottom.

Once mired in the sediment, some oil chemicals can persist for years or decades, depending on the environment. In areas swept by high-energy currents, the material may be dispersed. In areas where sediments accumulate (such as ship channels through urban harbors), the contaminated sediments become an environmental concern—both when simply lying on the bottom and when channels are dredged and the mud must be disposed.

For some types of oil inputs and some ecosystems, we now have enough information to model and simulate the fate of oil in the water. This scientific knowledge helps policymakers make better decisions on how to dredge harbors, control pollution sources, clean up contaminated areas, and locate petroleum facilities.

How does oil affect marine life?

From experiments and field measurements, we know that certain types and concentrations of petroleum chemicals can harm marine life. Long-term effects of oil exposure can alter the physiology and ecology of populations of marine organisms, especially those found in sensitive habitats.

Biological and physical processes can reduce the concentration of oil chemicals in an ecosystem, especially if the source of pollution is cut off. As concentrations decline and chemical compositions change, plant and animal communities usually rebound. But the recovery can range from months to decades depending on the chemistry, the conditions, and the organisms and ecosystems affected.

One of the significant advances in the 1970s and 1980s was the development of guides to the sensitivity of various types of coastal ecosystems to oil pollution. Maps of sensitive ecosystems are now used during responses to accidental oil spills, improving the ability of resource managers and engineers to assess where containment booms and other prevention and cleanup measures should be deployed.

There have been few studies, however, on the cumulative effect of chronic inputs of oil to the marine environment, including the many sources associated with oil consumption on land. Assessing these impacts is complicated because oil runoff is often accompanied by other polluting chemicals, making it difficult to tease out which ones have which deleterious effects. Limited experiments have taught us that the interactive effects among chemicals can either increase or decrease each chemical’s long-term effects, depending on the organisms and chemicals.

Much of our knowledge about the effects of oil is still limited. It has focused on biochemical and physiological effects on a few individual organisms and on the degradation of a few particular habitats. But we need a better understanding of the large-scale effects of oil on entire communities and populations, rather than individual organisms. The complexity of how species interact within ecosystems—such as how damage to one species can affect the other species that feed on it—leads to contentious debate whenever regulators start to weigh long-term impacts on marine life.

Fuel for further research

A high priority for the future should be better monitoring and research so we can better quantify how much oil is really entering our waters, how much of it is coming from each source, and what the effects may be.

We also need to expand research on oil pollution in deeper waters. Most concerns and research have traditionally focused on coastal waters. Yet new concerns arise as oil production moves offshore. We can only speculate on the impact of oil exploration and production in deeper waters until we have more detailed knowledge of the biological organisms in these habitats and the biogeochemical processes that govern their lives.

Developed countries and emerging countries consume more oil every year, and that consumption leads to more and more inputs of oil to the oceans. We have learned from our colleagues in developing countries that increased use of fossil fuels has not been accompanied by the sort of stringent regulations developed countries adopted after years of harsh lessons. Further research and education can help those countries minimize the adverse impacts of oil inputs on their oceanic ecosystems.

Editor’s Note: A recent study by the U.S. National Academy of Sciences, entitled Oil in the Sea III: Inputs, Fates and Effects, provided the foundation for this article. To read that report, visit www.nap.edu/books/0309084385/html.

The Remarkable Diversity of Seafloor Vents

The Remarkable Diversity of Seafloor Vents

February 13, 2004

In late summer of 1984, I anxiously awaited my first trip to the seafloor in the submersible Alvin. There was a delay in launching the sub, but I resisted the urge to have a drink, anticipating one final trip to the bathroom before crawling into Alvin’s three-person, 6-foot sphere for eight hours. I was excited not only about my first chance to dive, but about visiting the home of the seafloor rocks I had long been studying for my master’s thesis.

Since 1982, I had spent most of my waking hours examining pieces of seafloor vent deposits that had been recovered during a routine dredging operation along the Juan de Fuca Ridge off the Pacific Northwest coast. Expecting to find common seafloor rocks called basalts, scientists were surprised to pull up fragments and boulders of massive sulfide covered with small tubeworms. They had discovered the fourth and, at the time, newest site of hydrothermal venting on the seafloor, a place now known as the Main Endeavour Field. These rocks helped launch my scientific career.

In the years leading up to my 1984 dive, I had learned that hydrothermal vent systems played a significant role in transferring heat and mass from the solid Earth to the ocean, and that the vent sites host unusual biological communities, including tubeworms, bivalves, crabs, and fish that thrive in the absence of sunlight. It was also becoming clear that the relatively constant chemistry of the ocean was in part sustained by hydrothermal activity.

What my colleagues and I were only beginning to realize then was that hydrothermal vent systems are like snowflakes—no two are ever exactly alike.

Long-standing mysteries

When scientists in Alvin discovered the first active hydrothermal system in 1977 at the Galápagos Rift, they found warm fluids, later determined to be a blend of cold seawater and hot vent fluids, seeping from seafloor crust. Never-before-seen organisms were present at the vents, including large clams and tall, red-tipped tubeworms.

In 1979, a second active hydrothermal system was discovered along the East Pacific Rise. At that site, much hotter fluids (350°C) jetted from tall rock formations composed of calcium sulfate (anhydrite) and metal sulfides. When the clear hot fluid jetting from these chimney-like structures mixed with cold seawater, fine particles of dark metal sulfides precipitated out of solution, creating the appearance of black “smoke” (hence the name “black smoker chimneys.”)

The discovery of these vent systems immediately answered a question long posed by geophysicists: How is heat transferred from Earth’s interior to the oceans? Earlier studies had shown that, contrary to model predictions, not as much heat was being transferred by conduction (particle-to-particle transfer) near the ridge crests.

Scientists hypothesized that heat was also transferred by convection, as fluid circulated within the crust near mid-ocean ridges. Sure enough, cold seawater is entering cracks and conduits within seafloor crust. It is being heated by underlying rocks and rising and venting at the seafloor, carrying significant amounts of heat from Earth to ocean.

The chemistry of the ocean

The discovery of vents allowed scientists to begin to answer another major question: How does the ocean maintain its relatively constant chemical composition? Over time, rivers drain materials into the oceans, and winds blow in particles, some of which dissolve to add chemical elements to the oceans. Some of these elements, in turn, exit ocean waters, settling in ocean sediments, for example.

Many components of seawater—including lithium, potassium, rubidium, cesium, manganese, iron, zinc, and copper—enter the oceans via vents. They are leached from seafloor crust by subterranean chemical reactions with hot hydrothermal fluids. Other hydrothermal vent reactions draw elements out of seawater and place them back into the earth.

For example, magnesium eroded from land is carried to the ocean by rivers. Yet magnesium concentrations have not increased in the oceans. Scientists puzzled for decades over where all the magnesium could be going.

Scrutinizing hydrothermal vents, researchers found that seawater entering seafloor crust is rich in magnesium, but fluids exiting the vent are free of it. Oceanographers surmised that magnesium is left behind in the crust, deposited in clay minerals as seawater reacts chemically with hot rock.

Dive to Main Endeavour Field

In those early years when observations were few and samples fewer, my thesis rocks provoked considerable interest. In some ways, the rocks were similar to those recovered from the East Pacific Rise, but in other ways they were quite different.

The Main Endeavour Field samples were rich with amorphous silica, which should only precipitate if the hydrothermal fluid had cooled without mixing with seawater. And they did not contain anhydrite, a common vent chimney mineral that dissolves in seawater at temperatures less than about 150°C (302°F).

We theorized that the dredged pieces must have come from the low-lying mounds that lay beneath and around black smoker chimneys. Our dives to the Endeavour Field in 1984 would tell us if we were correct.

The trip to the seafloor took 90 minutes. As we approached the seafloor, the pilot asked me to look out my viewport and let him know when I saw bottom. Alvin’s lights were turned on. It was like the curtain going up in a dark theater and the stage lights going on. We were hovering over the same basaltic rocks that I had spent countless hours studying in photographs.

Then to my right, I could see a rock wall rising from the seafloor. It was obvious from the hedges of pencil-diameter tubeworms sticking out of the tops and sides of the cliffs that I was looking at large hydrothermal vent structures.The view was nothing like what had been described at the East Pacific Rise. Instead of low-lying mounds of sulfide debris topped by active smokers, we saw steep-sided structures standing 15 meters (50 feet) high.

Why was this site so very different? How could these chimneys stand there like multi-story buildings without collapsing?

A fluid environment

Answers to these questions came from studying new samples. We learned that the tall chimneys structures were essentially “cemented”—silica filled pores in the vent structure walls as the emerging fluids cooled, making the structures sturdy. But why was so much silica precipitating at this site?

The fluid chemistry provided answers: This vent site’s fluids were rich in ammonia (NH3). As the ammonia-rich fluids cool, NH3 takes up excess H+ ions to form NH4, which raises the pH of the fluids. The higher pH likely allows silica to precipitate within Main Endeavour Field structures; at sites with no NH3, low pH probably inhibits the formation of amorphous silica.

As with almost every visit to a new vent site, our survey of the Main Endeavour Field raised as many questions as it answered. The differences among known hydrothermal systems and the revelations that accompanied each new discovery provoked oceanographers to hunt for new sites. We were explorers trying to learn as much as we could. Hypotheses were advanced, only to be proven wrong by yet another discovery.

More visits to these seafloor hot springs made it clear that all vent fluids are not the same. Rather, the chemical composition changes from ridge to ridge—and from time to time.

Researchers returning to some vents found that the chemistry of vent fluids was not constant, changing on scales ranging from days to years. These vent sites were all associated with sites of recent magmatic activity, with recorded earthquakes and evidence that dikes had been intruded into the ocean crust and, at some sites, that lava had been extruded onto the seafloor. The vent fluid compositions were changing as these dikes cooled, and as fluids penetrated deeper into the crust.

