A 7000-YEAR OFFSET BETWEEN THE MAXIMUM EXTENT IN THE EASTERN AND WESTERN SECTORS OF THE SOUTHERN LAURENTIDE MARGIN

The Laurentide Ice Sheet (LIS) was the largest ice sheet at the Last Glacial Maximum, containing up to 120 m of sea level equivalent. Although the maximum extents are known, the maximums appear to be reached at different times. Available chronological data shows a major offset in the age of maximum extent along the southern margin, divided roughly by Lake Superior into eastern and western sectors. The eastern sector lobes, which include the Great Lakes and New England regions, have been dated using radiocarbon, surface exposure and OSL ages that point to a maximum extent at ~23,900 cal yr BP. Conversely, the Des Moines Lobe and margins further west possess less chronological controls. The limited data however, indicates that the maximum extent of at least some portions of this sector was reached ~16,900 cal yr BP.

At least three hypotheses might explain the temporal offset of maximum extent. One hypothesis is reorganization the LIS from a single dome over Hudson Bay to a multi-dome system, caused by increased discharge from the ice streams at the easternmost, marine-based LIS margin (Heinrich 1; ~17,000 yr BP). This reorganization led to the formation of the independent Keewatin dome over western Canada. However, reconfiguration of the dynamics of the ice sheet within dating resolution seems unlikely. A second hypothesis is the surging of individual lobes. Yet, a comparison between the Des Moines Lobe chronology and that of the Cordilleran Ice Sheet reveals similar timing of maximum extent. The Matanuska/Knik Glacier complex in the Anchorage Lowland of Alaska reached its maximum at the Elmendorf moraine ~16,700 cal yr BP. Similarly, the Puget Lobe of the Cordilleran ice sheet reached its maximum ~16,950 cal yr BP. This apparent synchrony of three independent ice masses over a large area allows for the possibility of external forcing of these scattered ice masses.

For example, Heinrich 1 and associated cooling coincides with a redistribution of moisture availability in western North America, caused by southerly depression of the summer jet stream. This may have caused local climate fluctuations along the southern LIS margin. Given this apparent correspondence with the Heinrich 1 event, it may be useful to document climate conditions along the southwestern sector of the ice sheet to see if they are compatible with a positive mass balance.