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
December 1997 — The monsoon, a giant sea breeze between the Asian massif and the Indian Ocean, is one of the most significant natural phenomena that influences the everyday life of more than 60 percent of the world’s population. In summer, heating of the land produces a region of intense low surface pressure over northwestern India, Pakistan, and northern Arabia. A broad region of southwesterly winds develops, quite different from the northeast trades found in the other oceans at the same latitudes. The elevated east African coastline intensifies the wind near the surface and directs it parallel to the coasts of Somalia, Yemen, and Oman. This strong flow, the Findlater Jet, is remarkable for its steadiness of direction and its strength, which can exceed 36 knots in July. The offshore Ekman transport (see figure opposite) that results gives rise to intense upwelling along the coast, where cold, nutrient-rich water is brought up to the surface, and to convergence and downwelling in the central and eastern part of the Arabian Sea. In response to the Southwest Monsoon and unlike conditions in any other ocean, the surface mixed layer in the central Arabian Sea deepens and cools during the summer. In midwinter, when the Eurasian continent cools, a high pressure region develops on the Tibetan plateau and northeast winds persist over southern Asia and the Arabian Sea.
These winds are not as strong as during the summer, but, combined with strong surface cooling, they lead to deepening of the mixed layer in the central and western Arabian Sea together with higher primary production over the entire Arabian Sea. Thus, the winter monsoon brings a second cycle of mixed layer deepening and cooling to the Arabian Sea.
Even at it coolest, however, the surface water of the Arabian Sea is relatively warm, about 25°C in the central Arabian Sea, and the strong sunlight found year-round in low latitudes can support high productivity in the upper ocean. Thus, nutrient availability is the limiting factor, and primary productivity depends largely on mixing processes that can bring nutrient-rich water to the surface from below. The objective of our work in the Arabian Sea was to understand this link between the physics of the ocean’s response to the monsoon and the biological and geochemical variability of the Arabian Sea. Our data show that nutrients brought to the surface by mixing drive productivity in the surface layer, which produces particulates that fall from the surface layer into the deep ocean, removing carbon dioxide (CO2) from the air to that “sink” region.
The task of observing this biological pump is not easy; the ocean is highly variable, with a wide range of space and time scales. We knew that we would learn very little from conventional sporadic, independent ocean observations. A well organized, interdisciplinary research program with continuous data collection was critical to success. The field study for this research program, a combined effort of the Joint Global Ocean Flux Study (JGOFS) program and the US Office of Naval Research, began in late 1994 and continued for about one year. It featured observations during all phases of the monsoon, including seven repeated occupations of a cruise track with numerous stops to sample the ocean in detail. In addition, several advanced, moored instrument systems were deployed for the duration to sample the changing Arabian Sea environment continuously, hour-by-hour, at all depths.
In the lull after the summer monsoon of 1994, the Upper Ocean Processes Group of the Woods Hole Oceanographic Institution deployed an advanced surface mooring in the western, central Arabian Sea where the Findlater Jet is strongest. This Air-Sea Interaction (ASI) buoy carried a complete meteorological station as well as an unprecedented number (32) and diversity of oceanographic instruments for measuring temperature, salinity, current, dissolved oxygen, chlorophyll fluorescence, light transmission, and photosynthetically available radiation. This instrument complement resulted from a collaboration of investigators at WHOI, the University of Santa Barbara (Tommy Dickey), and the Lamont-Doherty Earth Observatory (John Marra and Chris Langdon). This buoy was recovered in the spring of 1995 and a fresh set of equipment deployed at the same site until October 1995 in order to collect a full year of data. During the experiment, the meteorological data was telemetered via satellite and published on a World Wide Web site to be immediately accessible by researchers worldwide.
We also deployed in the 4-kilometer-deep ocean a mooring with threetime-series sediment traps at depths of 0.8 kilometers, 2.2 kilometers, and 3.5 kilometers. Located about 53 kilometers north of the ASI mooring, these instruments were part of an eight-mooring (24-time-series-sediment-trap) Arabian Sea network fielded by US, German, and Indian scientists during 1994 and 1995. All the sediment traps were set to open and close their collecting bottles at precisely the same time; every eight and a half days a new sample bottle automatically moved into place.