Searching for new vents

In the early 1980s, after a number of vent sites had been found in the Pacific, scientists began to wonder if hydrothermal activity and active black smokers might exist on the more slowly spreading Mid-Atlantic Ridge. The hydrothermal vent systems transfer large amounts of heat from magma or newly solidified hot rock, but on slow-spreading ridges the spreading rate, and magma delivery rate, is much less (about 1/3) of that on the northern East Pacific Rise and Juan de Fuca Ridge.

To our surprise, exploration from the mid-1980s through the early 1990s gradually made it clear that hydrothermal systems may be spaced further apart on the Mid-Atlantic Ridge, but they tend to generate much larger mineral deposits. Expeditions to the Southwest Indian Ridge in 2000 and the Gakkel Ridge (under the Arctic Ocean) in 2001 revealed that hydrothermal venting occurs on even the slowest-spreading portions of the mid-ocean ridge system.

New ways to find vents

Two decades of study have taught us that there is no single type of seafloor hydrothermal vent system. The plumbing systems beneath the seafloor are both diverse and incredibly complex.

Ocean scientists today are posing questions about the dimensions and evolution of the hydrologic systems beneath vent sites. We puzzle over how hot these fluids get, how deep into the crust they descend, and how far they travel before venting at the seafloor. And where does seawater enter these systems?

To answer these questions, we will need to continue exploring, not only over geographic space, but also over time. In the early years, most vents were discovered serendipitously, but as we’ve explored and learned more about these systems, we’ve been able to develop systematic methods for pinpointing sites.

For example, a technique of “tow-yowing” has been developed, where a conductivity-temperature-depth (CTD) sensor is raised and lowered through the water column in a saw-tooth pattern above the ridge to map the locations of plumes, and then to home in and map the buoyant portion of plumes coming directly from active vent sites. This technique was used successfully to find vent sites in the Pacific and Atlantic, and most recently in the Indian Ocean.

Seafloor observatories

The need to explore the dynamics of hydrothermal systems over time has led to new technologies and the development of seafloor observatories. New, more precise and durable instruments allow us to monitor temperature and fluid chemistry at vent sites for hours, days, or months—as opposed to observing those properties for brief moments and grabbing one-time samples.

The future of hydrothermal studies was displayed in a recent series of coordinated experiments. With support from the National Science Foundation’s Ridge Interdisciplinary Global Experiments (RIDGE) program, a team of researchers built a seafloor observatory on the Endeavour segment of the Juan de Fuca Ridge.

During the summers of 2000 and 2001, scientists made complementary and continuous observations centered around the Main Endeavour Field (the same site I first visited in 1984) and at vent sites to the north and south. The program goals included making more accurate measurements of the heat and mass flowing from the system, and observing how the hydrothermal plumbing is influenced by tides and by high-temperature reactions that separate elements into saltier liquids and more vapor-rich fluids (a process called “phase separation”).

Instruments were deployed to continuously monitor vent fluid temperatures, flow rates, and chemical properties. Scientists also used newly developed samplers to collect fluids at regular time intervals. While these instruments were in place, other researchers made acoustic images of vent structures and venting fluids. Still others used the Autonomous Benthic Explorer (ABE) to measure water column properties above the vent field, seafloor depth, and magnetic signatures.

Later in the program, the team deployed a systematic array of current meters, thermistor strings, magnetometers, and tilt meters. Scientists even tested techniques to “eavesdrop” on the data being collected and download it without removing the instrument from the vent. The result of this collective effort was the most comprehensive study of a hydrothermal system to date, and a model for future seafloor observatories.

A continually unfolding story

As we develop these new techniques and instruments, our ability to explore ongoing seafloor processes will grow. More than a quarter-century into our studies, we still find ourselves constantly revising and refining our ideas about hydrothermal systems.

At the same time, as we home in on the fine details of how these systems work, we continue to find new sites that completely break the mold. As recently as December 2000, researchers diving in the Mid-Atlantic discovered “Lost City,” a vent site located far away from the ridge axis, on old rather than nascent seafloor crust, and with 15-story-high white minaret-like structures made of carbonate—a mineral that is not found at most other known vent sites.

So after 32 dives to the seafloor to study vents, I am often still surprised, and I am always awed. Even when I return to a vent site that I’ve visited before, I still find it an unbelievably beautiful sight to watch jets of hot fluid mixing with seawater, and unusual organisms that make their homes near these vents. Like my colleagues, I look for ways to make our studies more precise, more methodical, and more continuous. But 20 years after my first dive, I still enjoy seeing it all live. It’s the difference between watching a movie of a waterfall and standing next to one.

When Seafloor Meets Ocean, the Chemistry Is Amazing

When Seafloor Meets Ocean, the Chemistry Is Amazing

February 13, 2004

Far more natural gas is sequestered on the seafloor—or leaking from it—than can be drilled from all the existing wells on Earth. The ocean floor is teeming with methane, the same gas that fuels our homes and our economy.

In more and more locations throughout the world’s oceans, scientists are finding methane percolating through the seafloor, bubbling into the water column, collecting in pockets beneath seafloor sediments, or solidifying in a peculiar icelike substance, called methane hydrate, in the cold, pressurized depths of the ocean.

Massive deposits of methane hydrates could prove to be abundant reservoirs of fuel. But in the past, these massive storehouses of methane also may have “thawed” suddenly and catastrophically, releasing great quantities of climate-altering greenhouse gas back into the atmosphere.

In some places, seeping methane sustains thriving communities of exotic organisms that harness the gas as an energy source in their sunless environment. Below the seafloor, an unknown but potentially vast biosphere of microbes may be making the methane that percolates upward. (See Is Life Thriving Deep Beneath the Seafloor?)

Other places on the seafloor show evidence that pockets of gas trapped beneath sediments have exploded to form “mud volcanoes,” or may have triggered seafloor avalanches and tsunami waves.

An underestimated phenomenon

Until recently, scientists have largely overlooked seafloor methane and its potentially dramatic impacts. The problem is that methane commonly vents out of isolated cracks in the seafloor—some so small that they are easily missed by oceanic surveillance systems. Once out into the ocean, the methane usually is diluted rapidly by seawater, or it dissolves in seawater and is consumed by microorganisms that convert it metabolically into carbon dioxide. Unless you happen to be looking in the right place at the right time, you’ll miss the show.

But evidence has steadily accumulated that natural seepage of methane from the seafloor is a large, continuous, and ubiquitous phenomenon. When oceanographers happen upon these vents (often called “cold seeps”), the scene is often spectacular.

Several researchers have documented large craters or pockmarks on the seafloor, while others have described huge carbonate mounds (formed by organisms that ingest methane and produce carbonate). Both are often relics of past seafloor gas venting. Sometimes gas simply seeps from the ocean floor and sustains communities of unusual tubeworms, mussels, and other creatures like those found at hydrothermal vents. (See The Evolutionary Puzzle of Seafloor Life.)

Gas frozen solid at the seafloor

The deep ocean floor around gas seep sites is often covered by methane hydrates. These are solid crystals of methane encapsulated in ice, which form under the low temperatures and high pressures typical of ocean depths greater than about 1,500 feet.

These hydrates look like seafloor carbonate, but when chunks are broken off, methane hydrates float upward (carbonates sink). As those hydrates rise into higher temperatures and lower pressures, they decompose, releasing methane gas into the ocean—a process akin to releasing the pressure on a bottle of soda.

Energy companies have been eyeing methane hydrates as a potentially tremendous new source of natural gas. Since the 1930s, the use of natural gas has increased fivefold to account for more than 25 percent of the world’s energy consumption. With existing technology, the world gas supply is estimated to be 5,300 trillion cubic feet (tcf), Robert Kleinberg of Schlumberger and Peter Brewer of Monterey Bay Aquarium Research Institute reported in American Scientist. At the current rate of global consumption (about 85 tcf per year), a 60-year supply remains.

But the amount of gas at various locations around the world varies widely. Russia and the Persian Gulf each have about 1,700 tcf, while the total for North America is about 260 tcf. Japan and Europe import nearly all of their natural gas, while India and China have very small domestic reserves.

A potential new energy resource

The untapped well of methane hydrates holds the promise of energy independence for nations close to oceans or permafrost regions (where conditions and consistently cold temperatures also create methane hydrates). Offshore methane hydrates would provide the U.S. alone an estimated potential natural gas reserve of 300,000 tcf. Projections of hydrate gas reserves in the ocean south of Japan are 2,000 times that country’s very small existing natural gas reserves, according to Kleinberg and Brewer.

Most of the world’s gas hydrates are sequestered in the deep ocean, presenting great challenges for potential commercial production. Hydrates dissolve quickly when removed from the unique conditions on the ocean bottom, so researchers must figure out how to either stabilize them or produce and transfer fuel directly from the seafloor.

Many known deep-water deposits, such as the Blake-Bahamas Plateau off the Carolinas, are very diluted or spread across relatively thin layers over wide areas, making them very difficult to “mine” economically. And deep-sea hydrates are often associated with complex biological communities that would be disrupted or destroyed by gas extraction and production.

Recharged oil wells

Recent work by a number of laboratories suggests that free gas streaming through the seafloor or seafloor hydrate deposits may constitute yet another large oceanic methane source. On the northern continental slope of the Gulf of Mexico, for instance, a process known as “gas washing” fills subsurface petroleum reservoirs with natural gas that flows upward from even deeper reservoirs in the Earth’s crust.

It has been estimated that less than 2 percent of generated oil and gas ever makes its way into commercial reservoirs. Of the residual oil, about half remains dispersed in the source rock and sediments.

