The Department of Geology and Geophysics (G&G) conducts research into a wide variety of topics aimed at furthering our understanding of the dynamic processes of the Earth/Ocean/Atmosphere system. Our research spans across land and oceans as we seek to understand connections between the continents and oceans, ice-sheet dynamics and the formation and evolution of the Earth as a whole. We study the structure and evolution of the oceanic crust from its formation at mid-ocean ridges to consumption at subduction zones, coupled with the dynamics of the mantle that drives seafloor spreading. We study a wide range of fluid-mediated processes, including those occurring at hydrothermal vents, at shelf-edge seeps and in subduction zone settings. Included in these processes are links to seismicity, fluxes of chemicals to the ocean and mantle, microbial activity and the subseafloor biosphere. We study the role of oceans both in relation to past climate change and as a driver of present day climate dynamics, and use natural archives like from sediments, corals, and tree rings to understand past climate. We study a wide range of coastal processes including the impacts of climate change and storms on coastal regions.
The Department today consists of about 30 Ph.D. level Scientific Staff and another 16 Technical Staff (many of whom hold Ph.D. degrees). In addition there are about 25 graduate students pursuing their Ph.D. through the WHOI/MIT Joint Program and roughly 8 Postdoctoral Scholars, Fellows and Investigators.
The Scientific and Technical staff carry out research that involves sea-going deployments of instruments built in house; laboratory studies using high precision analytical facilities; and theoretical and computational studies of ocean and climate processes and geodynamics. Examples of the facilities within the department include the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) and the Northeast National Ion Microprobe Facility (NENIMF). We now run the national Ocean Bottom Seismograph Instrument Center (OBSIC).
Geology & Geophysics Department
1998— The MELT Experiment was the largest seafloor geophysical experiment ever attempted, and one of its major components was MT, the magnetotelluric technique. MT offers a valuable tool toward the MELT Experiment’s goal of probing the earth’s inaccessible deep interior. But the technique remains something of a mystery even to many marine scientists. It has been used widely on land, particularly for regional-scale surveys, but only a few full-scale MT surveys have been carried out on the seafloor.
The primary data collected by marine MT experiments are measurements of changes in the earth’s electrical and magnetic fields at the seafloor. These fields are affected by electromagnetic currents within the earth, and here’s where MT’s apparent complexity starts—because the source of these currents is not within the earth, but rather in the ionosphere.
Charged particles, emitted from the sun as a solar wind, become trapped in the ionosphere by the earth’s magnetic field. These moving charges essentially create a variety of electric currents encircling the earth. If the earth were a perfect insulator, like space, that would be the end of the story. But the earth can conduct electricity. As these ionospheric currents flow around the earth, they generate a response within the planet itself. More specifically, the pattern of ionospheric currents induces almost a mirror-image pattern of currents within the earth.
These so-called “induced image currents” cause changes in the earth’s electric and magnetic fields. These changes depend on the conductivity of the earth’s interior, which, in turn, is determined by the composition and structure of the materials that constitute our planet’s interior. Thus, by measuring changes in Earth’s electric and magnetic fields at the surface, we can effectively deduce its electrical conductivity and reveal its interior structure. As CAT scans reveal images and frameworks that enable us to learn about the workings of the human body, MT experiments similarly provide essential cutaway views that allow us to learn about processes taking place within our planet.
Like standard alternating currents in most households, which have a frequency of 60 Hertz, or one cycle per 1/60 of a second, induced image currents also alternate—though they do so over a wide range of frequencies. The variations, or frequencies, we use in seafloor MT range from periods of about 100 seconds to several hours. These variations are caused by the chaotic nature of the events that entrap ions from the solar wind, as well as by more regular events, such as the earth’s daily orbit around the sun. The important point is that different frequencies penetrate the earth to different depths. If induced image currents came in only one flavor, we would be able to image the earth’s interior at only one depth. As it is, higher-frequency currents (with one cycle per 100 seconds, for example) don’t penetrate deeply and can tell us about structure 10 to 15 kilometers deep; the lowest-frequency currents (with one cycle per several hours) can tell us about depths of several hundred kilometers.