Data from the moorings confirmed our suspicion that productivity in the upper ocean was episodic rather than slowly varying as had been assumed for decades. Tommy Dickey and John Marra’s time series of chlorophyll content from their instruments on the ASI mooring showed that most of the annual primary production occurred during four phytoplankton blooms (see figure below right). Sediment trap data showed that particle flux to the deep interior of the ocean was also strongly episodic “timing as well as relative amplitudes of the increased particle flux closely followed the primary production blooms, offset by several days to a few weeks. In other words, the biological pump in this central Arabian Sea site works hard several times a year rather than operating continuously at a moderate rate. At this location, the biological pump transfers slightly less than one percent of the photosynthetically assimilated organic carbon to the ocean’s interior for long-term storage of CO2 carbon.
These findings have a number of important implications that are applicable to a more general understanding of the ocean. Conventional seagoing productivity measurements in a given ocean area have simply been too scarce to allow us to develop a description of the annual variability in primary production in much of the open ocean. Even the unprecedented, repeated ship survey in the Arabian Sea is hard-pressed to resolve the short, intense blooms seen in the moored time series.
As the figure at right shows, the day-to-day variability of the primary productivity estimated from Dickey and Marra’s instruments coincides strikingly with that of organic and inorganic carbon flux documented by the time series sediment trap array. By comparing the timing of events that occurred in the eupohotic layer with events in the deep ocean layers, we can determine the settling rate of carbon-carrying particles. We previously suspected that the sinking speed of carbon particles falling from the upper ocean is much faster than what had been estimated from Stoke’s law, which provides a relation between fall rate and the drag on particles in water. Our Arabian Sea results confirm this important hypothesis about the vertical transport of organic carbon in the ocean’s water column. The consequences of the short-lived primary production maximum of December 11, 1994, are evident in the 2.2-and 3.5-kilometer-deep sediment traps, whose sampling period centered on December 29. Settling carbon particles arrived at those depths two 8.5-day sample intervals after the maximum organic carbon production near the surface. However, during the December 7 to 16 open period, the biogenic silica export maximum (mostly from diatom frustules) appeared in the ocean’s interior earlier than the organic carbon export maximum. The large organic carbon export event that peaked during the March 11 to 19 open period corresponded to the primary production bloom that peaked on March 11 (though this was not as sharp as the December 11 peak). This indicates that the particulate organic carbon that escaped remineralization reached a depth of 3.5 kilometers within one 8.5-day interval.
During the summer monsoon, the primary production bloom that peaked on July 17 was modest compared to the very large export production event that peaked during the July 17 to 25 sampling interval. A small primary production maximum that peaked on June 28 was not seen clearly in the export flux, but the signal may have blended with the July 17 peak.
The last and largest productivity event of the 1995 summer monsoon began in late August and continued for nearly two months. The organic carbon export production maximum at 2.2 kilometers depth occurred during the September 14 to 22 period, earlier than that of the primary production maximum on September 23.
Thus, we have found the chemical signatures of large bloom events in the euphotic layer to be preserved in exported particles collected in the ocean’s interior. The production rate as well as the timing of the bloom is duplicated. Diatom production is a good indicator of the ocean’s fertility and the biological pump’s efficiency. When conditions that we refer to as “Silica Ocean” prevail, the biological pump functions more efficiently to remove CO2 carbon from the atmosphere; a Silica Ocean produces more diatoms than coccolitho-phorids at the initial stage of the biological pumping (see The Rain of Ocean Particles and Earth’s Carbon Cycle). The tropical Arabian Sea is more often a “Carbonate Ocean.” However, during the observed periods of high primary production, this sea temporarily becomes a “Silica Ocean,” resulting in transport of more CO2 carbon to the deep ocean sink.
Various physical processes may be involved in the biological pump. Priming the pump requires that nutrient rich water from below the surface layer be brought up into the euphotic region. Near the coast, upwelling does this. In the central Arabian Sea, the mixed layer deepens and cools primarily due to wind-driven entrainment of cooler, nutrient rich water across its base in the summer monsoon. In the winter monsoon, the surface water in the central Arabian Sea is made more dense (cooler and saltier) by surface cooling and evaporation. The dense water sinks and the layer deepens with only modest mixing across its base.
