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An early map of the extent of Lake Agassiz (by 19th century geologist Warren Upham). This map is now believed to underestimate the extent of the region once overlain by Lake Agassiz.

Lake Agassiz was an immense glacial lake located in the center of North America. Fed by glacial runoff at the end of the last glacial period, its area was larger than all of the modern Great Lakes combined, and it held more water than contained by all lakes in the world today.[1]

Contents

Conception

First postulated in 1823 by William Keating, it was named after Louis Agassiz in 1879 after he was the first to realize it was formed by glacial action.

Geological progression

Geologists have come to a consensus on the likely geological history of Lake Agassiz.

During the last Ice Age, northern North America was covered by a glacier, which alternately advanced and deteriorated with variations in the climate. This continental ice sheet formed during the period now known as the Wisconsin glaciation, and covered much of central North America between 30,000 and 10,000 years ago. As the ice sheet disintegrated, it created at its front an immense proglacial lake, formed from its meltwaters.[2]

Around 13,000 calendar years before present (almost 12,000 14C years before present), the lake came to cover much of Manitoba, western Ontario, northern Minnesota, eastern North Dakota, and Saskatchewan. At its greatest extent, it may have covered as much as 440,000 square kilometers, larger than any currently existing lake in the world (including the Caspian Sea).

The lake drained at various times south through the Traverse Gap into Glacial River Warren (parent to the Minnesota River, a tributary of the Mississippi River),[3] east through Lake Kelvin (modern Lake Nipigon) to what is now Lake Superior,[4] or west via the Mackenzie River through the Yukon Territory and Alaska.[1] Geologists believe that a major outbreak of Lake Agassiz about 13,000 BP drained through the Great Lakes and Saint Lawrence River into the Atlantic Ocean. A return of the ice for some time offered a reprieve, but after retreating north of the Canadian border about 9,900 years ago, Lake Agassiz refilled. The last major shift in drainage occurred about 8,400 calendar years before present (about 7,700 14C years before present). The melting of remaining Hudson Bay ice caused lake Agassiz to drain nearly completely. This final drainage of Lake Agassiz contributed an estimated 1 to 3 meters to total post-glacial global sea level rise. Much of the final drainage may have occurred in a very short time, in two or one events, perhaps taking as short as a year.

Lake Agassiz' major drainage reorganization events were of such magnitudes that they had significant impact on climate, sea level and possibly early human civilization. Major freshwater release into the North Atlantic is considered to disrupt oceanic circulation and cause temporary cooling. The draining at 13,000 may be the cause of the Younger Dryas stadial[1][5]. The draining at 8,400 may be the cause of the 8,200 yr climate event. A recent study by Turney and Brown links the 8,400 drainage to the expansion of agriculture from east to west across Europe; he suggests that this may also account for various flood myths of prehistoric cultures, including the Biblical flood.[6].

Remnants and effects

Lake Winnipeg, Lake Winnipegosis, Lake Manitoba, and Lake of the Woods, among others, are relics of the ancient lake. The outlines and volumes of these modern lakes are still slowly changing due to differential isostatic rebound.

Other geological and geomorphological evidence for Lake Agassiz can also be seen today. Raised beaches, many kilometers from any water, mark the former boundaries of the lake at various times. Several modern river valleys, including those of the Assiniboine River and the Minnesota River, were originally cut by water entering or leaving the lake. The fertile soils of the Red River Valley, now drained by the Red River of the North, are formed from lacustrine deposits of silt from Lake Agassiz.

See also

References

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Notes

  1. ^ a b c Perkins (2002)
  2. ^ Ojakangas & Matsch (1982), pp. 106-10. The retreat of glacial margins is not caused by a reversal of the glacier's flow, but rather from melting of the ice sheet. Id.
  3. ^ Fisher (2003), pp. 271–72
  4. ^ Leverington (2003)
  5. ^ Broecker (2006)
  6. ^ Turney (2007)

Sources

External links

Coordinates: 51°N 98°W / 51°N 98°W / 51; -98


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