Deglaciation of North America

Abstract

At the Last Glacial Maximum (LGM), the North American ice sheet complex consisted of three major ice sheets: the Laurentide Ice Sheet, which was centred on the Canadian Shield but also expanded across the Interior Plains to the west and south; the Cordilleran Ice Sheet, which inundated the western mountain belt between the northernmost co-terminus United States and Beringia (unglaciated Yukon Territory and Alaska); and the Innuitian Ice Sheet, which covered most of the Canadian Arctic Archipelago north of about 75°N latitude. The ice cover over Newfoundland and the Maritime Provinces of Canada during this interval is usually referred to as the Appalachian Ice Complex, because ice flowed out from local centres within the region rather than from the Canadian Shield. All of the peripheral ice sheets were broadly confluent with the Laurentide Ice Sheet at the LGM, and the Greenland Ice Sheet was confluent with the Innuitian Ice Sheet. The North American ice sheet complex was by far the largest of former Late Pleistocene ice sheets, with an area of about 15 million square kilometres (17.4 million, including Greenland ice). Its growth and decay thus account for more than half, possibly as much as three quarters, of global sea-level change during the last glacial cycle. Its presence profoundly affected the global climate system and the isostatic deformation of the planet.

The nucleus of this complex, the Laurentide, comprised three major sectors, the Labrador Sector, the Keewatin Sector, and the Baffin Sector. These sectors are named for areas of ice sheet inception and probable areas of outflow at LGM. They were located respectively east, west, and north of Hudson Bay. Continental scale syntheses of ice recession following LGM are given by Prest (1969), Bryson et al. (1969), Denton & Hughes (1981), Dyke & Prest (1987), and Dyke et al. (2003). The last is the source of the maps on this web site.

Deglaciation history is fairly well known because it is based on mapped glacial landforms, such as end moraines and drumlins and it is constrained by radiocarbon dates. The broad patterns have been known for decades from large-scale compilations such as the Glacial Map of Canada (Prest et al., 1968; Dyke & Prest, 1987) and the Glacial Map of the United States East of the Rockies (Flint et al., 1959). The fundamental assumption in interpreting these patterns, common to the present and previous efforts, is that where moraines or other ice marginal features do not record specific ice margins, the margins trended normal to ice flow directions. This common assumption accounts for the fundamentally similar patterns seen in most reconstructions of deglaciation, specifically recession back to the three main centres of Keewatin, Quebec-Labrador, and Baffin Island.

Most direct dates on ice marginal positions are from sites where moraines or meltwater features have been followed into shell-bearing marine deposits, such as commonly occur in ice-contact deltas. Opportunities to directly date ice marginal features landward of the limit of postglacial marine incursion are exceedingly rare, being limited to those few localities where ice readvanced across living or recently dead vegetation or where the varve chronologies of proglacial lakes can be tied into the radiocarbon time scale. The remaining age control is entirely in the form of minimum-limiting dates on lake sediments, peat, wood, plant macrofossils, foraminifera, and mammal bones. Reservoir corrections are applied to dates on foraminifera as they are to marine mollusks.

Recent improvements in age control and more detailed mapping of deglacial patterns have brought the North American deglaciation sequence into more evident correlation with the major climatic events recognized in the North Atlantic region and in the Greenland ice cores. Relatively little recession occurred prior to 14 000 years BP except in deep water on the continental shelf off New England and Atlantic Canada. Indeed, the southwestern margin of the Cordilleran Ice Sheet continued to advance through the Puget Lowland of Washington State until 14 500 years BP. The acceleration of retreat at 14 000 years BP corresponds to the sudden warming evident in the Summit, Greenland record. The Younger Dryas cooling (11 000 to 10 000 years ago) exerted an important control of ice marginal behaviour of the North American ice sheets, similar to that shown by the Scandinavian Ice Sheet. The prominent 8200 calendar years BP cold event (7500 radiocarbon years BP) is now firmly correlated with the deglaciation of Hudson Bay and drainage of glacial lakes Agassiz and Ojibway (Barber et al., 1999).

The world's largest ice sheet complex lost only <10% of its area prior to 14 000 years BP. It then retreated nearly linearly until 7000 years BP, by which time only 10% of the area remained more glaciated than today. Only two events perturbed the linear reduction of ice area: reduced recession during the Younger Dryas interval, and accelerated recession during the rapid opening of Hudson Bay.

References

Barber, D.C., Dyke, A.S., Hillaire-Marcel, C., Jennings, A.E., Andrews,
J.T., Kerwin, M.W., Bilodeau, G., McNeely, R., Southon, J., Morehead, M.D. &
Gagnon, J-M. (1999). Forcing of the cold event of 8,200 years ago by
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Denton, G.H. & Hughes, T.J. (1981). The Last Great Ice Sheets.
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Dyke, A.S. & Prest, V.K. (1987a). The Late Wisconsinan and
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Dyke, A.S., Moore, A. & Robertson, L. (2003). Deglaciation
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maps at 1:7 000 000 scale with accompanying digital chronological
database and one poster (two sheets) with full map series.

Flint, R.F., Colton, R.B., Goldthwait, R.P. & Wilman, H.B.
(1959). Glacial Map of the United States east of the Rocky Mountains. Geological
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Prest, V.K. (1969). Retreat of Wisconsin and Recent ice in
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Prest, V.K., Grant, D.R. & Rampton, V. (1968). Glacial Map
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