However, another finding of this work leads us to consider different physical processes as well. That finding stems from our observations that the abundance of biologically assimilated silica relative to the calcium carbonate material collected in the sediment traps varies. In practice, we often use the ratio of biologic calcium to silicon found in the flux and/or the ratio of organic carbon (from settling cell material) to inorganic carbon (from settling biogenic calcium carbonate) to quantify this ratio. We observe that the silicon to calcium ratio in the particles exported to the ocean’s interior in blooms in December 1994 and August 1995 was very high and that during these periods eddies were passing the mooring site. One apparent impact of these eddies is the introduction of nutrient rich water into the surface layer. The strong surface currents associated with these eddies were seen by the current meters on the ASI mooring. The eddies also have a sea surface elevation signature, and Tommy Dickey was able to correlate the passage of particular large eddies observed by satellite with the blooms he and Marra observed at the ASI mooring.
In the euphotic layer of the Arabian Sea, dissolved silica is depleted by the demands of organisms living in the upper ocean layers. As soon as the dissolved silica is supplied from deeper layers to the euphotic layer, diatom production accelerates. In the Arabian Sea, production of coccolithor-phorids also increases, but diatom production dominates. The primary production blooms of February, March and July 1995 were not linked to eddies, but were locked to the end of the periods of mixed layer deepening associated with each monsoon. There is also evidence that a dust storm caused by the strong winds in July 1995 covered the entire western and central Arabian Sea coincident with the July bloom. The ratio of organic carbon to inorganic carbon stood out from the background; but while the silicon to calcium ratio in the particles arriving at the ocean’s interior during these blooms was high, it was not as elevated as that seen during the eddy-related blooms.
Thus, perhaps one of our most intriguing results is that the chemical composition of what we find to be the rapid export flux from the surface layer may guide our efforts to understand the physics of the Arabian Sea. Convective deepening and wind-driven entrainment appear to result in only modest transport of nutrient-rich water into the surface layer compared to the eddy-related transport. We will now have to consider more carefully the role of eddies. By doing so we will build our understanding of how the biological pump now functions and be able to apply that understanding to the record of how the pump functioned in the geological past as revealed in the records provided by the ocean sediments.
Sus Honjo’s work was supported by the National Science Foundation, and Bob Weller’s by the Office of Naval Research. The data and other contributions to their understanding provided by colleagues, particularly Tommy Dickey and John Marra, is gratefully acknowledged.
Bob Weller is part of the Physical Oceanography Department’s Upper Ocean Processes Group. The talent and skill of this group’s technical staff have made it possible to deploy heavily instrumented surface moorings around the globe and to make significant progress toward understanding the interaction of the ocean and atmosphere. His most recent experimental project deployed two surface moorings in the eastern tropical Pacific as part of the Pan American Climate Study. In 1997, this put Bob and the rest of the group at sea and away from WHOI for six weeks in April and May on a Lima, Peru, to San Diego cruise aboard Roger Revelle (Scripps Institution of Oceanography) and again for six weeks in November and December on a Honolulu to Honolulu voyage on Thomas Thompson (University of Washington).
Spring 1998 — Submarine lava flows and their associated narrow feeder conduits known as dikes constitute the basic building blocks of the upper part of the ocean crust. We are only beginning to understand how lava erupts and forms on the seafloor by flooding topographic lows, flowing through channels or tubes, centralizing into volcanoes, or some combination of all of these. (See Smith/Cann article for a discussion of these volcanic processes.) The style of emplacement along with the extent and volume of individual lava flows and eruption rates are important parameters that help determine the initial properties of oceanic crust, its vertical and horizontal structure, and what processes control the magma supply to the crust.
