A CASE FOR STEP-WISE RETREAT OF THE LAURENTIDE ICE SHEET DURING THE YOUNGER DRYAS: CENTRAL UPPER PENINSULA OF MICHIGAN

The Younger Dryas (YD) is a well-known paleoclimatic event from 12.9-11.6 cal ka BP. Although the response of small glaciers to climate change is well established, far less is known how large glaciers like the Laurentide Ice sheet (LIS) respond to rapid climate change during events like the Younger Dryas. This study investigates the right lateral moraines of the Green Bay lobe of the Laurentide Ice Sheet in central Upper Peninsula of Michigan. Here and in adjacent Wisconsin, the LIS buried two forest beds (Two Creeks and Lake Gribben) and the ages of 13.7 and 11.6 cal ka BP, respectively, indicate that the ice sheet occupied this region during Younger Dryas time. Hypothesis for ice sheet behavior during this time include (i) retreat some 200 km across Lake Superior before the YD and the same length readvance during the YD and (ii) limited readvance at the end of the YD.

To test these possibilities this study employed the USGS 10 m digital elevation model to map surficial glacial landforms and added 25 new radiocarbon dates to refine the ice-sheet deglaciation chronology. Mapping revealed at least four and perhaps as many as seven successive ice-contact margin positions between the Two Creeks and Lake Gribben locations suggesting a step-wise retreat. Radiocarbon dates indicate the LIS retreated across the region from 12.8 to 11.4 cal ka BP and then readvanced at 11.3 cal ka BP. This implies that during most of the YD the ice sheet was in retreat with a readvance occurring after the YD. If the LIS retreated in a step wise manner during the YD, it may imply warmer summers or high solar insolation values as the mechanism controlling the ice sheet margin at this time.