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| Enlarge ImageAn ice dam forming a large Ice Age lake collapsed 13,350 years ago, sending a flood down the Hudson River Valley. The flood of fresh water into the ocean also caused dramatic climate changes. (Illustration by Jack Cook, WHOI) |
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Trapped behind the Adirondack
Mountains and a tremendous ice sheet, glacial Lake Iroquois was three
times the size of modern Lake Ontario. Then, 13,350 years ago, the
natural ice dam collapsed. Floodwaters rushed down the Hudson River
Valley; past modern Manhattan, Brooklyn, and Staten Island; through an
earthen dam where the Verrazanno-Narrows Bridge now stands; and across
another hundred miles into the North Atlantic. The water level in Lake
Iroquois dropped 120 meters (400 feet), and rocks the size of
Volkswagens moved hundreds of miles downstream.
Beyond reshaping the landscape, the catastrophic
flood also had dramatic impacts on Earth’s climate. It may have
triggered a brief but global period of colder climate known as the
Intra-Allerod Cold Period, said Jeff Donnelly, a geologist at Woods
Hole Oceanographic Institution. The torrent from the glacial lake would
have thundered into the North Atlantic, adding a vast freshwater "lid"
on the ocean surface that could have rearranged ocean circulation and
changed climate patterns, said Donnelly, who is a fellow of both the
Ocean and Climate Change Institute and the Coastal Ocean Institute at
WHOI.
Scientists have long suspected that
large discharges of glacial water into the ocean could drive climate
fluctuations, but linking discharge events like the Hudson flood with
individual climate changes has been difficult because of the challenges
in pinpointing the location, timing, and amount of the discharges.
Donnelly
and colleagues analyzed sediments from the Hudson River Valley, which
extends under water on the continental shelf (see map). On huge
sediment lobes on the shelf, where sediments are normally the size of
sand grains, they found car-sized boulders—which were likely pushed
there by the great flood.
The team also
analyzed ancient pollens deposited by the flood in sediments near the
Tappan Zee Bridge and the Holland Tunnel, as well as walrus fossils
buried by the flood in the offshore sediment lobes. The results
precisely dated—for the first time—the discharge from Lake Iroquois and
linked it to the Intra-Allerod Cold Period.
Donnelly and colleagues described the historic flood and its effects in a February 2005 paper in the journal Geology.
—Shelley Dawicki
Funding
for the research was provided by the John E. and Anne W. Sawyer Endowed
Fund, the Office of Naval Research, the WHOI Postdoctoral Scholar
Program, The J. Lamar Worzel Assistant Scientist Fund, and the WHOI
Ocean and Climate Change Institute.
Posted: June 10, 2005 [top] |