The residual oil and organic matter in deeper sediments is subjected to more heating and natural processing and is broken down into natural gas. The gas streams upward, washing out clogged pore spaces and recharging many fuel reservoirs. Evidence comes from oil wells in the northern Gulf of Mexico, where we have observed significant changes in oil compositions over time scales as short as 10 years. The wells continue to produce long after their expected lifetimes.

The other half of the residual oil leaks upward and out of the sediments into ocean bottom waters. Remarkable satellite photographs of the Gulf of Mexico and other regions reveal slicks extending for miles in areas where no oil production is occurring. Similar photographs are now being used to locate new oil and gas accumulations.

Methane-making microbes

Beyond the geological “cooking and squeezing” processes that produce petroleum and gas, large quantities of gas also are being produced biologically. Many gas hydrate accumulations and ocean-floor gas seeps consist of methane largely derived from microorganisms.

Bacteria living in oxygen-poor areas beneath deep-sea sediments on the seafloor produce methane as a major product of their metabolism. Some models suggest that bacteria in sediments may account for 10 percent of the living biomass on Earth. In addition, microbial communities beneath the seafloor, whose numbers are entirely unknown, may also be producing vast amounts of methane.

Global warming and tsunamis

The pervasive and ongoing movement of methane gas—from seeps, decomposing hydrates, gas washing, and microbial sources—leads to some fascinating phenomena and important questions.

Methane is a greenhouse gas that traps heat about 20 times more effectively than carbon dioxide. If methane deposits and seeps prove to be ubiquitous in the oceans, they are a potentially significant contributor to global warming.

Relatively modest changes in global ocean temperatures or sea level could trigger a massive release of oceanic methane. If a change in ocean bottom pressure or a rise in water temperatures passes a certain threshold, sizable methane hydrate deposits could decompose rapidly and release a large quantity of heat-trapping gas back into the atmosphere. This scenario has been proposed as a possible cause for some past episodes of rapid global warming.

Evidence from the past suggests that upward-seeping methane may pose another threat: underwater avalanches. Landslides at the edge of the continental slope just off the East Coast of the United States may have been triggered by pockets of methane gas that had built up pressure under a lid of overlying sediments and exploded. Similar landslides today might generate tsunamis that would hit the U.S. coast. An offshore oil-drilling platform that accidentally hit such a gas pocket would also be endangered.

A wide-open new field

Many challenges remain ahead for researchers. Methane seeps are widely distributed around the world’s oceans, yet their discovery remains mainly serendipitous. The volume of oil and gas seeping to the floor throughout the world’s oceans is also unknown, as most of the seafloor remains unexplored.

Even in the cases of known seeps—especially those found in and around known oil and gas fields—data on the rates of seepage are scarce. Yet evidence suggests that gas seeps and methane hydrate deposits may be even more pervasive than their known extent today and may play a fundamental role in regulating ocean chemistry, sustaining marine life, and shaping seafloor geology.

How to Build a Black Smoker Chimney

How to Build a Black Smoker Chimney

December 1, 1998

1998— Diving along the mid-ocean ridge at 21°N on the East Pacific Rise, scientists within the deep submersible Alvin peered through their tiny portholes two decades ago to see an astonishing sight: Clouds of billowing black “smoke” rising rapidly from the tops of tall rocky “chimneys.” The “smoke” consisted of dark, fine-grained particles suspended in plumes of hot fluid, and the “chimneys” were made of minerals that were rich in metals. Using specially designed fluid bottles and temperature probes, Alvin took samples of these black smoker chimneys, as well as the 350°C fluids venting from them. Since then, scientists have observed and sampled numerous active vent sites along portions of the mid-ocean ridge in the Atlantic and Pacific Oceans, and in back arc basins in the Pacific Ocean. It has become abundantly clear that these high-temperature seafloor hydrothermal systems are the analogs to systems that created some of the world’s economically valuable mineral deposits, including some that have been mined on land. In Cyprus and Oman, for example, ore deposits of millions of tons are found in ophiolites, portions of ancient seafloor thrust onto land by tectonic forces.

Scientists can gain much insight into hydrothermal processes through detailed studies of these exposed areas of fossil systems, but only by investigating active systems can they simultaneously examine hydrothermal fluids and the corresponding mineral deposits created by them. By analyzing these fluids and deposits, we have been able to formulate models to explain how submarine mineral deposits, from seafloor chimneys to great subseafloor depths, are initiated and how they grow in their early stages.

One of the most fascinating aspects of black smoker chimneys is how rapidly they form. They have been measured to grow (after upper parts of the chimneys are razed by sampling) as fast as 30 centimeters per day. Examination of young chimney samples, under the microscope and by X-ray diffraction, revealed that the earliest stage in the creation of a black smoker chimney wall involves precipitation of a ring of a mineral called anhydrite. The ring forms around a jet of 350°C fluid, which exits the seafloor at velocities of between 1 and 5 meters per second. Anhydrite, or calcium sulfate (CaSO4), is an unusual mineral because it is more soluble in seawater at low temperatures than at high temperatures. Seawater contains both dissolved Ca2+ and SO42- ions, and when it is heated to 150°C or greater, the ions combine and anhydrite precipitates. Hydrothermal fluids contain little or no sulfate, so the origin of the sulfate in the precipitated anhydrite is seawater. Calcium, however, is present in both seawater and hydrothermal fluid. That made it more difficult at first to determine whether the initial anhydrite chimney wall formed solely from seawater that was heated by hydrothermal fluids, or from the mixing of cold, sulfate-rich seawater with hot, calcium-rich hydrothermal fluid.

Strontium, which is present in seawater and hydrothermal fluid, was used to investigate this problem. Strontium has the same charge as calcium and a number of different, easily measurable isotopes. (Isotopes are elements having the same number of protons in their nuclei, but different numbers of neutrons. Thus they share chemical properties but have slightly different physical properties.) Strontium can readily take the place of calcium in the crystalline lattice that forms when anhydrite precipitates. The concentration of strontium, as well as the ratio of two of its isotopes, strontium 87 and strontium 86, were measured in both vent fluid and in seawater. Because the ratio of strontium 87 to strontium 86 is higher in seawater than in hydrothermal fluid, it is possible to determine whether the source of the strontium (substituting for calcium in newly formed anhydrite grains) is seawater or vent fluid. The answer is both: Anhydrite walls form from the turbulent mixing of seawater and hydrothermal fluid, not just from the rapid heating of seawater.

During this mixing, other processes occur during early stages of chimney growth. Metal sulfides and oxides (zinc sulfide, iron sulfide, copper-iron sulfide, manganese oxide, and iron oxide) precipitate from the vent fluids as fine-grained particles, most of which form a plume of “smoke.” Because bottom seawater is denser than the mix of seawater and hydrothermal fluid in the plume, the plume rises some 200 meters above the ridge to a level of neutral buoyancy. Some particles form close to the chimney and become trapped within and between grains of anhydrite within the nascent chimney walls. These particles give the anhydrite, which is white in its pure form, a gray to black color. Copper-iron sulfide (chalcopyrite, or CuFeS2) begins to precipitate and plate the inner surface of the chimney. The evolving chimney walls are thin, ranging from less than a quarter of an inch to a few inches, but on either side of them are large gradients of pressure, temperature, and concentrations of elements. Aqueous ions, including copper, iron, hydrogen sulfide, zinc, sodium, chloride, and magnesium, are transported from areas of high to low concentrations (by diffusion). These elements also are carried by fluids flowing back and forth across the wall from areas of high to low pressure (by advection). As a result of these processes, zinc sulfide, iron sulfide, and copper-iron sulfide precipitate in the interstices of the chimney walls. The chimney walls thus become less porous and more metal-rich over time.

Early examinations of black smoker chimneys resulted in a model of chimney growth that is still accepted nearly 20 years later. But in terms of size and ore grade, black smoker chimneys are not important mineral deposits. Most of the metals are lost into the plume that rises into the water column above the vents and is dispersed. In the last decade, the focus of study has shifted to larger deposits present along mid-ocean ridges.

In 1985, scientists aboard the National Oceanic and Atmospheric Administration’s ship Researcher discovered hydrothermal vents at the Trans-Atlantic Geotraverse (TAG) active mound, the single largest known active mineral deposit along the mid-ocean ridge. Roughly circular in plan view, the TAG site has a diameter of about 150 meters and rises some 50 meters above the seafloor, with an estimated mass of 3 million tons. TAG has been intensively sampled. The Alvin, Mir, and Shinkai submersibles have been used to recover vent fluids, chimneys, and other hydrothermal precipitates from the mound surface. And in 1994, 17 holes were drilled into the mound during Leg 158 of the Ocean Drilling Program (ODP). Drillcore was recovered from five mound areas to a maximum depth of 125 meters below the seafloor.

As in studies of black-smoker chimneys, the combination of vent fluid data and examinations of anhydrite played an important role in determining the processes involved in growth of the large TAG mineral deposit. In 1990, two distinct fluid compositions were observed to be exiting the TAG mound. From the so-called Black Smoker Complex in the northwest area of the mound, 366°C black smoker fluid billowed from an aggregation of chimneys in a huge black plume that shrouded nearly the entire complex. Approximately 70 meters southeast of this complex, however, clear fluid with temperatures of less than 300°C emanated from an area called the Kremlin, named for its 1- to 2-meter-high chimneys with their distinctive, onion-shaped Byzantine cupolas. The mineral composition of the black smoker chimneys was very similar to those of black smokers at other mid-ocean ridge vent sites. The chimneys forming from the cooler white smoker fluid, however, were quite different, containing significant amounts of zinc, as well as cadmium, silver, and gold. Analyses of the white smoker fluids indicated that they were more acidic and contained less copper, iron, calcium, and hydrogen sulfide, but 10 times more zinc, than the hotter black smoker fluids. However, concentrations of other elements, such as potassium, were identical, suggesting that the two fluids were related to one another.

