Paleoclimatologists trying to understand how the world drifted into the last ice age 70,000 years ago are getting new clues from isotopes preserved in deep-sea sediments. Based on this information, scientists argue that climate changed before ocean circulation did, ruling out a slowing of heat-carrying ocean currents as the immediate cause of glaciation.
The results are based on studies of the rare-earth element neodymium. Researchers at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, found that the ratio of neodymium-143 to neodymium-144 isotopes in North Atlantic and Pacific waters differs enough, thanks to the range of ratios of surrounding continental rocks, that it can be used to follow the mixing of waters as currents flow from basin to basin.
Ocean circulation changes should dominate the changes in the neodymium ratio, say Lamont group members headed by Alexander Piotrowski (now a postdoc at the University of Cambridge, U.K.). During each of four temporary warmings during the last ice age, the ratio swung down and then back up--just as it should have done if the warm, north-flowing Gulf Stream had temporarily sped up, delivering more heat to high latitudes and more North Atlantic water to the southward-flowing deep arm of the "conveyor belt" flow.
In the run-up to the ice age, by contrast, the core told a more complicated tale. Starting about 70,000 years ago, bottom waters cooled as glacial ice grew on the polar continents, as indicated by oxygen isotopes of microscopic skeletons of bottom-living organisms. Only several thousand years later did the conveyor belt flow slow down, according to neodymium. Given that millennia-long lag behind the growing cold and ice, "ocean circulation responded to climate change," not the other way around, says Steven Goldstein, also part of the Lamont team.
"It's groundbreaking work," says paleoceanographer Christopher Charles of the Scripps Institution of Oceanography at the University of California, San Diego. But a single core is not likely to win the day, says paleoceanographer Jerry F. McManus of Woods Hole Oceanographic Institution in Massachusetts, who notes that he and others will be looking for weaknesses in the theory.