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Vol.25, No. 13 Feb. 4, 2000

Innovative Research Proves Gulf Stream Slowed During Last Ice Age

By Kurt Sternlof

Scientists have long speculated that the Gulf Stream slows during glacial periods. An important component of the global oceanic "conveyor," the Gulf Stream carries tropical warmth up the eastern seaboard of the United States to the North Atlantic and Europe. Without its warming influence, a summer swim off the coast of Long Island would resemble a dip from an iceberg, and the perennial winter rains of London would likely all be snow.

This conveyor also moderates global climate. So while the notion of a diminished ice-age Gulf Stream makes sense, it has never been independently demonstrated—until now.

Lamont-Doherty Earth Observatory geochemist Jean Lynch-Stieglitz has found evidence that the Gulf Stream—which originates in the Gulf of Mexico and wraps around Florida to head northward—ran at about two-thirds of its present speed during the height of the last ice age. This in turn suggests that the entire oceanic conveyor also slowed, and so moved less heat into the icy upper latitudes.

In order to make this discovery, which was published in the Dec. 9 issue of Nature, Lynch-Stieglitz and her colleagues combined two established techniques of modern oceanography, one geochemical the other physical, in reconstructing the Gulf Stream as it flowed through the Straits of Florida more than 10,000 years ago.

"I think Nature accepted this article as much for the novelty of our methodology, as the importance of our result," she said. "As far as I know, this is the first time that oxygen isotope data from sedimentary deposits of foraminifera have been used to quantify the flow rate of an ancient ocean current."

Foraminifera are tiny, amoeba-like creatures that leave behind a record of ocean chemistry in the calcareous shells they construct from the seawater around them. Isotopes are atoms of a given element, like oxygen, with different numbers of neutrons in their nuclei.

The first step of the process involved measuring the ratio of oxygen isotopes recorded by foraminifera across the straits during the peak of the last ice age. These ratios are known to vary with the temperature and salinity of the water in which the creatures lived. The density of the water, in turn, increases with greater salinity and lower temperature. Working backwards from the isotope ratios, Lynch-Stieglitz was thus able to reconstruct a profile of the density contrast across the glacial-era Gulf Stream near Florida.

The second step consisted of feeding this water density data into a mathematical model that calculates density-driven current flow. Known generally as the geostrophic method, this technique is widely used to calculate the rate of modern current flow using direct water-density measurements.

"This is an elegant and powerful new tool that can and will be applied to reconstructing ancient shallow ocean currents in many places around the world," said Wallace Broecker, senior Lamont-Doherty geochemist and a founding father of the conveyor model of ocean circulation. "It's an entirely new way of using oxygen data that provides real flow-rate numbers, not just qualitative interpretations."

The elegance of the new technique aside, Lynch-Stieglitz's finding that the Gulf Stream was a third less vigorous during the last ice age is important in its own right, Broecker said.

The ocean conveyor belt operating today is driven by the sinking of dense water in the polar regions. These deep-water currents flow toward the equator, pulling warm surface water back toward the poles in compensation. This overturning conveyor mechanism mixes the oceans while distributing heat and moderating climate.

In trying to work out the complex relationship between patterns of ocean circulation and climate, it is extremely important to understand what happens during glacial periods—whether the conveyor slows, shuts down or even changes configuration. While other research has shown that the formation of deep water in the North Atlantic all but stopped during the last ice age, the question remained of whether the entire conveyor stopped or simply became shallower.

"Jean's work suggests that conveyor circulation virtually stopped during the last glacial maximum," Broecker said.

The Gulf Stream as it passes Florida today is powered approximately two-thirds by prevailing winds and one-third by conveyor circulation, Lynch-Stieglitz explained. During the last ice age, the winds—driven by equator-to-pole temperature differences—would likely have been stronger, if anything. Thus the one-third drop in the strength of the Gulf Stream can be directly attributed to a cessation in conveyor circulation.