A hypothetical series of steps was soon developed to explain these observations. The deficits of copper, iron, and hydrogen sulfide in the cooler white smoker fluid could best be explained by the precipitation of copper-iron sulfide and iron sulfide (that is, chalcopyrite and pyrite) within the mound. In addition, it was theorized that precipitation of anhydrite within the mound could explain the lower calcium concentrations in white smoker fluids. To trigger the deposition of sulfates and sulfides inside the mound, sulfate-rich seawater would have to percolate down into the mound and mix with the hotter hydrothermal fluid. The precipitation of sulfides would release hydrogen ions, making white smoker fluid more acidic. The increased acidity, in turn, would cause metals in the mound, such as zinc, cadmium, silver, and gold, to dissolve. Once dissolved in the fluid, these so-called “remobilized” elements could be transported toward the upper edge of the mound. This would explain the excess zinc observed in white smoker fluid. At the seafloor, the white smoker fluid, rich in remobilized metals, confronts 2°C seawater just outside the chimney wall. Crossing this thermal gradient, the fluid cools, and some metal-rich minerals precipitate. This process, known as “zone refinement,” explains how some ore deposits are separated into large-scale zones containing different metals, with copper in the center of the deposits, for example, and zinc at the edges.

To determine whether the scenario described above was reasonable, we used geochemical modeling calculations, which take into account the thermodynamics of a range of different chemical reactions at high temperatures. The theoretical reactions had to reproduce the already-well-documented composition of the less-than-300°C fluid from combinations of the 366°C fluid and seawater. These calculations demonstrated that the composition of the cooler white smoker fluid we observed could theoretically result from mixing 86 percent black smoker fluid with 14 percent seawater, which would result in the precipitation of 19 parts anhydrite, 8 parts pyrite, and 1 part chalcopyrite within the mound, as well as the remobilization of zinc and other metals by the resultant acidic fluid.

These predictions were put to the test when the TAG mound was drilled in the fall of 1994. One of ODP Leg 158’s major findings was that significant amounts of anhydrite are present throughout the mound. Anhydrite has not been seen in analogous ophiolite structures probably because it dissolves at lower temperatures and essentially has disappeared from land-based deposits. But anhydrite was recovered from three of the five sites drilled at TAG. It was present at the base of the deepest hole drilled at the TAG site, to a depth of 125 meters below the seafloor. The drilling revealed that the mound contained wide, complex, anhydrite-rich veins, ranging in size from 1 millimeter to 1 meter wide, which formed as anhydrite precipitated in cracks within the mound.

The discovery of anhydrite confirmed the prediction that seawater was entering and traveling through the mound. It also let us determine the proportions of seawater that were mixing with hydrothermal fluid within the mound. Analyses of strontium isotopes from anhydrite grains recovered from various depths in the mound demonstrated that anhydrite was forming from mixtures that ranged from 99.5 percent seawater and 0.5 percent hydrothermal fluid to 52 percent seawater and 48 percent hydrothermal fluid.

Samples of anhydrite grains recovered from drilled cores were also used to determine the temperature and salinity of fluids at various points within the mound. When mineral grains form, small amounts of the fluid from which they are forming can be trapped and enclosed within the precipitating grain. As a result, small cavities can form within the mineral, which may contain one or more phases (liquid, vapor, or solid). These are called fluid inclusions. The cavities can range in size from about 1 micron to greater than 1 millimeter, though 3- to 20-micron inclusions are most common. The anhydrite grains from the TAG mound all contained abundant fluid inclusions with two phases: liquid and a vapor bubble.

We analyzed fluid inclusions within mineral grains by using a heating-freezing stage attached to a high-magnification microscope. The salinity is determined by freezing the fluid within the inclusion at temperatures of less than -50°C and then slowly heating until the last ice melts. The temperature of final melting is a function of the salt content in the fluid. The temperature of the fluid when it was trapped within the mineral also can be determined by heating the fluid inclusion and measuring the temperature at which the vapor bubbles disappear. With adjustments for undersea pressures, measurements of fluid inclusion trapping temperatures from a number of different anhydrite grains recovered from within the TAG mound indicated very high temperatures (greater than 337°C) throughout most of the mound. At depths greater than 100 meters below the seafloor, trapping temperatures were in excess of 380°C.

The combination of strontium-isotope and fluid-inclusion analyses of TAG anhydrite grains not only demonstrated that large amounts of seawater are being entrained into the mound, they also showed that anhydrite (and chalcopyrite and pyrite) precipitated in the mound from seawater-hydrothermal fluid mixtures that are greater than 50 percent seawater. That led to a dilemma and to another discovery. The dilemma was that our geochemical modeling calculations predicted that if the mixing proportions were greater than 50 percent seawater, and if mixing alone determined the temperature of the fluids in the mound, the fluid temperatures in the mound should be relatively low (5° to 250°C). Our fluid inclusion measurements, however, indicated that the anhydrite grains were nearly all precipitating at much higher temperatures, 187°C to 388°C.

The logical conclusion is that the seawater and seawater-hydrothermal fluid mixtures that are entering and travelling through the mound are being heated by conduction. Hot black smoker fluids are flowing rapidly along a direct, highly focused route up through the mound to the Black Smoker Complex. Drilling within the mound revealed extensive accumulations of breccia—rocks made of sulfide-rich fragments that conduct heat well. So it is reasonable to conclude that cold seawater is being conductively heated as it flows through channels bounded by breccias. If enough seawater is being heated in this way, black smoker fluid may be cooled slightly as it rises through the mound to the Black Smoker Complex—from the 388°C temperatures found in the anhydrite grains near the base of the mound to the 366°C temperatures of the exiting black smoker fluids.

Results of drilling revealed other important information on the internal structure of the TAG active mound. By dating mound materials, it is possible to reconstruct a history of the mound, showing that it has undergone repeated episodes of high-temperature fluid flow, punctuated by quiescent periods, over a roughly 20,000-year interval. When the high-temperature fluid flow ceases, the anhydrite dissolves, and the chimneys that the anhydrite supports collapse, scattering fragments of rock. When high-temperature fluid flows resume and percolate through these fragments, anhydrite precipitates and serves as a matrix that cements together fragmented chimney pieces and mound materials into breccia deposits. Textures within the TAG breccias indicate that there have been multiple cycles of mound material reworking—a likely consequence of repeated episodes of anhydrite deposition and dissolution.

Information from the TAG mound shows how this kind of intermittent activity can, over long periods of time, result in the gradual formation of a large hydrothermal mineral deposit similar to the ore bodies preserved in Cyprus. Studying the large TAG active mound has greatly increased our understanding of how large mineral deposits like these can form.

Meg Tivey’s research on hydrothermal deposits has been funded through the National Science Foundation and the Joint Oceanographic Institutions/US Science Advisory Committee. Her work on the TAG active mound has benefited from collaborations with many scientists, including Susan Humphris and Geoffrey Thompson (WHOI), Rachel Mills (University of Southampton), and Mark Hannington (Geological Survey of Canada).

Meg Tivey chose to major in geology after taking a course with five field trips to local beaches and fault zones. She then worked as a physical science technician at the US Geological Survey before deciding to pursue graduate studies in marine geology at the University of Washington. She now specializes in studies of active seafloor hydrothermal systems. Her current projects include examining the formation of polymetallic sulfide deposits, continuing work on linking measured vent fluid compositions to observed mineralogy of vent deposits, using X-ray computed tomography (CAT scans) to examine seafloor sulfide samples in three-dimensions, and working with engineers to build instruments capable of measuring temperatures and flow rates on the seafloor in high-temperature and low-pH fluids.

The Cauldron Beneath the Seafloor

The Cauldron Beneath the Seafloor

December 1, 1998

1998— Just over 20 years ago, scientists exploring the mid-ocean ridge system first made the spectacular discovery of black smokers—hydrothermal chimneys made of metal sulfide minerals that vigorously discharge hot, dark, particulate-laden fluids into the ocean. The ultimate source of the fluid venting from these smokers is seawater, but a comparison of chemical compositions shows that seawater and hydrothermal fluid are distinctly different. The vent fluids are not only far hotter than surrounding seawater, they are also more acidic and enriched with metals, and have much higher concentrations of dissolved gases, such as hydrogen, methane, and hydrogen sulfide (see table below right). The metals transported by the fluids frequently form ore deposits at the seafloor, and the dissolved gases support a prolific biological community that derives its energy from chemical reactions rather than sunlight. By what processes is seawater turned into this remarkable fluid that emanates from black smokers?

The answer lies beneath the seafloor, within the oceanic crust. The mid-ocean ridge system, which forms where the earth’s tectonic plates are spreading apart, is volcanically active and the site of numerous heat sources, which induce seawater to circulate through the permeable oceanic crust. It is estimated that the equivalent of an entire ocean’s worth of water circulates through the mid-ocean ridge hydrothermal systems every 10 million years or so. As the seawater percolates through subseafloor rocks, a complex series of physical and chemical reactions between seawater and volcanic rocks drastically changes the chemical composition of both the seawater and the rocks. These chemical reactions not only influence the composition of the oceanic crust, they also play a role in regulating the chemistry of the oceans.

The history of these chemical reactions is recorded in the minerals and chemical composition of the rocks. By investigating samples of rocks that have been altered, we can learn about the sequence of water-rock interactions taking place in the subsurface. We can then begin to understand the processes responsible for the chemical composition of vent fluids, the formation of sulfide-rich mineral deposits, and the existence of biological communities at hydrothermal vents.