The goal of the MELT Experiment was to map basaltic melt, from its source within the mantle to the base of the oceanic crust at the mid-ocean ridge crest. While the earth can conduct electrical currents, most rocks, including those comprising the mantle, do not conduct particularly well. This situation changes considerably when melt is present: Pure basaltic melt is several orders of magnitude more conductive than olivine, a common mantle mineral. In the mantle melting column, we do not expect to see pure melt, nor anything like it, but rather some distribution of streams and pools of liquid melt within a matrix of solid mantle rocks. In this case, how the melt is distributed is important. It is possible to think of the melt as a network of wires that connect parts of the mantle. If the melt forms a well-connected network through the rock, electric currents can flow and the mantle will be electrically conductive. Of course, reality is more complicated and other factors, such as water dissolved in the mantle rock, can affect conductivity. These other factors are also important for understanding the whole process of melt production.
The MT component of the MELT Experiment was a truly multinational effort involving more than a dozen scientists from Woods Hole Oceanographic Institution and Scripps Institution of Oceanography in the US, and from France, Japan, and Australia. Each group contributed instruments to the array and played a role in the data analysis. From June 1996 to June 1997, 47 instruments were deployed at 32 seafloor sites to measure the time variations of the electric and magnetic fields. Two lines were set out. The main southern line had 19 sites and crossed a magma-rich segment of the East Pacific Rise ridge crest, extending 200 kilometers on either side of the crest. The second line of 13 sites crossed the ridge to the north on a magma-starved ridge segment, extending 100 kilometers on either side of the axis.
Each group’s instruments essentially did the same thing: measure changes in the electric and magnetic fields at the seafloor. But each group accomplished this in slightly different ways, deploying very different-looking instruments. As in all marine experiments, the environment makes seafloor MT measurements more difficult to make, but in one way nature helps us. The ocean is electrically very conductive and acts as a screen against electromagnetic noise—extraneous signals from other sources that would confuse interpretation of the data. On land, power lines, for example, can be a nuisance. The seafloor, however, is electrically quiet, making it possible to measure very small electric field variations. The other part of the MT signal is the seafloor magnetic field—not the steady field trapped in lavas and used to identify magnetic reversals, but the magnetic field variations linked to ionospheric currents.
To a first order, the ratio of the electric to the magnetic field at the earth’s surface is a direct measure of the earth’s electrical conductivity. We calculate this ratio for a range of current frequencies using modern processing techniques. To produce a model of the earth, data from all instruments have to be examined through a process of numerical inversion. The interaction of induced currents in the earth with the conductive bodies we hope to image (such as the melt column) affects the electric and magnetic fields over a wide region of seafloor. Generally, it is not possible to look at data from a single instrument and interpret the underlying structure. Instead, we have to use computer modeling to predict the fields that the mantle would create and compare these answers to data from all the instruments. The model is updated to improve the agreement and the process is repeated until a satisfactory model is found. There are many pitfalls involved in this process, as well as different ways of carrying it out. The groups involved in the MELT Experiment have been using a variety of methods over the past few months, and we are in the process of comparing results and discussing their implications.
The MT analyses are still in their early stages, but some first-order results are beginning to come through. The MT data show an asymmetrical distribution of melt between the areas west and east of the ridge crest, with a more extensive region to the west. The melt column also appears to be a broader feature, with a low percentage of melt in it, rather than a narrow vertical column of melt directly beneath the ridge. This indicates a more passive flow of mantle toward the ridge crest. Deeper, we see some evidence for a conductive mantle at depths greater than 150 kilometers. If this proves to be true, it could be evidence for deeper melting—deeper than the part of the mantle generally believed to be responsible for most melt generation. However, in the final analysis, water dissolved in the mantle rock may prove an important factor in mantle conductivity at this depth.
