Catastrophic flood: A NASA satellite image of the great lakes of North America that were gouged out by the retreat of an ancient ice sheet starting 10,000 years ago. Hudson Bay is shown to the top right.

Credit: SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE

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PARIS: Canadian geologists have shed light on how a vast lake, trapped under the ice sheet that once smothered North America, drained into the sea – an event that cooled Earth's climate for hundreds of years.

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 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.

Massive drainage

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.

By some estimates, sea levels rose 14 m as a result. How the great flood was unleashed has been a matter of debate. Some experts suggest an ice dam was smashed down, or the gushing water spewed out over the top of the icy lid.

Now, researchers Patrick Lajeunesse on the University of Laval and Guillaume Saint-Onge believe of the University of Quebec in Rimouski, have found evidence that the outburst happened under the ice sheet, rather than above it or through it.

In a study in the journal Nature Geoscience, the pair describe how they criss-crossed more than 10,500 km of the Hudson Bay on a research vessel, using sonar to scan the seafloor.

In the south of the bay, they found lines of deep waves in the sandy bed, stretching more than 900 km in length and some 1.7 m deep. These are signs that the bay's floor, protected by the mighty lid of ice, was swept by a mighty current many years ago but has been still ever since, they say.

Curious marks and channels

In the west of the bay, they found curious marks in the shape of parabolas twisting around to the northeast. The arcs were chiselled as much as three metres into the seabed and found at depths of between 80 to 205 metres.

The duo believes that this part of the bay had icebergs that were swept by the massive current. The bergs' jagged tips were trapped in the seabed and acted like a pivot. As the icebergs swung around, other protruding tips ripped arc-like tracks on the bay floor.

Also presented as evidence are deep submarine channels and deposits of red sediment that stretch from land west of Hudson Bay right across the northwestern floor of the bay itself – both point to a current that swept all before it. "Laurentide ice was lifted buoyantly, enabling the flood to traverse southern Hudson Bay under the ice sheet," the study authors write.

Previous work reported that the flood was so huge that it affected world climate. 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 to 400 years, creating a mini-Ice Age.