Radiocarbon Isochrones on the Disintegration of the Laurentide Ice Sheet

The last great event of the Wisconsin Glacial Stage in North America was the disintegration of the Laurentide Ice Sheet. This occurred between 13,000 and about 5,000 years ago and had a profound effect upon the paleogeography of the continent. Analysis of present-day distribution of fauna and flora, the archaeological record, and climatic and sea-level fluctuations are intimately bound up with ice sheet disappearance, yet there has been no systematic attempt to utilize existing radiocarbon and geological data to attempt a plot of the ice sheet perimeter at specific intervals through time. The present paper makes this attempt in the form of two maps (Figures 1 and 2), the first being an objective portrayal of iso-lines drawn on the radiocarbon data, the only assumption being that the ice sheet perimeter tended to parallel the coastline or, more particularly, the trend of the outer edge of the continental shelves and the southern limit of Wisconsin till on land; the second map is a subjective interpretation of the first based upon geologic field evidence and the climatic and geomorphic intuition of the writers. Some of the immediate implications raised by the maps are discussed and a series of significant conclusions are derived: (1) The northern limit of the Laurentide Ice Sheet proper was close to the arctic mainland coast of Canada. (2) There was a dramatic change from an east-west ice barrier near latitude 60°N in late-glacial time, to a broad low corridor from the Arctic Ocean to the Great Plains in Boreal time. (3) The Laurentide Ice Sheet retained its identity as a distinct unit until about 8,400 years BP (Cockburn Stade) and had catastrophically disintegrated during Atlantic time, within a few centuries of 8,000 years BP. (4) The three remaining ice remnants centered over Keewatin, Labrador-Ungava, and Foxe Basin-Baffin Island persisted through the Atlantic climatic episode (alti-thermal), that on Baffin Island surviving to the present day in the form of the Barnes Ice Cap.