Funding for the MELT Experiment was provided by the National Science Foundation through the RIDGE Program. The many people involved in the MT component of MELT include: Alan Chave, Bob Petitt and John Bailey (WHOI), Jean Filloux and Helmut Moeller (SIO), Pascal Tarits (UniversitÉ de Bretagne Occidentale), Martyn Unsworth and John Booker (University of Washington), Graham Heinson and Anthony White (Flinders University, South Australia), and Hiroaki Toh, Nobukazu Seama and Hissashi Utada (University of Tokyo).
December 1997 — Phytoplankton photosynthesis has provided Earth’s inhabitants with oxygen since early life began. Without this process the atmosphere would consist of carbon dioxide (CO2) plus a small amount of nitrogen, the atmospheric pressure would be 60 times higher than the air we breathe, and the planet’s air temperatures would hover around 300°C. (Conditions similar to these are found on Earth’s close sibling Venus).
As phytoplankton grow through the process of photosynthesis, they fix CO2 in their cells as organic carbon and thus absorb atmospheric CO2 into the upper ocean layers. Animal plankton graze the phytoplankton, employing most of the organic carbon as their energy source and oxidizing the rest of it back to CO2, which eventually returns to the atmosphere. A small portion of this photosynthetic carbon escapes the oxidation process by settling, or “sedimenting,” through the water column in particles, which are often called “marine snow” and include pelletized feces of small animal plankton. CO2 carbon is also transported to the deep ocean in another way: Some plankton, such as coccolithophorids, planktonic foraminifera, and pteropods produce beautiful calcite and aragonite shells or tests that sink toward the seafloor when the organisms die.
Once the organic carbon and calcium carbonate particles reach the ocean interior at a depth of a few kilometers, they are “stored” there and will not return to the atmosphere for a relatively long period of time. This complex carbon-transporting ocean process, often called the “biological pump,” is a critical mechanism in preventing what we now know as the “greenhouse effect,” the collection of gases in the atmosphere that hinders upward transport of heat.
Understanding of Earth’s carbon cycle is one of humankind’s great scientific questions. Sediment traps are an important tool for studying the spatial and temporal variability of sinking particles (and carbon) in the ocean. The idea behind these devices is very simple: Vertically settling particles are collected at a specific area during a specific time period by providing a stable collection area at a depth along a mooring. The collected particles are then recovered and weighed, and the vertical flux of particles can be calculated as weight or volume per unit area in a unit of time—milligrams per square meter per day. Because the export flux of carbon is usually highly seasonal and often episodic, a short-term measurement produces data that is only useful for limited special purposes. It is therefore critical to collect sediment in time series for at least a year. Though this is not as technologically easy as this simple description may sound, the time-series sediment trap array method, together with multidisciplinary ocean measurements, has recently brought large leaps in the understanding of basinwide dynamics of the biological pump in relation to such global oceanographic phenomena as El Niño and the Asian monsoons. Sediment trap experiments have come to be one of the principal methods for understanding global CO2 cycles in the ocean.
We now have 15 years of time-series, sediment-trap data collected from the interior of the world’s open oceans through the collaborative effort of an international group of scientists, including the WHOI PARFLUX group. We are finally beginning to understand the pattern of basin-to-basin export-flux variability and to make intelligent estimates of the flux of CO2 carbon in particulate matter to the ocean’s interior. A sediment trap collects not only carbon products but also other kinds of particles: For example, we have found that the export flux of biogenic silica produced by plankton with siliceous frustules and tests provides important information for understanding another type of basin-scale biological pump (see Arabian Sea article).
By integrating sediment trap based measurements (figure above), we estimate the global flux of carbon to the ocean’s interior at 0.8 gigaton, nearly one billion tons, per year—0.35 gigaton in organic carbon and 0.44 gigaton in inorganic carbon (calcium carbonate). The global calcium in carbonate and silicon in biogenic opal export fluxes are 2.9 and 1.4 gigatons per year, respectively. Interannual fluctuation of global export production is unknown, as we are far from understanding the interannual changes in overall primary production.