Gaining access to investigate the subsurface portion of a hydrothermal system is, of course, a difficult problem, and scientists must employ several different strategies. The most direct approach is to find techniques to collect and analyze altered rocks. One way is to drill a borehole through a seafloor hydrothermal mineral deposit and recover samples from the oceanic crust beneath. Over the past few years, the international Ocean Drilling Program (ODP) has conducted drilling operations in two hydrothermal areas—one on the Juan de Fuca Ridge off the northwestern US coast, and one on the Mid-Atlantic Ridge about halfway between Florida and West Africa. The drill cores recovered from these sites allow scientists to study the variability in rock-water reactions that occur under the different physical and chemical conditions found at different depths within the earth’s crust. Ocean drilling operations, however, are extremely expensive and consequently have been carried out at only a few locations.

Scientists can also collect seafloor rock samples by using dredges and small submarines (“submersibles”) in areas where faults and fractures have exposed rocks on the seafloor that were once in the deep subsurface. The disadvantage of this method is that the same processes that expose the rocks may also muddle the spatial and temporal relationships among individual samples. Nevertheless, much has been learned about the chemical effects of water-rock reactions from dredge and submersible samples. In many samples, the outer rim, which has been altered by exposure to circulating hydrothermal fluids, can be compared to the fresh, unaltered interior of the rock in order to learn how the rock has been changed by the fluid (see rock below).

While drilling, dredging, and submersibles can be used to collect rocks to study the shallower portion of the ocean crust, scientists have had to turn to rocks on land to investigate deeper sections of the hydrothermal system. In a few locations, including sites in the western US, Oman, Cyprus, and the west coast of Newfoundland, sequences of rocks exposed on land resemble what scientists believe to be the structure of the oceanic crust. Many geologists think that these rocks represent sections of oceanic crust that have been thrust onto the continents by tectonic movements. Within these so-called “ophiolite” sequences are ancient analogs of seafloor hydrothermal mineral deposits, and these sites provide another source of hydrothermally altered rocks for study. But this method, too, has pitfalls: In some cases, ophiolite rocks have been altered during the tectonic processes that uplifted and thrust the oceanic crust onto land. This subsequent alteration often obscures the original alteration that took place on the seafloor, making it difficult to use the rocks to study submarine hydrothermal processes.

Scientists also employ experimental strategies in laboratories to understand fluid-rock interactions, setting up reactions between rocks and seawater under conditions simulating those in a seafloor hydrothermal system. The earliest of these experiments actually pre-dated the discovery of seafloor hydrothermal systems. In the experiments, crushed rock samples and seawater in varying proportions are placed in a sealed reaction vessel (commonly referred to as a “bomb”!), which is then subjected to high temperatures and pressures. These “cook-and-look” experiments provide a way to explore how reactions change as physical and chemical conditions are varied, and they help scientists determine how the chemistry of the fluid and the rock evolves as the reactions proceed. Over the years, experimentation has progressed to include “flow-through” models that examine the changes in chemistry and physical state as fluids migrate through a system. While laboratory simulations often result in end-products that are somewhat different from those observed in rock samples from actual altered ocean crust, scientists have gained insights that have been critical in deciphering the complex set of water-rock reactions taking place in natural hydrothermal systems.

A third approach to understanding the chemistry of hydrothermal systems is geochemical modeling. Scientists have used models to investigate the sequence of minerals that dissolve and precipitate during fluid-rock reactions, as well as to examine how the fluid changes its composition as it circulates through the crust. These efforts depend on the availability of good thermodynamic data at the temperatures and pressures that occur in hydrothermal vent systems, much of which has been generated only in the past few years. The models provide a framework for integrating the observations made from rock samples and experimental studies, and they have proven to be a powerful tool to relate the changes in fluid chemistry to the alteration mineralogy of the rocks.

By integrating results from these different investigative strategies, a model is beginning to emerge of how seawater chemistry changes in an active seafloor hydrothermal system, from the time it enters the oceanic crust until it is discharged as a vent fluid. Conceptually, the circulation of seawater through the oceanic crust can be divided into three parts (see figure below right):

A recharge zone, where seawater enters the crust and percolates downward;
A reaction zone at the maximum depth of fluid penetration, the site of the high-temperature reactions that are thought to determine the final chemical characteristics of the hydrothermal fluid; and
An upflow zone, where the buoyant hydrothermal fluids rise and are discharged at the seafloor.

The Recharge Zone
Seawater percolates readily into the upper layer of the oceanic crust, which is constructed of highly porous and permeable volcanic rocks that are broken apart in many places by cooling cracks and tectonic fractures. As a consequence, reactions between seawater and the exposed rocks at relatively low temperatures up to about 60°C are pervasive. Although reactions are relatively slow at these low temperatures, they nevertheless begin to change the composition of the seawater through two processes. First, the seawater partially oxidizes the crust, resulting in the removal of oxygen from the seawater. Minerals containing iron in the rocks are replaced by iron oxides and hydroxides (a process analogous to the formation of rust), which also fill veins and pore spaces in the upper crust. Second, the reactions with seawater break down the original rock minerals, replacing them with alteration minerals such as mica and clay. In the process, potassium and other alkali elements, such as rubidium and cesium, are transferred from seawater into the rocks.

Beyond about 300 meters into the oceanic crust, penetration of seawater becomes more and more restricted as the rocks’ permeability decreases. Larger fractures and fissures are more likely to become the main conduits for fluid flow. As the fluid (already oxygen- and alkali-depleted relative to seawater) continues to penetrate downward toward the heat sources, it becomes heated further, and other reactions occur. At temperatures above about 150°C, clay minerals and chlorite precipitate out of the fluid, essentially removing all of the magnesium originally present in the fluid. The formation of clay minerals and chlorite also removes hydroxyl ions from the fluid, resulting in an increase in acidity (that is, a lower pH). This increase in acidity, in conjunction with the breakdown of the original minerals in the rocks, causes calcium, sodium, potassium, and other elements to be leached from the rock into the fluid. Hence, the removal of potassium (and the other alkalis) from the fluid at lower temperatures is partially reversed at higher temperatures at greater depths!

Another important reaction results in the formation of the mineral anhydrite (calcium sulfate). This mineral possesses something called “retrograde solubility,” which means that instead of becoming more soluble with increasing temperature as most minerals do, it becomes less soluble. At the pressures found at the bottom of the ocean, this results in anhydrite precipitating from seawater when temperatures rise above about 150°C. This process removes about two-thirds of the sulfate initially present in the seawater and also limits the calcium concentration of the fluid. At temperatures greater than 250°C, the remaining sulfate in the fluid reacts with iron in the crust to form metal sulfide minerals.

The Reaction Zone
The “reaction zone” designates the region where high-temperature, water-rock reactions occur. This zone is near the heat source that drives the circulation system. The depth of the reaction zone depends on the depth of the heat source and varies from one mid-ocean ridge to another. On the fast-spreading East Pacific Rise, the presence of a magma lens at a depth between 1.5 and 2.4 kilometers defines the lower limit of circulation, but seawater may penetrate deeper on slower-spreading ridges where no melt lens has been detected. Scientists think reactions in this zone determine the final chemical characteristics of the hydrothermal fluid. Reactions at such high temperatures (up to 350° to 400°C) produce a characteristic suite of alteration minerals (chlorite, sodium-rich feldspar, amphibole, epidote, and quartz), which, in turn, controls the fluid composition. Metals, such as copper, iron, and zinc, as well as sulfur, are leached from the rock by the acidic fluid. This provides the source of metals for the massive sulfide deposits observed at the seafloor, as well as the hydrogen sulfide to support the chemosynthetically based hydrothermal biological community.

The Upflow Zone
Buoyancy forces cause the hot fluids to rise rapidly toward the seafloor, much as hot air causes a balloon to rise in the atmosphere. Initially, the upflow is focused along a conduit of high permeability, such as a fault surface. As it reaches shallow depths, the flow may continue to be focused and may discharge through a chimney, or it may follow more tortuous pathways and be discharged as a more diffuse flow (like water flowing through a sponge). Continued high-temperature reactions between the rock and the upward-flowing, metal-rich, magnesium-depleted hydrothermal fluid produce an “alteration pipe” of highly altered rocks with an interconnected network of veins filled with sulfides, silica, and chlorites. As focused high-temperature (350° to 400°C) fluids discharge at the seafloor as black smokers, mixing with the surrounding seawater causes metal sulfides to precipitate and form massive sulfide deposits rich in iron, copper, and zinc (see article, page 22). However, at locations where the volcanic pile is especially permeable, the upflowing hydrothermal fluid will mix with colder seawater in the shallow subsurface, resulting in the metal sulfides being precipitated beneath, rather than at, the seafloor. The resulting lower-temperature fluids, depleted of metal sulfides, vent as “white smokers,” rather than particulate-laden black smokers. Shallow subsurface mixing may also heat seawater to form anhydrite and cool hydrothermal fluids to precipitate silica, both of which cement the metal sulfides or seal fluid conduits.

Together, all the hydrothermal water-rock reactions that occur—from the time seawater enters the system to the time hydrothermal fluid leaves it—play a role in regulating the chemistry of seawater. But the relative importance of hydrothermal reactions must be balanced with other factors that influence ocean chemistry—particularly, rivers, which are the principal conduits by which most (but not all) chemical elements enter the ocean. River input provides a good measuring stick by which to compare the relative contribution of hydrothermal activity to the fluxes of elements in and out of the ocean. Hydrothermal vents are a source to the ocean of alkali elements that leach from the crust during hydrothermal alteration (although this process may be tempered somewhat by lower-temperature weathering of the shallow crust away from the ridges, which removes alkali elements from seawater).Vents also represent a significant source of manganese input to the ocean. Most of the metals present in hydrothermal fluids (iron, copper, zinc, etc.) are removed rapidly by precipitation, either at the seafloor or by mixing with seawater in the subsurface, so most of the metals do not enter the oceans. On the other hand, hydrothermal circulation removes magnesium and sulfate from seawater, so the crust acts as a sink for these elements. The magnesium loss is perhaps the most significant, and hydrothermal activity may be a major mechanism of balancing the magnesium budget in the ocean.

