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9 December 2007
Ancient flood triggered 'Big Chill,' study says

An epic gush of fresh water into the North Atlantic slowed a deep ocean current and triggered a century-long chill in Europe and North America some 8,200 years ago, according to a new study. The finding confirms scenarios suggested by previous models of the ancient climate and should raise confidence in predictions made about how the oceans will respond to Greenland's rapidly melting glaciers, an outside expert said.
    Scientists suspect the sudden draining of North America's ancient glacial lake Agassiz—which was seven times larger than all of the Great Lakes combined—caused a well-studied cold snap about 8,200 years ago. But evidence that the deep-water current slowed was lacking. Now, a team of European scientists has found the evidence in the contents of a 39-foot (12-meter) plug of seabed mud pulled from some 11,200 feet (3,400 meters) deep in the northwest Atlantic. "We show that there's a sudden disruption in the deep circulation which takes place just at the time of the flood outburst," said Helga Kleiven, a paleoclimate expert at the Bjerknes Center for Climate Research in Bergen, Norway. She and her colleagues reported the finding in latest issue of the journal Science.
    Kleiven and her colleagues drilled the core off the southern tip of Greenland, where sediment-rich deep waters slow down and deposit their loads. "The sedimentation rate is 10 to 15 times higher at these drift sites than they are in the rest of the North Atlantic," Kleiven said. The researchers identified a section of the core that corresponds to a hundred-year period around 8,200 years ago. The chemistry of the sediment there is unlike that from any other time over the past 10,000 years, Kleiven said. The sediment grains in this section are also much smaller, suggesting the larger, heavier grains had already fallen out of slower-moving waters or were never picked up. In addition, oxygen isotopes in the shells of microscopic bugs found in this section suggest the surface water temperature was markedly colder.
     Richard Alley is a glaciologist at Pennsylvania State University who originally proposed in 1997 that changes in ocean circulation could have triggered the cooling that occurred 8,200 years ago.
He said the core preserves the short-term record beautifully and corresponds well with the climate record detected in ice cores pulled from Greenland. The new findings suggest that the changes in the ocean circulation pattern and cooling of the ocean surface happened over the course of a few decades at most, Kleiven noted. "The response we see in these deep-ocean changes [is that] they occur on timescales which are rapid enough [that] they could impact human societies," she said.
     While no immediate freshwater supply the size of lake Agassiz exists today, Greenland's rapidly melting ice sheets could potentially slow the deepwater current and affect global weather patterns.

Source: National Geographic News (6 December 2007)

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