Less than a handful of seafloor eruptions have ever been monitored in real time or near real time, so when a swarm of seismic events showed characteristics of an eruption off the west coast of the US in July 1993, the American science community mobilized to take advantage of this unique opportunity. The seismic activity was initially detected by seafloor sensors on the CoAxial ridge segment of the Juan de Fuca mid-ocean ridge system at 46°15’ N, 129°53’ W, but then over a period of just two days the activity marched 40 kilometers north along a narrow band of the seafloor to center on 46°31.5’ N, 129°35’ W, where activity finally dissipated after a few more days.
Research cruises sent to this latter site discovered a seafloor eruption had indeed occurred, forming a new lava flow up to 30 meters thick, 2,500 meters long, and 400 meters wide. To the north and south of the lava flow, a linear narrow fault-bounded valley, called a graben, was also discovered, oriented along the same trend as the lava flow. This narrow graben is the surface expression of the subsurface feeder conduit or dike zone that fed the lava to this eruption site from the magma chamber located some 40 kilometers to the south.
We were fortunate that the Juan de Fuca region had been mapped relatively recently with modern bathymetric systems and thus repeat bathymetric mapping after the CoAxial eruption allowed the pre-eruption topography to be subtracted from the post-eruption topography to obtain an estimate of the new lava flow’s thickness. Pre-eruption bathymetric surveys are not always available, however, which precludes using differential bathymetric mapping to determine lava flow thickness for many regions. One possible solution is to measure a property that is proportional to the volume of the new lava, such as its magnetic anomaly. Newly erupted lava is thought to be initially highly magnetized before it degrades to less magnetic minerals through sea-water alteration. Highly magnetized lava thus should produce a distinctive magnetic signature relative to the older lava. Individual lava flows are typically thought to be on the order of a few tens of meters thick and a few kilometers long, requiring close-up, near-bottom magnetic surveys rather than distant surface surveys to obtain the requisite resolution for detecting such features. Magnetic surveys also offer some potential advantages over differential bathymetric mapping. Surface ship bathymetry has a relatively large footprint of 100 meters square and a limiting depth resolution of 5 to 15 meters, although near-bottom bathymetric mapping could improve on this resolution by an order of magnitude. Depending on the geometry and density of the magnetic survey tracks, near-bottom magnetic mapping could have both a small effective footprint and the ability to map flows thinner than 5 meters. The magnetic mapping method also has the advantage that it can be done after the lava flow has erupted and does not require pre-eruption surveys.
Six months after the eruption, in late 1993, we used the submersible Alvin to carry out an initial survey of the new floor and found that the new lava flow did indeed have a strong magnetic anomaly associated with it and that the anomaly could be directly attributed to the new lava. For a more extensive survey, I turned to some colleagues here at Woods Hole Oceanographic Institution who are developing small autonomous underwater vehicles (AUVs) as new platforms for science. AUVs are untethered robotic vehicles that can be programmed to operate independently, for example, to execute a series of geophysical tracklines over the seafloor while computing their position, depth, and altitude off the seafloor. The AUV currently under development at WHOI is nicknamed “ABE” for Autonomous Benthic Explorer, and the elegance and simplicity of making measurements from such a platform belies the technological achievements needed to make it possible. (See Oceanus Vol. 38, No. 1, 1995, for a discussion of ABE’s development.)
In 1994, we outfitted ABE with a magnetometer sensor to carry out a detailed magnetic survey of the CoAxial ridge lava flow. ABE collected additional data in 1995, providing approximately 35 kilometers of tracklines over the new flow. The surveys cover the majority of the lava flow and provide a broad view of its magnetic field. The new lava flow produces an extremely strong near-bottom magnetic field anomaly of approximately 15,000 nano Teslas (units of magnetic field intensity), which is almost 25 percent of Earth’s main magnetic field of 55,000 nano Teslas. The observed anomaly arises from a combination of the magnetization of the new flow and the topographic effect of the ridge upon which the new flow erupted. The ABE magnetic field data were first corrected for the magnetization of the vehicle and then corrected for the variations in observation depth of ABE.