Susan Humphris’s research is supported by the National Science Foundation. Thomas McCollom is an NSF Earth Sciences Postdoctoral Fellow.

Susan Humphris first came to Woods Hole from England in 1972 to enter the MIT/WHOI Joint Program. For her Ph.D. thesis, she studied some rocks dredged from the ocean floor that had reacted with seawater and determined the reactions that must have occurred. Six months after she completed this work in 1976, the first hydrothermal vents were discovered. She has spent more than three years of her life on research vessels of various kinds, ranging from traditional sailing ships, when she worked at the Sea Education Association teaching oceanography to undergraduates, to drilling vessels as a participant in the Ocean Drilling Program. She has completed about 30 dives in submersibles and has used ROV Jason to study new hydrothermal sites. In her spare time, Humphris and her husband tend a large vegetable garden and raise chickens and the occasional pig.

Tom McCollom’s interests are in the organobiogeochemistry of seafloor hydrothermal systems. He manages to squeeze in a little research now and then between running around on the soccer and ultimate fields, pedaling his bicycle, climbing up (or skiing down) hills, dancing to his favorite bands, and birdwatching with his wife, Ifer.

A New Way to Catch the Rain

December 1, 1997

December 1997 — The carbon budget of the upper ocean includes an important loss to the deep ocean due to a very slowly falling rain of organic particles, usually called sediment. As this sediment falls through the upper water column it is consumed, mainly by bacteria, and the carbon is recycled into nonsinking forms (dissolved or colloidal organic carbon or inorganic forms). Thus the sediment rain decreases with increasing depth in the water column, and only a tiny fraction reaches the deep sea floor, less than about one percent.

It is of great interest to observe the sediment rain at different depths in the ocean so that the recycling rate can be quantified. There is a long (and contentious) history of sample collection in simple, open-topped cylinders or “traps.” For reasons of technical convenience, these traps have been either attached to moorings or tethered from surface floats. The ongoing Bermuda Atlantic Time Series (BATS) program utilizes surface-tethered traps to catch sediment falling through the upper 300 meters of the ocean. These traps are deployed for three to five days during the regular monthly BATS cruises, and the contents are then analyzed for the carbon, nitrogen, and other elements being carried to the deep ocean by the sediment rain. BATS investigators, especially Tony Michaels, now at the University of Southern California’s Wrigley Institute, who first interested us in this problem, have been unable to close the upper ocean carbon budget using these data. It seems that on annual average, there is an unaccounted loss in the carbon budget. Perhaps there is an important unknown process acting to deplete the upper ocean carbon budget, or perhaps the surface-tethered sediment traps may be undercollecting the sediment. The former would be the most exciting result, but there is good reason to suspect a role for collection error, and that is what we have begun to address.

A collection error by a moored or a surface-tethered sediment trap is easy to imagine; the sediment falls through the water at a rate of about 10 to 200 meters per day, while it is carried horizontally by currents at a rate of from 5 to 50 kilometers per day (typical of currents in the upper ocean-deep ocean currents may be much less). A fixed or surface-tethered trap will thus be immersed in a nearly horizontal flux of sediment. Flow past the trap would set up vortices that could either enhance or reduce the collection of sediment, depending upon the fall speed of the sediment, the speed of the current, and the tilt and geometry of the trap. Similar collection problems are known to afflict most rain gauges (we mean water rain!), and the phenomenon can be reproduced and studied in controlled, laboratory experiments. But even if we knew in great detail the dynamics of this collection error, we could not apply this knowledge to correct the presently measured sediment flux since the currents and the sediment falling speeds are highly variable and, in practice, not easily measured.

An obvious solution is to make a sediment trap that drifts freely with the currents, so that the flow past the trap is effectively zero (imagine the wind blowing past a hot air balloon). Based upon this idea, and armed with a grant from the Green Foundation, we have developed and recently begun to test a new sediment trap that we call the Neutrally Buoyant Sediment Trap (NBST). The NBST is built from components and techniques familiar to us from our work with a variety of neutrally buoyant float systems. The main challenge for the NBST was to float at a prescribed depth, with shallower depths, 150 to 300 meters, being the most difficult. Around Bermuda, where we have started NBST testing, the ocean is very weakly stratified at these depths, and thus the ballasting of an NBST becomes extraordinarily sensitive. An error of just one gram in the weight of a 16 kilogram instrument leads to a depth error of about 40 meters, which is unacceptable. The NBST traps also carry a significant load of denser water (an unpleasant solution of saline and formaldehyde to preserve the samples and discourage theft by midwater scavengers, mainly small shrimp) that can be partially flushed out during launch. To overcome these ballasting issues we decided to endow the NBST with a variable displacment device (a cylinder and piston in contact with the sea) and a very modest brain, or microprocessor, so that it could be self-ballasting. Thus the NBST measures pressure, and if it finds that it is deeper, say, than its target depth, then it increases its displacement by forcing the piston outwards, something like a miniature submarine blowing ballast. By repeated checking and correcting at hourly intervals, the NBST can be made to float at a prescribed depth almost anywhere in the water column.

While the NBST idea is inherently simple, it still happens that testing the prototype NBSTs at sea has been hair raising, since they are autonomous and not command-recoverable once
they are launched. On our first field test in summer 1996 we deployed a glass-hulled instrument (glass has several important advantages as a hull material, including being able to endure very high pressures). Unfortunately, the sea state was high at the time of deployment, and we bumped the NBST with the ship. This cracked the hull, and led to a prompt sinking (we now make the hulls from aluminum tubing that is better able to take such abuse). On a later trial we lost two out of two instruments due, we think, to electrical interference between a new high voltage light, intended to be used as a recovery beacon, and the microprocessor (we have redesigned the light).

These failures were a loss of time and resources, and they were also intensely disappointing. But we were confident that our basic idea had merit, and we persisted in building and deploying NBST prototypes until finally achieving real success during this past summer and fall. With assistance from Debbie Steinberg and co-workers at the Bermuda Biological Station for Research, we obtained our first NBST samples during the summer of 1997. During this test an NBST at 150 meters collected approximately the same sediment flux as a surface-tethered trap at the same depth; however, a second NBST at 250 meters depth collected significantly more material than the comparable surface-tethered trap. Thus the profile of the sediment flux, which is a direct consequence of the recycling process, appears very different when measured by an NBST compared with conventional traps (the NBST-measured flux appears to fall off less rapidly with depth). On still another successful deployment this past fall, an NBST at 150 meters collected significantly less material than did a comparable surface-tethered trap, and we found that the kind and quality of the collected material was markedly different.

Our story of upper ocean sediment trapping is still unfolding, but we can already see that it will not be as straightforward as a simple under- or over-collection error. If indeed there is a collection error by surface-tethered traps (and there probably is, given our results to date), then it likely depends upon the season, since the kind of material that makes up the sediment flux changes with season, and also with depth. During the next two years we hope to make a series of comparisons over a full annual cycle as part of the BATS program. These new data are sure to spark a great deal of interest among the geochemists who have grappled with these difficult problems, and it may, perhaps, inspire other engineers to conceive still better means to measure this very gentle but crucially important rain of organic material that falls through the upper oceans.

The authors are grateful to the Green Foundation for a technology development grant that made their floating sediment trap project possible.

Authors Valdes, Buesseler, and Price collectively have 58 years of service to WHOI science (23, 17, and 18 years, respectively), including many months at sea for each. Buesseler, who is a 1987 graduate of the MIT/WHOI Joint Program is currently in Arlington, VA, at the National Science Foundation, where he is Associate Program Director in Chemical Oceanography.

Geochemical Archives Encoded in Deep-Sea Sediments Offer Clues for Reconstructing the Ocean's Role in Past Climatic Changes

Geochemical Archives Encoded in Deep-Sea Sediments Offer Clues for Reconstructing the Ocean’s Role in Past Climatic Changes

December 1, 1997

December 1997 — Paleoceanographers are trying to understand the causes and consequences of global climate changes that have occurred in the geological past. One impetus for gaining a better understanding of the factors that have affected global climate in the past is the need to improve our predictive capabilities for future climate changes, possibly induced by the rise of anthropogenic carbon dioxide (CO2) in the atmosphere.

The relatively recent geological past (the last 1.6 million years), known as the Quaternary period, is characterized by large climatic swings from ice age to warmer interglacial periods similar to the present one. During the ice ages, large ice sheets accumulated on the northern continents, and sea level dropped by as much as 120 meters. Analysis of air bubbles trapped in the Antarctic and Greenland ice caps documents significantly lower atmospheric CO2 levels during the cold glacial periods compared to the warm interglacials (Fig. 1). Since carbon dioxide is a well known “greenhouse” gas, whose presence in the atmosphere traps heat near the earth’s surface, its lower concentration in the glacial atmosphere could have contributed to the cold climate of the ice ages.

Establishing the exact role that atmospheric CO2 played in past natural climatic oscillations, however, is not a simple matter. Changes in atmospheric CO2 may have been more a response to climate change than a forcing mechanism. On the other hand, while we know that the pace of Quaternary glaciations was primarily driven by variations in Earth’s distance from the sun and in the angle of Earth’s axis of rotation, the resulting changes in incoming solar radiation to the planet’s surface are too small to account for the large climate variability observed. This implies that the effect of these orbital parameters must have been amplified by some internal feedback mechanisms within the earth’s environment, and we suspect that atmospheric CO2 may be a major factor. In view of its obvious connection to present societal concerns, this particular problem has elicited a lot of attention in the paleoceanographic community.

