Monday, February 25, 2008

When The Ice Levee Breaks

The subglacial origin of the Lake Agassiz–Ojibway final outburst flood. 2008. P. Lajeunesse and G. St-Onge. Nature Geoscience, published online 24 February.


During the last ice age, the Laurentide Ice Sheet once covered most of Canada and parts of the northern United States with a frozen crust that in some places was three kilometres (two miles) thick. As the temperature gradually rose some 10,000 years ago, the ice receded, gouging out the hollows that would be called the Great Lakes.

Beneath the ice's thinning surface, an extraordinary mass of water built up -- the glacial lake Agassiz-Ojibway, a body so vast that it covered parts of Manitoba, Saskatchewan, North Dakota, Ontario and Minnesota.

And then, around 8,200 years ago, Agassiz-Ojibway massively drained, sending a flow of water into the Hudson Strait and into the Labrador Sea that was 15 times greater than the present discharge of the Amazon River.

The influx of freshwater into the North Atlantic reduced ocean salinity so much that this braked the transport of heat flowing from the tropics to temperate regions. Temperatures dropped by more than three degrees Celsius in Western Europe for 200-400 years -- a mini-Ice Age in itself.

Abstarct: Deglaciation of North America resulted in the development of the ice-dammed lake Agassiz–Ojibway along the southern margin of the Laurentide Ice Sheet and its catastrophic northward drainage 8.47 kyr ago. This sudden outburst of fresh water may have weakened the Atlantic ocean overturning circulation and triggered the cold event that occurred 8.2 kyr ago.

Geological evidence of this flood has been documented in a red sedimentary bed in cores collected in Hudson Strait and by submarine features in Hudson Bay. However, there have been few constraints on the manner in which the lake drained: for example, by flow over the ice sheet or beneath it, in one or several pulses and where the flood routes were located.

Here we present seafloor images obtained using multibeam sonar, which reveal that the outburst flood displaced icebergs to produce arcuate (arc-shaped) scours on the seafloor with a dominant east-northeast–west-southwest orientation. The flood also produced sandwaves in areas unaffected by the arcuate scours, indicating they were protected from iceberg scouring by overlying ice during the event. We suggest that these sandwaves, along with submarine channels inferred from the data, indicate that Laurentide ice was lifted buoyantly, enabling the flood to traverse southern Hudson Bay under the ice sheet.