Hudson Bay gouges shed light on flood

Quebec researchers Patrick Lajeunesse and Guillaume St-Onge have reconstructed the "catastrophic outburst" of a vast meltwater lake at the end of the last ice age, an event already known to have disrupted global climate and which, according to some theorists, triggered flooding around the world -- perhaps even inspiring the biblical story of Noah's Ark.

The sudden, massive discharge of freshwater from prehistoric Lake Agassiz -- which at its greatest extent reached from Quebec to Saskatchewan -- has long been blamed for kick-starting a period of global cooling and raising sea levels around the world by at least a metre in a matter of months.

The lake, which had a maximum volume equivalent to 15 Lake Superiors, is generally thought to have drained abruptly into the North Atlantic Ocean when an ice dam around Hudson Bay finally collapsed as warming temperatures brought an end to the ice age.

But the new study by Lajeunesse, a Laval University geoscientist, and the University of Quebec's St-Onge, says an array of arc-shaped "scours" at the bottom of Hudson Bay, created when icebergs were driven along the seabed by water rushing out of Lake Agassiz into the ocean, shows the final outburst occurred below the glacier rather than at the surface.

The glacier, known as the Laurentide Ice Sheet, was under such pressure from the gushing lakewater that it was actually "lifted buoyantly" by the massive outflow, "enabling the flood to traverse southern Hudson Bay under the ice sheet," the scientists conclude in a paper published online at Nature Geoscience.

The scientists used sonar images to map the seabed scours, ancient channels and sand features believed to have been created by two distinct pulses of Agassiz flood water.

The existence of the super-size Agassiz, named for a leading 19th-century geologist, has been known since the late 1800s. Formed some 12,000 years ago from glacial meltwater, the lake was encircled by beaches still visible today as sandy ridges throughout Central and Western Canada.

Lake Winnipeg is considered a modern-day remnant of the Agassiz superlake.

Initially centred around the present Ontario-Manitoba border, Lake Agassiz formed, at its greatest extent, a 1.5 million-square-kilometre freshwater basin larger than the combined areas of Saskatchewan and Manitoba.

University of Manitoba geologist Jim Teller's initial reconstruction of the lake's dying throes initiated a worldwide wave of research into what was certainly one of the most awesome natural events in Canadian prehistory.

Teller has also suggested that rising seas caused by the Agassiz flood may have sent water rushing into the Persian Gulf basin, giving rise to Sumerian and Mesopotamian flood legends that were set down in writing about 5,000 years ago.

The most famous of those stories is the one many scholars believe is the basis of the biblical flood: The Epic of Gilgamesh, a 4,500-year-old clay tablet chronicle unearthed a century ago by archeologists near the city of Ur, at the north end of the Persian Gulf.

-- Canwest News Service

Republished from the Winnipeg Free Press print edition February 25, 2008 $sourceSection$sourcePage