The results would be catastrophic if it happened today, but it did happen some 13,400 years ago during the retreat of glaciers over North America and may have triggered a brief cooling known as the Intra-Allerod Cold Period.

Assistant Scientist Jeffrey Donnelly of the Woods Hole Oceanographic Institution presented the findings at the American Geophysical Union's fall meeting in San Francisco last Thursday.

Donnelly and colleagues analyzed data from sediment cores, walrus fossils and pollen to precisely date the discharge from Glacial Lake Iroquois down the Hudson River Valley at 13,350 years ago. The flood waters broke through a spot of land where the Verazanno Narrows Bridge now stands to reach the North Atlantic.

The discharge of glacial freshwater into the North Atlantic has long been thought to drive fluctuations in past climate because the huge volume of freshwater would alter thermohaline circulation in the ocean.

Directly linking discharge events with individual climatic changes has been difficult because of the challenges in pinpointing the location, timing and amount of the discharge.

The Intra-Allerod Cold Period lasted only about 150 years and occurred just before the Younger Dryas, a sudden cold climate period lasting some 1,200 years and ending about 11,000 years ago.

Many scientists believe the Younger Dryas was caused by the shutdown of the Gulf Stream in response to a sudden influx of fresh water from deglaciation in North America. Global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic Ocean.

The team compared their evidence for the massive flood down the Hudson Valley with data from sediment cores taken from the Cariaco Basin off Venezuela in the Caribbean, which show a slowing of thermohaline circulation and heat transport into the North Atlantic at that same time.

Donnelly and his colleagues were able to determine the timing of this event by analyzing data from sediment cores from the Hudson River Valley and the continental shelf. Sediment samples collected near the Tappan Zee Bridge indicate that ocean water flooded the lower Hudson Valley just after the flood event occurred.

Pollen data from the first marine sediments deposited near the Holland Tunnel correlate with those from radiocarbon-dated sediments from nearby Sutherland Pond in New York and provide further constraint on the timing of the flood.

Walrus remains recovered from gigantic sediment lobes deposited offshore during the flood were carbon dated to further pinpoint a precise time period.

Large rocks the size of Volkswagens, also associated with these sediment lobes, have been photographed on the outer continental shelf off the mouth of the Hudson River, where sediments normally are the size of grains of sand or smaller. Donnelly says the large rocks most likely came from the melting glacier and were carried down to the Atlantic in the floodwaters.

Glacial Lake Iroquois, in the same location and about three times the size as modern day Lake Ontario, was formed as the Laurentide Ice Sheet receded from its maximum extent along southern Long Island, New York, and northern New Jersey to southern Canada from about 21,000 to 13,000 years ago.

Several other glacial lakes, Glacial Lake Albany and Glacial Lake Vermont, existed for several thousand years and deposited thick layers of silt and clay in the Hudson River Valley and Champlain Lowlands.

Donnelly says a dam north of the Adirondack Mountains in upstate New York holding back the ancient lake collapsed, allowing lake water to drain into the Hudson River Valley and the North Atlantic, dropping the level of Glacial Lake Iroquois some 120 meters (about 400 feet).

Following the collapse of Glacial Lake Iroquois, another lake, Glacial Lake Candona, formed in the Ontario, Saint Lawrence and Champlain Lowlands, controlled in level by a sill or rock dam near Fort Ann, Vermont.

Lake Candona existed only about 100 to 200 years before it drained to the Atlantic when the ice sheet blocking the St. Lawrence Valley collapsed. Following the drainage of Lake Candona, seawater invaded the St. Lawrence and Champlain Lowlands and formed the Laurentian Seaway and the Champlain Sea. Glacial Lake Candona dropped about 40 meters (125 feet) as it drained into the North Atlantic via the Saint Lawrence River Valley.

This opening of the St. Lawrence Valley as a conduit for glacial meltwater about 13,000 years ago likely played a role in causing the onset of the Younger Dryas cold interval.

The team will publish the results of the complete study in the February 2005 issue of the journal Geology. Donnelly's research was funded by the Postdoctoral Scholar Program, The John E. and Anne W. Sawyer Endowed Fund, The J. Lamar Worzel Assistant Scientist Fund, and the Ocean and Climate Change Institute at the Woods Hole Oceanographic Institution.

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