Laurentide Ice Sheet: Estimated volumes during Late Wisconsin

Abstract

Estimates are made of the volumes of the Laurentide ice sheet and the Innuitian ice sheet (covering the Canadian Arctic Islands north of latitude 74°N) from 18,000 to 6000 B.P. Relevant parts of the existing theory of flow in an ice sheet are first reviewed. Two limiting models are considered: a steady‐state ice sheet, the dimensions of which do not change with time, and a stagnant ice sheet that thins at the same rate everywhere. Formulas relating ice thickness to radius and volume to area are given. The relation between area and volume of six existing ice sheets is examined to guide the choice of numerical parameters. The history of the Laurentide and Innuitian ice sheets is reviewed to help decide which model (or whether a combination of the two) is more appropriate at different times. Their volumes and thicknesses are then calculated from areas measured on a recent map of ‘speculative’ positions of the ice margins. Volume estimates for the Laurentide ice sheet are 26.5 × 106 km³ at maximum, 17.5 × 106 km³ at 11,800 B.P., and 6 × 106 km³ at 8500 B.P. The sea entered Hudson Bay shortly before 8000 B.P. and Foxe Basin a few hundred years thereafter. This divided the ice sheet into separate Keewatin, Labrador, and Foxe‐Baffin ice sheets, which had an estimated total volume of 106 km³ at 7500 B.P. The Keewatin and Labrador sectors had disappeared by about 6000 B.P. The volume of the Innuitian ice sheet at maximum is estimated at 106 km³. It had decreased to its present‐day value of 5 × 104 km³ by about 8000 B.P. Maximum errors in these figures are estimated at between. ±16% and ±20% at different times, plus an unknown amount for inaccuracies in the map of the ice margins. From maximum until about 12,000 B.P. the volume of the Laurentide ice sheet decreased much more rapidly than its thickness. If, at the ice sheet maximum, bedrock below its center in Hudson Bay was in isostatic equilibrium, between 150 and 390 meters of uplift must have occurred between 9000 and 8000 B.P. Melting of ice in the Laurentide and Innuitian ice sheets between 15,000 and 6000 B.P. can account for a rise in sea level of between 56 and 76 meters.