The catastrophic draining of two gigantic glacial lakes in Canada's Hudson Bay region some 8,200 years ago appears to have caused the most abrupt, widespread cold spell on Earth during the last 10,000 years, according to a group of scientists.
Don Barber, a geological sciences doctoral student at the University of Colorado at Boulder, said the lakes, Agassiz and Ojibway, contained more water than all of the Great Lakes combined. Barber and his colleagues estimated that when an ice dam from a remnant of the Laurentide Ice Sheet collapsed, the flow of lake water rushing through the Hudson Strait and into the Labrador Sea was about 15 times greater than the present discharge of the Amazon River.
The fresh water probably gushed into the Labrador Sea in the North Atlantic for about a year, reducing sea-surface salinity and altering ocean circulation patterns at the time, said Barber, also a researcher at CU-Boulder's Institute of Arctic and Alpine Research. Ocean circulation models suggest massive influxes of freshwater can disrupt heat transport in currents flowing from the tropics to temperate regions.
Ice core data taken by scientists in Greenland show temperatures dropped by as much as 15 degrees Fahrenheit in central Greenland and by nearly 6 degrees F in Western Europe following the catastrophic lake drainage. "This was the coldest climate event in the last 10,000 years," Barber said.
A paper on the subject by Barber, published in the July 22 issue of Nature, was co-authored by INSTAAR's John Andrews, Anne Jennings, Mike Kerwin and Mark Morehead. Other co-authors include John Southon of the Lawrence Livermore National Lab in Livermore, Calif., Art Dyke and Roger McNeely of the Geological Survey of Canada, Claude Hillaire-Marcel and Guy Bilodeau of the University of Quebec and Jean-Marc Gagnon of the Canadian Museum of Nature.
The surface currents of the Atlantic act much like conveyor belts, carrying salty, warm water from the tropics to the temperate regions. The water cools in the temperate North Atlantic, then becomes dense enough to sink and send heat into the atmosphere, said Barber.
Under normal conditions, winds blowing from the west across the Atlantic send air warmed by the sea toward Western Europe, said Barber. About one-third of the heat that warms Western Europe is delivered by the ocean, while the other two-thirds comes from the sun, he said.
Although southern Greenland and northern Canada are at about the same latitude as Sweden and Norway, Greenland is almost uninhabitable because of its colder temperatures and lack of viable agricultural land for crops and livestock.
But if an enormous amount of freshwater is suddenly infused into the temperate Atlantic waters as it apparently was 8,200 years ago, Western Europeans could suffer severely. The Laurentide lakes drainage seems to have halted the sinking of surface waters in the Labrador Sea, temporarily crippling the water conveyor belt and causing the Western European cold snap to last for about 200 to 400 years, according to ice-core data.
"If the scenarios of extreme global warming in the future come true, it could lead to significant melting of the Greenland Ice Sheet and create more precipitation at high latitudes," said CU-Boulder geological sciences Professor John Andrews.
"Adding very large amounts of freshwater to large rivers could conceivably close down the vertical circulation system in the North Atlantic, leading to another extreme cooling event."
Evidence for the catastrophic Laurentide lakes drainage comes from "red bed" sediments underlying the ancient glacial lakes that were carried some 800 miles through the Hudson Strait by the massive freshwater plume, said Barber. In addition, fossil clams from the Labrador seabed corresponding to the freshwater flood were radiocarbon-dated to about 8,200 years ago.
In addition, oxygen isotopes from the shells of tiny, plankton-like organisms from the same age of sediments showed the creatures lived in less salty water about 8,200 years ago, indicating the Laurentide lake drainage made the Labrador Sea significantly fresher.
At its peak about 20,000 years ago, the Laurentide Ice Sheet that covered much of North America dipped down south as far as Ohio, and the ice is estimated to have been a mile deep at present-day Detroit, said Barber. At the time of glacial lake draining, the Laurentide Ice Sheet probably had retreated by about 80 percent.
Journal
Nature