In the modern ocean, factors affecting atmospheric CO2, such asexport flux of organic carbon and carbonate to the deep sea, dissolution of calcium carbonate shells in the deep sea, and deep water circulation, can be measured directly. A variety of incubation techniques are used to measure production of organic matter in surface waters, and broad views of surface water production at a given time can now be obtained from satellite imagery (Fig. 2). As several articles in this issue attest, sediment traps are deployed to estimate the export and recycling of organic matter and calcium carbonate from surface waters to the deep sea, and thermohaline circulation is becoming increasingly well constrained, both in terms of flow rates and pathways. For past oceans, however, these variables cannot be measured directly but must be inferred from proxy analysis (a marker in the sediments from which the variables can be inferred indirectly). A fraction of the biogenic particles produced in surface water survives degradation or dissolution in the deep sea and gradually accumulates on the seafloor. As many oceanic processes leave a chemical imprint in this material, a very complex but rather comprehensive chemical archive, which can be dated and deciphered, is continuously buried in deep-sea sediments.

It is probably safe to say that every element of the periodic table, every isotope, and an assortment of specific organic molecules that survive sediment burial have some potential for providing information on how past oceans operated. It is for us to discover the processes that regulate their distribution in the sediment, how well the chemical signals are preserved during burial, and whether they can then be used to infer past changes in the processes that generated them. Often, a specific element or isotope is affected not only by one but by several oceanic processes. Different elements can also be affected by the same processes, but with a somewhat different response.

Decoding this chemical message is rather like solving a jigsaw puzzle in which each piece is a puzzle in itself. You first have to build the pieces of the overarching puzzle one at a time—that is, you try to develop proxies for oceanic processes that you think may play an important role in climate control. As you keep building new pieces, you make preliminary tests to see how they come together. Oftentimes, you find that some pieces don’t quite fit and need to be adjusted. Sometimes several pieces will fit nicely together until an additional piece discloses that this was just an illusion. On good days, the added information allows you to rearrange the pieces into a more convincing pattern. On bad days, the whole thing falls apart and you are back to square one. Actually, you never go back to square one, even on very bad days, as a lot is learned from failures as well. This process, which is typical for many scientific endeavors, sounds rather daunting, at least in the initial stages, but as it gains momentum (and we have now reached this exciting part in our game), the overall picture becomes gradually clearer, and there is an inherent synergism in the process so that the emerging picture itself is giving you hints on how to build important missing pieces or rearrange those that don’t quite fit.

There is a good illustration of this process in the ongoing development of paleoceanographic tools that use natural radioisotopes formed in seawater from the decay of dissolved uranium. In view of the recognized importance of the biological pump in reducing atmospheric CO2, past change in ocean productivity is an obvious part of the “puzzle” for which a “piece” needs to be “built.” Different approaches are being explored to reconstruct that important characteristic of past oceans. Ideally, one would want a synoptic (snapshot) view of past ocean productivity, similar to what satellite data offers for the modern ocean. In one of these approaches, paleoceanographers are attempting to make use of the contrasting behavior of the two natural radionuclides thorium 230 and protactinium 231.

Weathering removes uranium from crustal rocks and rivers transport it to the ocean, which has a uniform uranium concentration everywhere. Two primary uranium (U) isotopes, U-238 and U-235, initiate different decay series and produce daughters with distinct properties (Fig. 3). Among these daughters, thorium 230 (Th-230) and protactinium 231 (Pa-231) are particularly useful for late Quaternary paleoceanography. Unlike their parent uranium isotopes, Th-230 and Pa-231 are very insoluble in seawater. They are rapidly adsorbed onto settling particles and removed to the underlying sediments through a process called “scavenging.” Such particle-reactive isotopes can be removed directly to the underlying sediment, or they can be transported laterally with seawater to be removed in regions of higher particle flux where the rate of scavenging is higher (Fig. 4). Partitioning between these two removal pathways depends on the residence time of the insoluble element in the water column. Of the two, Th-230 is the most particle-reactive. It resides in the water column for a very short period before removal, so there is very little time available for its lateral transport. Th-230 is, then, primarily removed to the sediment underlying the region where it is produced. In contrast, Pa-231 is less particle reactive and resides in the water column long enough for lateral transport over substantial distances before removal.

As a result, a significant fraction of the Pa-231 produced in low productivity, low export flux regions is removed to the sediment underlying more productive regions with high export flux (Fig. 4). Sediments underlying productive regions should thus have a high Pa-231/Th-230 ratio, while those underlying low productivity regions should have a low ratio. The validity of this model was checked by analyzing surface sediments and comparing the results to productivity maps of the modern ocean. We first looked at the Pacific Ocean, where the distribution of Pa-231/Th-230 in surface sediments compares very well with satellite imagery of ocean color and productivity (Fig. 5). Next we reconstructed export production during the last glacial period in the southern ocean (the seas around Antarctica). Comparing the distribution of Pa-231/Th-230 in modern and glacial sediments (Fig. 6) suggests that while there was relatively little change in the total productivity of the southern ocean, there was a clear northward migration of the belt of high values (and presumably high production), which is most apparent in the Atlantic sector. However, while the distribution of Pa-231/Th-230 in modern sediments again fits expectations reasonably well, the conspicuous maxima imprinted in the sediment are not as pronounced in satellite imagery. We now know that Pa-231/Th-230 in settling particles can also be significantly affected by the abundance of biogenic opal, which has a high affinity for protactinium. This piece of the “puzzle” will thus have to be adjusted accordingly. The reason this problem does not occur in the equatorial Pacific is that opal content is comparatively low in this region.

A more substantial but constructive readjustment resulted from measurement of the Pa-231/Th-230 ratio in Atlantic sediment. Just as in the Pacific, export production is high in the eastern equatorial upwelling region (Fig. 2).

We expected to find very high Pa-231/Th-230 in this region and much lower values in the central open ocean. Instead, however, low values were invariably found throughout the Atlantic, with only a few marginally higher values off West Africa (Fig. 7). This finding, surprising at first, brought to the fore the importance of an additional variable that had been largely neglected in previous work. The Pa-231/Th-230 ratio depends not only on the particle flux and scavenging intensity, but also on the rate of Pa-231 transport by currents moving from low flux to high flux regions (Fig. 4). A water mass called the North Atlantic Deep Water is produced by cooling in the North Atlantic, where it sinks before flowing southward to mix with the circumpolar deep water around Antarctica. As a result of this water entrainment, much of the Pa-231 produced in North Atlantic Deep Water as it transits the Atlantic is flushed into the southern ocean, and very little can accumulate in the Atlantic’s high productivity regions.

While it complicates the interpretation of sedimentary Pa-231/Th-230 in terms of export production, this finding also provides a means of constraining past changes in the rate of thermohaline circulation, another important piece of the atmospheric CO2 “puzzle.”

This approach was actually used to establish that thermohaline circulation was as vigorous during the last glacial period as it is today. The Pa-231/Th-230 ratio is still useful to constrain export production, but exchange of Pa-231 between ocean basins must be taken into account. Other paleoproductivity proxies, based on different principles and biased by oceanic processes other than circulation, are also being developed. By combining these different approaches, we better document and quantify glacial productivity. In turn, better glacial productivity estimates will allow us to better constrain thermohaline circulation rates from the distribution of Pa-231/Th-230.

Such iterative improvements of complementary proxies for related oceanic processes gradually lead us to an increasingly refined understanding of the mode of operation of past oceans and its influence on climate. Moreover, the paleoceanographic perspective that is being generated also provides important new insights into the working of the modern ocean and improves our forecasting ability for the future.

Funding for research described in this article was provided by the National ScienceFoundation.

Roger François started his career as a textile engineer in Zaire, printing faux-batiks on the shores of Lake Tanganyika. Looking for a more exotic occupation, he moved to Vancouver to learn the trades of oceanography and ski mountaineering, when he noticed that he was prone to being seasick and was afraid of heights. Receiving better reviews for his oceanographic accomplishments than for his telemark turns, he moved to Woods Hole to become a full-time oceanographer.

An MIT/WHOI Joint Program graduate, Mike Bacon has been on the WHOI Scientific Staff since 1977. He specializes in the use of uranium-series nuclides to study ocean processes. In his spare time he also enjoys active participation in the education of his six-year-old daughter Alexandra. Mike also enjoys studying and trading the financial markets, where he says he is still hopeful of eventually making a profit.

Transient Tracers Track Ocean Cimate Signals

Transient Tracers Track Ocean Cimate Signals

December 1, 1996

December 1996 — Transient tracers provide us with a unique opportunity to visualize the effects of the changing climate on the ocean. They trace the pathways climate anomalies follow as they enter and move through the ocean and give us valuable information about rates of movement and amounts of dilution. This knowledge is important for developing ocean-climate models to predict long term climate changes.

Humankind’s activities have resulted in the release of a number of globally distributed substances into the environment. These substances enter the oceans, and, although they have little, if any, impact on the environment, they travel through and “trace” the biological, chemical, and physical pathways of the ocean. The distributions of these “tracers” change with time. For example, isotopes created by atmospheric nuclear weapons tests in the 1950s and 1960s were introduced in a pulselike fashion, while atmospheric concentrations of chlorofluorocarbons (CFCs), which threaten the earth’s ozone layer, have been increasing with time. We refer to these substances in the ocean as “transient tracers” because their distributions are evolving.

