The Wisconsin Laurentide Ice Sheet: Dispersal Centers, Problems of Rates of Retreat, and Climatic Implications

Abstract

Isochrone maps on the late Wisconsin deglaciation of the Laurentide ice sheet enable estimates to be made of changes in the volume and area of the ice sheet. The average marginal recession between 12,000 and 7,000 BP is estimated as 260 m year-1 and varies little between the northwest and southern margins. The northeastern margin retreated at 20 m year-1 and the overall tendency was for the Laurentide ice sheet to migrate toward Baffin Island. Vertical mass loss required to produce marginal retreat of over 200 m year-1 indicates values between 10 and 50 m of vertical ice wastage per year. Consideration of atmospheric energy sources provides approximate ablation season inputs of 60, 14, and 6 kcal · cm-2 · year-1 for the south, northwest, and northeast margins compared to required totals of 72 to 360, 72 to 360, and 8 to 16 kcal · cm-2 · year-1, a deficiency in the energy sources by a factor of between 1 and 17.Attention is directed to the great extent of lacustrine and marine environments during deglaciation so that extensive sectors of the Laurentide ice sheet terminated in water; calving is considered the most likely additional ablation process that would explain both the high rates of marginal retreat and the lack of difference in retreat rates of margins at 72°N and 45°N.Large end moraines are commonly interpreted as the response of an ice sheet to climatic change; however, the ice sheet responds to climate through the associated mass balance fluctuations. Major end moraines within the borders of the Laurentide ice sheet are frequently preserved immediately above the local marine limit or glacial lake level, suggesting that moraines may be caused by a lag in response of the ice sheet to the reduction in frontal calving, thus resulting in a limited re-advance or stillstand. Such moraines are not associated with climatic change.