Our 15-year export flux data set reveals distinct “biogeochemical regions” where the biological pump operates in two significantly different modes known as “Carbonate Ocean” and “Silica (Opal) Ocean” (see figure above right). A Carbonate Ocean is defined as the condition in which sinking particles deliver more carbonate than silica to the ocean interior, as well as more inorganic than organic carbon. In a Silica Ocean, these ratios are reversed. To date, these conditions are only observational, and we are anxious to determine the reasons for this partition in the ocean. Global distribution of dissolved silica and other essential nutrients, which is controlled by very large scale ocean circulation, might be the deciding factor in this partition. The majority of present seas are Carbonate Oceans while Silica Oceans account for less than 20 percent of the world ocean. The export flux of organic carbon in a Silica Ocean is usually higher than that in a Carbonate Ocean because their biological pumps function differently: Photosynthesis of diatoms, whose shells are largely composed of silica, is the mechanism for primary production in a Silica Ocean, and photosynthesis of coccolithophorids, whose shells are largely composed of calcium carbonate, is the primary producer in a Carbonate Ocean.
At present, one of the most typical Silica Oceans is the sub-Arctic Pacific north of 45°N, an area that includes the Bering Sea, the Sea of Okhotsk, and the northern East Sea (Japan Sea). By contrast, most of the North Atlantic is a Carbonate Ocean. Nutrient availability is the major oceanographic difference between these seas. Layers rich with dissolved nutrients underlie the sub-Arctic Pacific at about 600 meters; at 2,000 meters the dissolved silica maximum is reached. In contrast, the amount of silica in the northern North Atlantic is an order of magnitude less than in the sub-Arctic Pacific.
In the northern North Atlantic, upper ocean waters sink due to thermal exchange and are replaced with nutrient-depleted surface waters supplied from the south. This results in a negative silica mass balance in the upper layers in this area and allows coccolithophorids to take over the niche from the diatoms because coccolitho-phorids need no dissolved silica to grow. On the other hand, the upper layers of the sub-Arctic Pacific are enriched by the upwelling of dissolved silica. A large flux of biogenic opal particles is supplied to the relatively shallow but vast shelves and slopes in the Sea of Okhotsk and the Bering Sea and is recycled to the upper waters, generating the positive feedback loop of a “silica trap.” Thus the ecosystem is essentially occupied by diatoms, and relatively scarce coccolithophorids appear only when the surface ocean is temporarily depleted of silica at the end of the export-flux bloom.
The biological pump operating in a Silica Ocean removes CO2 carbon from the upper to the deep ocean in the form of particulate organic carbon on a short time scale of a few to several weeks (see Arabian Sea article). The biological pump in a Carbonate Ocean also removes carbon from the atmosphere and upper oceans in the form of organic carbon. However, when a molecule of calcium carbonate is formed in the upper ocean as part of a coccolith or planktonic foraminifer test, a molecule of CO2 is also formed by this chemical reaction. Therefore, there is no net removal of CO2 carbon from the upper oceans by the settling of inorganic carbon as calcium carbonate. Secretion of calcium carbonate in the upper ocean by organisms reduces alkalinity, resulting in an environment less capable of absorbing atmospheric CO2. Therefore, a Carbonate Ocean does not play a large role in removal of atmospheric CO2. The significance of the biological pump in a Carbonate Ocean is principally concerned with the long term CO2 balance in Earth’s atmosphere (see figure above).
As soon as they leave the upper ocean where they are produced, almost all of the coccoliths and foraminifera settle to the deep ocean floor within a short time. There the oxidation of organic matter that begins in the upper ocean causes an increasingly acidic condition, which results in dissolution of the coccoliths and foraminifera. Indeed, researchers find it strange to retrieve a large quantity of perfectly preserved coccoliths and foraminifera tests from a sediment trap that has been moored above a deep Pacific seafloor that consists of nothing but red clay and a few fragmented calcareous remains.
