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. …