Transient tracers are valuable tools for studying ocean climate. First, because they are new to the ocean environment, they are indicators of “ocean ventilation.” Ventilation is the imposition of atmospherically derived properties on water masses. For example, waters in contact with the atmosphere will have dissolved oxygen concentrations increased to equilibrium values with the atmosphere. Providing their time history in the atmosphere is known and the manner in which they are transferred to the ocean is understood, they can be used to construct and test models of ocean ventilation and circulation. Observations of their distributions in the ocean and time series measurements of how they change with time are powerful tools: They provide direct visualization of climate changes, and they trace the pathways along which ocean climate perturbations propagate into the oceans. That is, changes in characteristics and volumes of water masses due to climate variations ultimately influence deeper, more isolated regions of the oceans. How these changes move to the deep ocean from regions of contact with the atmosphere must be understood. This process is an important mechanism whereby the oceans couple to the atmosphere on longer time scales, and probably plays a role in determining the interannual to decadal variations in global climate.

Observations of tracer distributions provide information on processes that are very difficult to observe any other way. Mixing and dilution, for example, play a dominant role in the southward transport of material along the deep western boundary of the North Atlantic. It has long been known that newly ventilated North Atlantic Deep Water travels southward in a concentrated current, hugging the western edge of the Atlantic basin. Although direct current measurements indicate velocities of tens of centimeters per second, the actual average propagation rate of tracers down the western boundary is only one or two centimeters per second. This is because there is a tremendous amount of entrainment and mixing associated with water recirculating within the rest of the basin. The mixing slows the progress of tracers and climate anomalies. Transient tracers are perhaps the only tools for measuring the amount of interior exchange and downstream propagation rates.

Tritium: the Cold War Legacy
It is said that every cloud has a silver lining, and that seems to be true even if it is a mushroom cloud. Although the atmospheric testing of nuclear weapons released alarming amounts of radioactive debris into the environment, and caused untold damage to both the environment and human health, it also provided oceanographers with some unique tools to study ocean circulation and ventilation. We have had the opportunity to observe the entry of these substances into the oceans, and to see how they are moved around by physical, chemical, and biological processes.

One of these bomb-produced tracers is tritium, a radioactive isotope of hydrogen. There is very little natural tritium in the world (the entire global inventory would only weigh a few kilograms!), but several hundred kilograms were created in the hydrogen bomb explosions. This tritium was injected into the stratosphere, almost immediately oxidized to water, and fairly rapidly rained out onto the surface of the earth. Since it is chemically bound up as part of the water molecule (it is, after all, hydrogen), tritium is an ideal tracer of the hydrologic system. Further, since the bulk of the atmospheric weapons testing was done in the northern hemisphere, most of the tritium was deposited in the high latitude northern hemisphere. Thus it is an ideal tracer of ventilation and water mass formation in the North Atlantic.

We can see this in the first figure above, which provides a bird’s eye view of tritium distribution in the North Atlantic in 1981. Picture yourself hovering somewhere over Norway, at an altitude of a few hundred miles. You are looking southwestward and downward on an isosurface of tritium in the North Atlantic. An isosurface is the two-dimensional analog of a contour line on a map, shown here in a three-dimensional ocean. The blue “blanket” you see in the picture is the 1 Tritium Unit isosurface, where we find a ratio of 1 tritium atom to 100,000,000,000,000,000 hydrogen atoms. This isosurface corresponds to about 5 or 10 percent of the maximum surface water concentrations of tritium during the mid 1960s, when it was at its peak. All the water beneath this blanket has remained relatively isolated from the tritium invasion, and, conversely, all the water above this blanket has been at the sea surface, exposed to the atmosphere, and thus ventilated or otherwise involved in interaction with the surface ocean in the 15 to 20 years between the bomb tests and this survey.

The blanket lies at about 500 to 1,000 meters depth in the subtropics, but deepens to 1,500 to 2,000 meters just south of the Gulf Stream off the New England coast. This is the effect of the Gulf Stream recirculation, a tight gyre that effectively ventilates the upper part of the ocean in this region. There is also a fold extending southward from this region at about 1,200 meters depth, marking the intrusion of intermediate depth waters toward the tropics. Most notably, however, is the dramatic dive that the “blanket” takes to the north, disappearing into the ocean floor. The track along which this happens parallels the Gulf Stream Extension/North Atlantic Drift. All the waters north of this line have been ventilated to the ocean floor on 10 to 20 year time scales. This is a powerful statement regarding the time scales of ocean ventilation, and has profound implications concerning how rapidly climatic variations can propagate through the oceans.

We can see this in yet another way. The figure at right is a plot of tritium vs. depth for a time series near Bermuda from the late 1960s to the late 1980s. The tritium data has been adjusted for decay to the same date (January 1, 1981) to allow us to more clearly see the time trends. The most obvious trend is the downward propagation of a tritium maximum (deeper red) from the surface in the late 1960s to about 400 meters depth in the late 1980s, a downward penetration rate of 18 meters per year. However, from the perspective of climate change, the most important signal is the sudden increase in tritium (green) at about 1,500 meters in the late 1970s and at 2,500 meters in the mid 1980s. Both of these features correspond to the onset of significant cooling events seen in the deep water at this station. The correspondence between the sudden tritium increases and cooling offsets is highly suggestive of significant changes in deep water ventilation at those times. Indeed, if we could take another “picture” of the tritium blanket shown in the figure on page 29, we would see that it has been pushed further downward and southward by a climatic event.

Another Kind of Cold War Legacy
The development and manufacture of chlorofluorocarbons (CFCs) for use in refrigerators and air conditioners (and later as spray can propellants) seemed like a good thing at the time: CFCs were easy to manufacture, nontoxic, chemically inert, and stable. Production, use, and ultimate release of CFCs into the atmosphere increased annually in an almost exponential fashion from their introduction in the 1930s. The unfortunate influence of these compounds on the ozone layer, however, has lead to international reduction in their manufacture and use. In 1990, the US and 55 other nations agreed to end CFC production by 2000. Meanwhile, however, oceanographers have found another “silver lining” in this ecological cloud, which has permitted us to study ocean ventilation and circulation: Waters that have been in contact with the atmosphere in the past few decades have taken up some of these compounds, and hence have been labeled in a distinctive way. Thus the distribution of CFCs in ocean water provides us with important clues regarding the pathways of newly formed water masses.

In the 1970s and early 1980s, there was not much winter time convection and formation of Labrador Sea Water. In fact, tracer sections (lines of stations) taken across the Deep Western Boundary Current to the south showed a decided lack of newly ventilated Labrador Sea Water. This hiatus is somehow related to the complex interplay between changing climatic conditions in the area and the freshwater outflow and budgets of the Arctic. The late 1980s and early 1990s have heralded a dramatic change in climatic conditions in the Labrador Sea. These changes have resulted in the production of a large amount of Labrador Sea Water, which is now invading the ocean interior. You can see this beginning to happen in the figure at right, a time series of CFC sections made along 55°W south of the Grand Banks. Labrador Sea Water occurs in these sections at a depth of about 1,500-2,000 meters (the middle heavy dashed line). Notice that there was very little CFC-11 in this water in the 1983, although there is a CFC tongue at a shallower level characteristic of waters that are formed in the southeastern corner of the Labrador Sea (the shallowest heavy dashed line). Below the Labrador Sea Water core, there is a weak but detectable CFC core in waters characteristic of Denmark Straits Overflow water (marked here by the deepest dashed line). Combined, these three water masses form the Deep Western Boundary Current system of the North Atlantic, and are responsible for the southward transport of newly ventilated waters.

However, in the 1990s, there is a sudden increase in the amounts of CFCs in the Labrador Sea Water core, as well as a steady increase in the deeper core associated with waters from the Denmark Straits overflow. This increase is continuing through the 1990s and is direct evidence of the newly ventilated waters’ arrival. This again is a signature of the penetration of climatic anomalies into the ocean interior.

While the 55°W sections capture the invasion of the newly formed waters into the northern Sargasso Sea, the pathway southward is not a simple one. The Deep Western Boundary Current is not a continuous ribbon of flow extending all the way from the Grand Banks to the equator, but rather a composite consisting of series of interconnected gyres lined up along its path. A fluid parcel that passes by the Grand Banks may spend most of its time looping through these gyres, and only part of its time in the Deep Western Boundary Current. This is why the mean propagation speed of tracers down the western boundary is only 1 to 2 centimeters per second, while velocities in the actual core of the Deep Western Boundary Current are 10 to 20 centimeters per second.

The figure at right is a plot of tritium and tritium-helium age in the core of the Deep Water Boundary Current vs. distance downstream from its origin. We see that the core becomes progressively older (about 20 years in 10,000 kilometers) corresponding to a speed of about 1.5 centimeters per second. Notably, the tritium concentration in the core decreases more than tenfold downstream, partly due to decay, but largely due to dilution and mixing with older, tritium-free waters. This process of mixing is an important mechanism for ventilating the abyssal ocean. It’s through this process that climate anomalies make their way into the deep ocean.

Thus the transient tracers are telling us something very important about the propagation of climatic changes into the deep ocean. They highlight the pathways and give us the rates of movement and dilution in the ocean. This information is valuable because the ocean provides the long term memory and feedback in the coupled ocean-atmosphere-climate system, and is the key to beginning to make long term predictions in our ever changing climate.

The research discussed in this article was supported by the National Science Foundation and the Office of Naval Research. Bill Jenkins started life as a nuclear physicist but drifted into environmental sciences out of a secret yearning to become a forest ranger. Not having a good sense of direction, and fearing black flies, however, he ended up as an oceanographer on Cape Cod. He joined the WHOI Chemistry Department (now the Department of Marine Chemistry and Geochemistry) in 1974. Bill Smethie’s interest in oceanography began during childhood summers spent at his grandfather’s log cabin on the Virginia side of the Potomac River. He embarked on his first oceanographic cruise at age 7 when he attached a makeshift sail to his inner tube and set sail for the other side of the river. His doting aunts prevented him from making it to the other side, but ever since he has had a never-ending curiosity for what lies beyond the horizon. He joined the Geochemistry Division of Lamont-Doherty Geological Observatory in 1979.