The deep ocean water, whose increased alkalinity is due to the dissolution of calcium carbonate, moves in such deep currents as the thermohaline driven circulation that runs south from the northern North Atlantic, proceeds as a bottom current through the Indian Ocean, and eventually upwells in the northern North Pacific (See Oceanus Vol. 39, No. 2). This mass of formerly deep water that has moved to the surface is thought to be a major absorber of atmospheric CO2. This mechanism is often called the Alkalinity Pump (different from the Biological Pump). Note that this process is too slow to affect the CO2 cycle of today’s Earth—it takes several centuries for the ocean’s deep-water circulation system to complete a turnover. Our present climate is greatly dependent on foraminifera that lived, died, and dissolved when Vikings roamed the ocean! In other words, calcium carbonate that sinks to the deep corrosive ocean is a “savings deposit” regulating future greenhouse effects of planet Earth well into the next millennium.
Major funding sources for Sus Honjo’s sediment trap work have been the National Science Foundation and the Office of Naval Research.
During his long research career at WHOI, Sus Honjo has concentrated on understanding the processes and rates of ocean sedimentation and the removal of carbon in the form of carbon dioxide from the atmosphere to its deep ocean “sink.” He recalls trembling with excitement when he looked into a microscope aboard R/V Knorr to examine the first sample collected by a deep ocean sediment trap in the central North Atlantic: Abundant fecal pellets produced by surface organisms proved his hypothesis that the majority of ocean sediment is delivered to the deep seafloor by large, rapidly settling aggregates. His global research efforts to link upper and deep ocean processes bridge marine geology, ocean chemistry, and biology and enhance understanding of the biogeochemistry of the ocean. As we prepared this issue, he was sending cruise reports from the southern ocean near Antarctica where he was leading a team of “trappers” and seafloor sediment research groups from WHOI and other US institutions in rough seas aboard NSF’s ice breaker/research vessel Nathaniel B. Palmer.
October 1997 —Deployment of a deep-ocean sediment trap mooring begins with the ship heading slowly into the wind. The mooring line, with the various instruments attached in top-to-bottom order, is paid out over the stern from a winch and pulled away from the ship, straight behind, seemingly almost on its own power. First over the stern is the top of the mooring, whose radio beacon and flashing light are designed to operate only when they are on the surface. Next comes a cluster of 20 floats, each capable of supporting 50 to 60 pounds in water, to provide upper-mooring buoyancy. These floats are all made of thick, plastic-covered glass spheres designed to withstand the great pressure of the deep ocean.
Next on the mooring line comes the shallowest sediment trap. Additional instruments such as a current meter are often deployed with the trap. This float/trap sequence is repeated until the end of the mooring line is reached and all the instruments are laid out along the surface. An acoustic release is connected near the end of the mooring. This critical device will be commanded to release the mooring from the anchor, allowing it to return to the surface at the end of the collection period.
Lastly, an anchor, usually about a ton of used freight car wheels, is connected to the lower end of the release with a long, highly flexible nylon line that stretches to absorb the landing impact. While a team of technicians and deck personnel are engaged in the laying out process, the bridge maneuvers the ship to the designated mooring location, passing a certain distance beyond where the anchor is to be set. When ready, the anchor is dropped into the ocean with a sizable splash. As the anchor sinks, the mooring line is pulled into the water, and one by one the traps and other equipment disappear from the surface. Finally, the radio signal goes silent as it slips beneath the waves. Resistance from the mooring pulls the anchor back a certain distance that we have allowed for from our modeling. The moment the anchor arrives on the ocean floor, its precise location can be calculated through acoustic communication from the ship to the release.
When the deployment period is over, usually a year, a ship will again be exactly positioned over the mooring, using satellite navigation. Technicians “talk” with the acoustic release near the anchor with a transducer—a microphone for communicating through the water column—and tell it to free the mooring from the anchor. In a short time, the bright orange floats at the summit of the mooring appear on the surface and the radio beeps. The rest of the recovery protocol reverses the deployment procedure. Traps and floats are picked up one by one and the mooring wire is wound on the winch. Finally, the acoustic release is recovered and the mooring crew can take a break—until it’s time to redeploy the moorings.
December 1997 — Until about 130 years ago, scholars believed that no life could exist in the deep ocean. The abyss was simply too dark and cold to sustain life. The discovery of many animals living in the abyssal environment by Sir Charles Wyville Thompson during HMS Challenger’s 1872–1876 circumnavigation stunned the late 19th century scientific community far more than we can now imagine.