For magnetic anomaly data to provide an independent estimate of flow thickness, crustal magnetization over the old and new lavas must be determined by representative rock sampling. Measurements of rocks sampled by Alvin and Jason revealed the new lava’s magnetization to be about 60 amps per meter on average, while the older surrounding lava has a magnetization of only 26 amps per meter. To estimate lava flow thickness from the ABE magnetic anomaly data, a forward modeling approach allows us to fit the observed magnetic anomaly for each ABE profile by iteratively incrementing the thickness of the new lava and calculating the resultant magnetic anomaly. The final profiles are then translated into a lava flow thickness map (see figure at right) which shows a maximum of almost 30 meters.
This “magnetic” thickness map is remarkably consistent with both the observed flow boundaries and the lava thickness derived from differential bathymetry. A total erupted lava volume of 8.8 million cubic meters can be estimated from the thickness map, which includes a correction estimate of 1.8 million cubic meters for the northern and southern extremes of the flow and the eastern tongue not traversed by ABE. This volume is consistent with differential bathymetry data on the 1993 CoAxial lava flow and represents the volume equivalent of a small Hawaiian eruption. The magnetic thickness map identifies the presence of the 1993 lava flow where it is too thin for detection by the differential bathymetry method. Differential depth measurements are also less reliable in areas of steep slopes because small shifts in navigation can produce artificial depth anomalies (see the overestimated differential depth anomaly in the figure below, where the south end of the eruption abuts the side of the small seamount). Magnetic thickness estimates are unaffected by these bathymetry problems.
The magnetic estimates do, however, have their own set of resolution limitations and problems. The error in the magnetic thickness estimate depends primarily on the estimates of both the old and new lava magnetization. The extent to which magnetization varies within a single flow is not well known, but it is reasonable to assume that the 1993 lava flow has relatively homogeneous magnetization because it erupted as one single flow and cooled relatively quickly. There is greater likelihood of magnetization variation for the older lavas because they may be composed of several flows from different ages and eruption events. By combining error estimates from the measured rock samples of the old and new lava, we obtain an uncertainty in the thickness of the new flow of about 30 percent. It is difficult, however, to estimate how well the magnetic modeling fits the true lava flow thickness because the only constraints are the differential bathymetry thickness data, which represent a spatially filtered and thickness truncated version of reality. Nevertheless, differential bathymetry does provide an upper constraint on lava thickness, and this is compatible with the magnetic thickness estimates.
The ABE survey shows that high-resolution, near-bottom magnetic field surveys can help to quantify the geometry of young, recently erupted seafloor lavas and can provide thickness estimates that are consistent with other observations such as differential bathymetry. Magnetic fields offer some advantages, for example, in mapping lava flows that are too thin for detection by the differential bathymetry method and where steep topography causes the differential bathymetry method to be unreliable. The most important advantage, however, is that magnetic surveys can be undertaken after an eruption, removing the need for preexisting data over the site.
For magnetic anomaly data to provide independent estimates of flow thickness, crustal magnetization over the old and new lavas must be determined by representative rock sampling. Finally, the magnetic survey provided an opportunity to test a newly developed autonomous underwater vehicle. AUVs represent the newest wave of technology that promises to revolutionize oceanography and marine geoscience and make the oceans more accessible to scientific study. Just as Alvin, deep-towed sensors, camera sleds, and remotely operated vehicles such as Jason revolutionized the way geologists and geophysicists viewed the seafloor in the 1980s, so now AUVs are poised to make as big an impact in the 21st century.
The research described in this article, as well as the development and operation of ABE, were supported by the National Science Foundation.
December 1996 — New Englanders claim a birthright to complain about the weather. As we note that the summer of 1996 was coolest and wettest in recent memory, most of us have already forgotten that summer 1995 was unusually warm and dry. Such variability in weather is normal, yet in historical times there have been truly exceptional events. For example, 1816 is known as the “Year Without a Summer.”* During that year, there were killing frosts all over New England in May, June, and August. July 1816 was the coldest July in American history, and frosts came again in September. Crop failure led to food shortages throughout the region. Although the immediate cause of cooling has been ascribed to the volcanic eruption of Tambora in Indonesia the year before, the Year Without a Summer occurred during a time when weather was generally more harsh than today. Persistently harsher weather suggests a change in climate, and the late 16th through the 19th centuries have become known as the “Little Ice Age.”
