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Science 21 April 2006:
Vol. 312. no. 5772, p. 350
DOI: 10.1126/science.312.5772.350a
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News of the Week

GEOPHYSICS:
Opening the Door to a Chilly New Climate Regime

Richard A. Kerr

Earth's early Eocene epoch 50 million years ago was a paradise for warmth-loving life. Back then, alligators basked in the high Arctic on Canada's Ellesmere Island. Today, for better or worse, they cannot venture farther north than the U.S. Deep South. Why did the planet cool so much?

Many paleoclimatologists suspect at least part of the answer lies in the way the supercontinent Gondwanaland fell apart. As the fragments that became South America and Antarctica dispersed, they opened the way for a climate-making ocean current that now encircles Antarctica. By cutting Antarctica off from warm currents flowing from the tropics, the Arctic Circumferential Current (ACC) could have helped bring on the continent's massive, permanent glaciation, with worldwide consequences. On page 428, paleoceanographers report new evidence that the oceanic Drake Passage between the two continents began opening--and changing climate--twice as long ago as once thought.

Figure 1 Locked tight. Analyses of fossil fish teeth (inset) show that Drake Passage began opening 41 million years ago.

CREDITS: (MAP) ADAPTED FROM H. D. SCHER; (PHOTO) H. D. SCHER

Paleoceanographers Howie D. Scher of the University of Rochester in New York and Ellen Martin of the University of Florida, Gainesville, found the clues in fossil fish teeth recovered from sediment cores from the far South Atlantic Ocean. Fish teeth, researchers have shown, absorb the rare-earth element neodymium from seawater shortly after they settle to the bottom. But the proportion of two neodymium isotopes in Pacific seawater differs from that in Atlantic seawater, because rivers carry differing isotopic ratios from the rock surrounding the two ocean basins. A varying isotopic ratio in the Atlantic is a sign that Pacific water has managed to mix into the Atlantic.

Some marine geologists, judging the size of the growing gateway by the crustal record of drifting continents, have argued that Drake Passage did not reach its modern depth and breadth until 20 million years ago. That's the earliest that the ponderous, wind-driven ACC could have first encircled the continent, they say.

Scher and Martin, however, found isotopic traces of Pacific water leaking through Drake Passage beginning about 41 million years ago. That was the time of a short-lived glacial advance that other paleoceanographers recently discovered, they note. Flow surged again at the time of the first substantial, long-lasting glaciation of Antarctica, 34 million years ago. That step, the researchers say, could have resulted from the simultaneous opening of the Tasmanian Gateway upstream. Opening that gateway would have allowed more water into the already-deepening Drake Passage and then the Atlantic.

Paleoceanographer James Kennett of the University of California, Santa Barbara, who suggested the gateway-opening hypothesis of climate change 30 years ago, says the early opening in the neodymium record doesn't really contradict the late opening in the crustal record. "I'd prefer to read the [neodymium] record as a more gradual increase in Pacific waters into the Atlantic," he says. "Everything's progressive; it doesn't all happen at once." Twenty million years or more may well have been required to crank up a full-blown ACC, he says, and to help usher in the global chill felt of late.

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Science. ISSN 0036-8075 (print), 1095-9203 (online)