Major questions immediately emerged: How do deep sea animals obtain food so far from the ocean’s surface where plants, the base of the ecosystem, grow? Do they all just wait until a whale corpse is occasionally delivered to the abyss? These questions were only answered fairly recently.
Twentieth century progress in oceanography resulted in further confusion. Microscopic particles suspended in the water column seemed so small and light that it was believed they should take hundreds, perhaps thousands, of years to settle through the water column. And what happens to labile (unstable) matter, particularly organic particles from dead, broken plankton cells? Scientists could not understand why coccoliths, the delicately architectured calcite shells of phytoplankton, only several micrometers in size, were preserved on the deep ocean floor just beneath the area where they were produced. Why were they not carried far from their source by currents, and how could they even exist there when chemistry clearly indicates they should be dissolved during their several-century trip to the bottom?
A WHOI experiment in the deep Sargasso Sea two decades ago shed light on this century-old question. Shipboard observation of the first successfully recovered sediment trap samples from 5 kilometers deep revealed that particles originating in the euphotic (light) zone aggregate: The fine, light particles do not settle individually but are repackaged into larger particles that settle to the deep sea at a much greater speed. Among the Sargasso Sea aggregates, we found an abundance of fecal pellets from upper ocean zooplankton. Viewing one of the fecal pellets under an electron microscope, I was fascinated to find it full of perfectly preserved coccoliths and undigested, many-celled organelles. Some oil droplets were obviously much lighter than seawater!
Because filter-feeding zooplankton are concerned only with the size of their food and graze phytoplankton almost indiscriminately, indigestible coccoliths and diatom frustules are concentrated in their fecal pellets. Many zooplankton fecal pellets are covered with a thin coating material. Although individual particles sink very slowly or are even buoyant, when they are bundled into a tight package and ballasted with particles of calcite—one of the densest materials produced in the ocean—they sink as rapidly as 100 to 200 meters a day.
Soon after World War II, scientists at Hokkaido University built an early submersible, named Kuroshio, to dive in the ocean north of Japan. Wherever they beamed a search light, they saw “snowflakes” dancing from the disturbance caused by the submersible. K. Kato and N. Suzuki named this phenomenon “marine snow.” More recently, the author and former MIT/WHOI Joint Program student Vernon Asper, now at the University of Southern Mississippi, have worked with WHOI engineers to construct an optical instrument to measure the size and density of marine snow all the way down through the water column. Cindy Pilskaln, who did her Ph.D. research at WHOI while a Harvard University Student and is now at the University of Maine, has found that fecal pellets alone cannot transport the amount of organic carbon known to exist in the deep ocean, and that the density of suspended particles in the water column remains at a steady state. One hypothesis is that the vertical transport mechanism may be the combination of rapidly descending fecal pellets and aggregates as well as individually settling, relatively large particles such as planktonic foraminifera and diatoms. All of these together form “marine snow.”
Many marine snow “flakes” are sticky and fibrous like a crumbled spider net, and particles easily adhere to them, forming agregates. An aggregate begins to sink when it attracts fecal pellets, foraminifera tests, airborne dust, and other heavier particles. As it descends, more suspended particles are added, making the aggregate even heavier and thus faster moving. An aggregate may break apart, spilling its contents into the water, but soon the spilled particles are picked up or “scavenged” by other falling aggregates. Thus aggregates are reorganized constantly with individual particles jumping on and off them before they arrive on the ocean floor. Meanwhile, a large portion of the organic matter in marine snow is recycled by microorganisms and upper and middle water column animals who again generate fecal pellets.
The removal of carbon from the ocean’s euphotic layer to its interior carbon “sink” is critical to the process that keeps Earth’s carbon cycle in order. We have learned that the speed of carbon settling to the ocean’s interior is very rapid: Particles can travel from surface waters to the abyss in only a few days or weeks (see Arabian Sea article). Nature accomplishes this process ingeniously by wrapping labile organic carbon up in a package and ballasting it with calcium carbonate, which causes it to settle at high speed to the deep ocean environment.