The Little Ice Age, and several preceding centuries, which are often called the “Medieval Warm Period,” are the subject of controversy. Neither epoch is recognized at all locations around the globe, and indeed at some locations there is clear evidence of warming while others show distinct cooling. One author titled a paper: “Was there a Medieval Warm Period, and if so when and where?” Nevertheless, when data from all Northern Hemisphere locations are considered, the annual average summer temperature proves to be a few tenths of a degree lower during the coldest part of the Little Ice Age in the late 1500s and early 1600s. Various forcing mechanisms have been proposed for such changes, including variation in the sun’s energy output, volcanic eruptions, and mysterious internal oscillations in Earth’s climate system, but none satisfy all of the data.
Natural climate changes like the Little Ice Age and the Medieval Warm Period are of interest for a few reasons. First, they occur on decade to century time scales, a gray zone in the spectrum of climate change. Accurate instrumental data do not extend back far enough to document the beginning of these events, and historical data are often of questionable accuracy and are not widespread geographically. Geological data clearly document globally coherent climate change on thousand-, ten thousand-, and hundred thousand-year time scales, so why is the record so confusing over just the past 1,000 years? Second, as humanity continues to expand and make more demands on our planet, annual average temperature changes of a degree could have considerable social and economic impacts. Third, as there is widespread agreement among climatologists that changes due to human impacts on atmospheric chemistry will eventually lead to global warming of about two degrees over the next century, it is important to understand the natural variability in climate on the century time scale. Will the human effects occur during a time of natural warming or cooling?
Of several approaches to studying climate on decadal to century time scales, here I will touch on the study of long series of measurements made at sea and the study of deep sea sediments. Ordinarily, there is little overlap between these two approaches. Reliable and continuous hydrographic observations rarely extend back beyond several decades, and deep sea sediments usually accumulate too slowly to resolve brief climate changes. However, the northern Sargasso Sea is a region where we have five decades of nearly continuous biweekly hydrographic data, a long history of sediment trap collections to document the rain of particles from the sea surface to the seafloor, and exceptional deep sea cores of sediment. The co-occurrence of these three elements has led to one of the first reconstructions of sea surface temperature for recent centuries in the open ocean.
Oceanographically, Station S in the western Sargasso Sea is important because temperature and salinity change there is typical of a large part of the western North Atlantic, and it is exclusively western North Atlantic water that is transported northward and eastward by the Gulf Stream. These are the waters that eventually cool and sink in the Norwegian and Greenland Seas, flowing southward to complete a large-scale convection cell that plays a fundamental role in regulating Earth’s climate.
In addition to long time series of hydrographic data from Station S, the site is remarkable for the long series of sediment trap data collected by WHOI’s Werner Deuser, beginning in the 1970s. Those traps have recovered nearly continuous samples of the seasonally changing rain of particles that settle from surface waters to the seafloor. An important component of those particles is the calcium carbonate shells of planktonic protozoans known as foraminifera. There are about 30 species of foraminifera, or “forams,” and Deuser’s investigations have established the seasonal change in species abundance and their stable isotope composition. We now know from these studies that only one species of planktonic foram, Globigerinoides ruber, lives year-round at the surface of the Sargasso Sea, and it happens to deposit its calcium carbonate close to oxygen isotopic equilibrium with seawater. This means that G. ruber is ideal for reconstructing past changes in the temperature and salinity of Sargasso Sea surface waters, as the figure at right illustrates.
Note that the average sea surface temperature and salinity from near Bermuda display some systematic variability on an annual average basis since 1955. These changes reflect a decade-long variability in the North Atlantic climate regime that is known as the North Atlantic Oscillation (see Deser article inset by McCartney). In this time series, the most severe climate occurred in the 1960s when annual average sea surface temperatures were depressed about half a degree by extreme storminess in the western North Atlantic. Cold, dry winds during winter storms also probably raised surface ocean salinity in the 1960s by promoting increased evaporation. If we had “annual average forams” from the 1960s, their oxygen isotope ratio would look like the time series shown in purple. The biggest climate change of the past five decades could indeed be recorded by the forams.
