ASSESSING THE TIMING OF NONGLACIAL CONDITIONS IN CENTRAL CANADA, WITH IMPLICATIONS FOR THE LAURENTIDE ICE SHEET EVOLUTION DURING MIS 3

The Laurentide ice sheet (LIS) repeatedly waxed and waned over the North American continent during the Quaternary period. Yet, because dating the sediments remains complex, even the young MIS 3 interstadial extent and configuration of the LIS is debated. Reconstructions range from an LIS restricted to the core regions (i.e., Keewatin and Quebec-Labrador) to an extensive ice sheet reaching into the upper Mississippi River Basin. Constraining the MIS 3 ice margin is essential to modelling ice volume of the LIS and its critical contribution to sea level change during this stage. At the heart of the LIS, nonglacial sediments in the Hudson Bay Lowland (HBL) were recently reported as MIS 3 in age, based on near-finite radiocarbon ages and optical ages. The ages obtained have been used to imply a reduced LIS, and possibly a deglaciated Hudson Bay during MIS 3 - despite paleoenvironmental data suggesting a climate similar to an interglacial. Here, we report new western HBL optical ages, stratigraphy and paleoenvironmental data that indicate the region was likely last deglaciated during MIS 5 instead of MIS 3. We also re-examine MIS 3 radiocarbon ages from southeast Manitoba and confirm, through optical age and stratigraphic evidence, a temporary retreat of the LIS during this time period. These new results challenge the view of a MIS 3 deglaciated Hudson Bay, and instead constrain the ice sheet margin to a position between the Hudson Bay Lowland and southeast Manitoba.