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Plankton chronicles icy rerun.

Plankton chronicles icy rerun

Sometime after 16,000 years ago, the great ice sheets that covered much of North America and Europe began to retreat, signaling the gradual return of warmer temperatures. Then around 11,000 years ago, a cold spell hit Northern Europe, Greenland and the Atlantic coast of Canada, sending these regions back into near glacial conditions for a geologically brief interlude of 700 years, called the Younger Dryas period. An oceanographer reports that ancient plankton from the Gulf of Maine can help trace the cause of this cold spell.

Many scientists believe the Younger Dryas cooling resulted from a radical redirection of meltwater flowing from the North American ice sheet. At first, most water from the melting ice drained into the Gulf of Mexico, via the Mississippi. Then between 12,000 and 11,000 years ago, most of this water detoured down the St. lawrence and into the Atlantic. According to theory, the influx of cold, fresh water into the North Atlantic would have rerouted ocean circulation and pushed the Gulf stream south, thereby cooling land around the North Atlantic. This theory explains why geologic records from the interior of the United States do not show any cooling.

The Younger Dryas, however, did leave its mark in the Gulf of Maine. In particular, the history of a certain plankton species supports the idea that water from the St. Lawrence suddenly cooled the North Atlantic, according to Detmar Schnitker, from the University of Maine's I.C. Darling Marine Center in Walpole. Called Thalassiosira gravida, this species lives along ice edges. Sediment cores show that Th. gravida thrived when the North American ice sheet first started to retreat from the Gulf, but the plants disappeared from this area as the water warmed. Then, about 11,000 years ago, Th. gravida bloomed again, signaling that ice had returned. Since low-salinity water freezes more easily than salty water, this plankton pattern suggests fresh water from the St. Lawrence flooded the nearby Gulf of Maine at this time, Schnitker says.
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Title Annotation:Earth Sciences; Younger Dryas period
Author:Monastersky, Richard
Publication:Science News
Date:May 27, 1989
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