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In-Depth Information
the younger dryas cooling event
Just before the Pleistocene ended, about 11,000 years ago, the steady
shrinking of lakes in the Great Basin briel y halted and reversed for sev-
eral hundred years. Lake Bonneville expanded during this period, creating
the so-called “Gilbert Shoreline.” h is short-lived return to greater mois-
ture in the American West coincided with a brief advance of glaciers in
Europe, during what has come to be known as the Younger Dryas event (see
i gure 16B). Evidence from Greenland ice cores suggests it lasted about 1,300
years in the North Atlantic region, though its duration appears signii cantly
shorter in the American West. In the broad timescales of climate, this return
to glacial conditions was so brief that paleoclimatologists think of it as an
“event,” though one that evidence suggests may have been hemispheric, if
not global.
One hypothesis explaining the cause of the Younger Dryas relates to
another great Pleistocene lake. At its peak, the area of Lake Agassiz was larger
than all the Great Lakes combined. Named at er Louis Agassiz—the father
of glacial geology—this lake owed its massive size to an ice dam that backed
up the glacial meltwater as the Pleistocene ice age was ending. h is enormous
volume of water was abruptly and catastrophically released at er the ice dam
collapsed, with the resulting wave l owing into Hudson Bay, through the
St. Lawrence Seaway, and into the North Atlantic, a critical location for
deep ocean circulation. In this region, salty Gulf Stream waters, which have
been transported northward, are cooled, becoming dense enough to sink to
great depths, forming deepwater that l ows slowly southward. But when the
rush of fresh lake waters burst out into the North Atlantic, they would have
l oated on the surface (freshwater being less dense than saltwater, therefore
more buoyant), preventing the sinking of cold, salty water that forms the
deepwater. h is, in turn, slowed the normal northward l ow of warm, salty
Gulf Stream water to the region.
Today, this warm current heats the atmosphere above it along the coast-
lines of the continents surrounding the North Atlantic. When the current
was slowed, or even stopped altogether, the result would have been a dra-
matic cooling. h ese changes in atmospheric and oceanic circulation that
occurred during the Younger Dryas appear to have af ected regions as far
away as California and the American West, where the Great Basin lakes once
again grew large.