Long before I knew that G. ruber was the best possible foram for reconstructing sea surface temperatures, I selected that species for my stable isotope studies because of its consistent abundance on the Bermuda Rise, in the northern Sargasso Sea to the east of Station S. At the time (the early 1980s), the Bermuda Rise was under consideration as a possible site for burial of low level nuclear waste, and it was necessary to know how rapidly and continuously the sediment accumulates. It turns out that because of the action of deep ocean currents, fine-grained clay and silt particles are selectively deposited there, resulting in very high rates of sedimentation. And whether samples are of modern or glacial age, G. ruber is consistently present. Much of my work over the past decade has documented the climate changes that occur on thousand year time scales and are preserved in foram isotope ratios and other data from Bermuda Rise sediments.
Until recently, the available data from the Bermuda Rise showed evidence of century- to thousand-year climate change continuing right up to about a thousand years ago, the age of the sediment at the tops of our cores. Because these samples were recovered with large, heavy tools that free fall into the seafloor, I suspected that they might have pushed away sediments of the last millennium without actually coring them. As a test of this idea, we acquired a box core from the Bermuda Rise (box cores penetrate the sea-floor slowly and disturb surface sediments little) and radiocarbon dated its surface sediment at the National Ocean Sciences Accelerator Mass Spectrometry Facility located at WHOI. Results showed that the sediment was modern, and additional dates were used to construct a detailed chronology of the past few millennia. When temperatures were calculated from oxygen isotope results on G. ruber from the box core, and when data were averaged over 50 year intervals, I found a consistent pattern of sea surface temperature change (see figure at right). The core-top data indicate temperatures of nearly 23 degrees, very close to the average temperature at Station S over the past 50 years. However, during the Little Ice Age of about 300 years ago sea surface temperatures were at least a full degree lower than today, and there was an earlier cool event centered on 1,700 years ago. Events warmer than today occurred about 500 and 1,000 years ago, during the Medieval Warm Period, and it was even warmer than that prior to about 2,500 years ago.
These results are exciting for a few reasons. First, events as young and as brief as the Little Ice Age and the Medieval Warm Period have never before been resolved in deep sea sediments from the open ocean. Because the Sargasso Sea has a rather uniform temperature and salinity distribution near the surface, it seems that these events must have had widespread climatic significance. The Sargasso Sea data indicate that the Medieval Warm Period may have actually been two events separated by 500 years, perhaps explaining why its timing and extent have been so controversial. Second, it is evident that the climate system has been warming for a few hundred years, and that it warmed even more from 1,700 years ago to 1,000 years ago. There is considerable discussion in the scientific literature and the popular press about the cause of warming during the present century. Warming of about half a degree this century has been attributed to the human-induced “greenhouse effect.” Although this is not universally accepted, it is widely accepted that eventually changes to Earth’s atmosphere will cause climate warming. The message from the Bermuda Rise is that human-induced warming may be occurring at the same time as natural warming—not an ideal situation. Finally, building on the studies of physical oceanographers and climatologists, marine geologists and paleoclimatologists may use the North Atlantic Oscillation as a model for understanding North Atlantic climate change on longer, century and millennial time scales.
This work was funded by the National Oceanic & Atmospheric Administration’s Atlantic Climate Change Program. We encourage Oceanus authors to include a bit of humor in the short biographies we request. Lloyd Keigwin claimed to be”a humorless scientist” who doesn’t like writing bios, so we asked Eben Franks, a research assistant in Lloyd’s lab, to provide some information. Here’s what Eben wrote: In addition to running a demanding research program, Lloyd Keigwin is also a Commander in the Navy Reserve. Despite nearly 30 years of sea-going experience he still finds himself subject to seasickness. [Editor’s note: This is not unusual among oceanographers!] Lloyd has been deeply affected by episodes of the popular PBS series “This Old House” and has spent 14 years (and counting) demolishing two perfectly adquate houses in the name of renovation. His limited spare time is consumed with multifarious projects ranging from attempting to convince the Navy to convert a nuclear sub for oceanographic research to casting longing looks at the antique German and British sports cars collecting dust in his barn.