THE LAURENTIDE ICE SHEET IN THE GREAT LAKES REGION DURING THE YOUNGER DRYAS

The Younger Dryas cold interval is often taken as major climate signal although its geographical coverage is often debated. One especially problematic association is how, or if, this well recognized climate signal is expressed on the Laurentide Ice sheet. Here we examine this question by review of chronology of the ice sheet from ~14.5 to 11 ka yr BP as the ice sheet underwent expansion and contractions in the northern Lake Michigan and southern Lake Superior basins.

New dating of the Two Creeks forest bed (Rech 2010 et al.) indicates the ice margin lie had retreated north of Two Creeks from 14.5 to 13.5 ka yr BP. The following readvance extended across northern Michigan to at least the western side of the Lake Huron basin. Retreat subsequent to 13.5 ka yr BP appears to be stepped as the ice margin moved northward into the Lake Superior basin. The traditional view that the ice sheet retreated to the northern shore of Lake Superior during this time is difficult to accommodate with our present understanding of ice calving dynamics. However a forest growing along the southern shore of Lake Superior indicates a limited interstadial that ended about 11.5 ka yr BP as the ice sheet shed outwash and formed a series of ice contact slopes.

The retreat during the Two Creeks time corresponds with the Bolling-Allerod warm interval. The readvance and stepped retreat may correspond to the cooling prior and perhaps during the YD. Although more work is needed the general view is that the ice sheet had a limited response to the Younger Dryas cooling. Perhaps the high solar insolation values at that time delivered sufficient energy to become the primary control on the ice margin location.