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.

The entire conveyor works to distribute heat from the equator toward the poles, thus moderating global climate. So while the notion of a diminished ice-age Gulf Stream makes sense, it has never been independently demonstrated -- until now.

In the Dec. 9th issue of Nature, Lamont-Doherty Earth Observatory geochemist Jean Lynch-Stieglitz presents evidence that the Gulf Stream, which originates in the Gulf of Mexico and wraps around Florida to head northward, operated at about two-thirds its current rate 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, 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 deep water in the North Atlantic all but stopped during the last ice age, the question remained as to 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.

"Without compensatory return flow in surface currents like the Gulf Stream, the conveyor had to be weak or shut down altogether, regardless of whether it was shallower than today," she said. "Of course, we want to corroborate this by applying the technique elsewhere, but it is now valid to ask, 'how does a weak conveyor contribute to or result from glacial climates?'"