Deep-Sea Sediments Reveal the History of the Great Ocean Conveyor

Article excerpt

ONE IMPORTANT PREREQUISITE for understanding humankind's impact on Earth's climate is making a determination of the nature and causes of substantial climate changes known to have occurred in the past, prior to human intervention. Although often difficult to obtain, information about ancient climates is recorded in deep-sea sediments, lake sediments, and ice. For instance, recent studies of Greenland ice cores revealed that large, swift changes in atmospheric temperature occurred at the end of the Last Ice Age. Average temperatures changed about 7 |degrees~ C in 50 years, a rate of more than 1 |degree~ C per decade. Similar shifts characterize the ice-core record every few thousand years between about 80,000 and 8,000 years ago, and thus appear to be a characteristic feature of Earth's climate. Though temperature changes can be caused by orbital variations that affect the amount of solar radiation Earth's surface receives, this phenomenon occurs too slowly to account for the frequency or abruptness of the changes seen in ice cores.

According to a theory popularized by Wallace Broecker (Lamont-Doherty Geological Observatory), sudden transitions between warm and cold climate may have been driven instead by changes in the heat-carrying capacity of the Atlantic Ocean, at the driving end of whole-ocean circulation scheme he has called the "Great Ocean Conveyor." The conveyor's major features are set up by sinking of northward flowing, warm Gulf Stream waters in the northern Atlantic Ocean and Norwegian Sea. Gulf Stream waters become enriched in salt due to evaporation as they pass through warm latitudes. As these waters flow toward cooler latitudes they release heat to the atmosphere, become dense, and sink. The newly formed deep waters flow south (Upper and Lower North Atlantic Deep Water), filling much of the deep ocean, and more warm water is drawn northward at the surface to replace the water exported at depth. This "heat engine" drives the northward penetration of relatively warm surface waters in the northeastern Atlantic Ocean and Norwegian Sea, and results in the presently hospitable conditions in Britain and Norway, as contrasted to those at equivalent latitudes in Greenland and Canada. However, under the conditions that prevailed during much of the Last Ice Age, certain elements of the conveyor were shut down, depriving the northern Atlantic region of ocean-borne heat. If Broecker is right, and shutdowns of the conveyor promoted the sudden temperature changes seen in ice cores, they ought also to be evident as changes in deep-sea sediments. Until now, however, directly verifying these changes has not been possible because typical oceanic sediments accumulate much too slowly to resolve such brief events. However, by studying sediments recovered from a deep channel off the southwestern coast of Norway, where mud accumulated at rates some 100- to 500-times greater than the ocean average (due to glacial erosion on the adjacent continent), we have recently been able to read the record of the shifting conveyor.

Planktonic Records Help Reconstruct Conveyor History

To track past changes in the Gulf Stream's strength and trajectory, we use a conventional technique based on the present-day temperature tolerances of living communities of planktonic Foraminifera (microscopic shell-forming animals living near the surface of the open ocean; see page 98). Today the action of the conveyor draws warm Gulf Stream waters, and hence temperate Foraminifera, into high latitudes in the northeastern Atlantic Ocean and eastern Norwegian Sea. The frigid waters around Greenland and in the Arctic support only polar Foraminifera. By counting the abundance of different types of foraminiferan shells in sediments deposited throughout the northern Atlantic during the Last Ice Age, other researchers have shown that polar Foraminifera had greatly extended their range to the south; the Gulf Stream must have then flowed more-or-less straight across the Atlantic toward Portugal, rather than northward, toward Norway. This largely cut off the poleward flux of oceanic heat, contributing to the establishment of frigid air temperatures and the growth of continental ice caps and sea ice.

Our studies of planktonic Foraminifera in sediments from the Norwegian Channel revealed changes in surface circulation during the recovery from the Last Ice Age with almost unheard-of resolution. We dated the changes in Foraminifera using a recently developed radiocarbon-dating technique called accelerator mass spectrometry, which permits direct counting of carbon-14 atoms in the shells (see Oceanographer's Toolbox, page 93). Using the carbon-14 dates to plot our results against age, we found that the history of circulation change was remarkably similar to the air-temperature changes recorded in Greenland ice cores. Even more remarkably, we found that the most abrupt transitions in our record spanned fewer than 40 years of sedimentation. As polar Foraminifera live only in waters colder than about 10 |degrees~ C and constitute more than 95 percent of the population in waters colder than about 5 |degrees~ C, the largest changes in Foraminifera correspond to temperature shifts of 5 |degrees~ C or more. Using these estimates along with the dating, we calculated rates of sea-surface temperature change in excess of 1 |degree~ C per decade! These phenomenal rates of change are similar to those recorded in the ice cores and, now, in models of ocean circulation played out on supercomputers.

Why Does the Great Ocean Conveyor Change Over Time?

Early in his effort to understand the machinations of abrupt climate change, Broecker noted that the salt content of Gulf Stream waters was critical to the sinking and deep-water formation that drives the conveyor. Perhaps excess freshwater runoff (from increased precipitation, decreased evaporation, or melting of snow and ice) might reduce surface-water salt content to levels below those compatible with sinking, turning off the conveyor and the associated northward flow of heat. Following his lead, ocean modelers found that once they were able to simulate deep-water formation on their computers, only very slight freshening of the surface brought the conveyor to a halt. The models are likely to be more sensitive than the real ocean; left unattended they run too fresh and tend not to form deep water (so much so that at an international meeting, Broecker presented one of the leading modelers with a household salt shaker!). Nevertheless, the models point out that fresh water may have a powerful influence on the conveyor's vitality.

Of the various limbs of the ocean conveyor, the Norwegian Sea limb is most effective in warming the atmosphere as surface waters undergo a larger degree of cooling (heat release) prior to sinking. It may also be the most vulnerable. Our studies show that as the Last Ice Age was coming to a close and continental ice caps were melting, the Norwegian limb of the conveyor was periodically shut down, leading to sudden shifts in sea and air temperatures. The open-ocean limb appears to have survived in some form even during ice ages. One possible explanation for this difference is that runoff is concentrated in the restricted basin of the Norwegian Sea rather than being mixed by the currents of the open Atlantic Ocean.

Our results provide direct evidence that changes in the Great Ocean Conveyor governed air temperatures around the Atlantic region. The magnitude of these changes was some three- to five-times greater than experienced during the Little Ice Age (generally considered to be 1450 to 1890), a time during which those who were not starving enjoyed good skating on the Thames and ice fishing in Scotland. We marvel at the development of human civilization during the climatically quiet times of the last eight millennia, and we take for granted the constancy of the oceans. But our results indicate that there have been times when the ocean dealt hard. It will be interesting, and possibly scary, to see how the ocean will respond to the new "greenhouse world." Such changes in atmospheric chemistry have not been witnessed since the end of the Last Ice Age.

In response to ineluctable demographic forces, Scott Lehman was conceived not far from New York in an atmosphere of post-war euphoria. Life was comfortable but boring in suburbia. Following a somewhat-delayed pubescence, the growing tide of testosterone was sublimated in the mountains (which even his parents admitted was more constructive than burning down ROTC buildings). Climbing and a few other minor personality flaws have confounded his academic and personal life ever since. He is currently an Assistant Scientist in the Geology and Geophysics Department at Woods Hole Oceanographic Institution (WHOI).

Lloyd Keigwin is an Associate Scientist in the Geology and Geophysics Department at WHOI, and a sometime naval officer. With undergraduate training at Brown University and graduate degrees from the University of Rhode Island, he grew up by the sea and likes chowder--two things that should never be admitted to when applying to graduate school in oceanography.

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