How it happened: The catastrophic flood that cooled the Earth

PARIS (AFP) - Canadian geologists say they can shed light on how a vast lake, trapped under the ice sheet that once smothered much of 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 (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.

By some estimates, sea levels rose 14 metres (45 feet) 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.

Quebec researchers Patrick Lajeunesse and Guillaume Saint-Onge believe, though, that the outburst happened under the ice sheet, rather than above it or through it.

In a study appearing on Sunday in the journal Nature Geoscience, the pair describe how they criss-crossed Hudson Bay on a research vessel, using sonar to scan more than 10,500 kilometres (6,000 miles) to get a picture of the bay floor.

In the south of the bay, they found lines of deep waves in the sandy bed, stretching more than 900 kilometres (562 miles) in length and some 1.7 metres (5.5 feet) 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.

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 (10 feet) into the sea bed and found at depths of between 80 and 205 metres (260 and 666 feet).

The duo believe that this part of the bay had icebergs that were swept by the massive current.

The bergs' jagged tips were trapped in the sea bed 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 suggests.

Previous work suggests the flood was so huge that it affected climate around the world.

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 (5.4 degrees Fahrenheit) in Western Europe for 200-400 years -- a mini-Ice Age in itself.

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