THE TIMING OF LAURENTIDE ICE SHEET RETREAT FROM ISLE ROYALE IN LAKE SUPERIOR BASED ON 10BE EXPOSURE AGES

Patterns and the timing of Laurentide Ice retreat from the Lake Superior basin following the Marquette Readvance are largely unknown because of the lack of datable material in sediment cores extracted from the lake bottom and the paucity of subaerial landforms between Lake Superior’s north and south shores. Here, we present new cosmogenic ¹⁰Be exposure ages of glacial erratics (n = 11) from two recessional moraines on Isle Royale, the largest island in Lake Superior. The average exposure age of these erratics is centered on 10.1 ka, which is consistent with two previously published radiocarbon ages from the bottoms of two of Isle Royale’s many inland lakes and an age determined for a sediment core pulled from Lake Superior southwest of Isle Royale. The timing of deglaciation we present for Isle Royale is furthermore consistent with age estimates for a subaqeous moraine that spans Lake Superior from Isle Royale to Michigan’s Keweenaw Peninsula. New ¹⁰Be data from Isle Royale, along with spatial patterns of Lake Superior’s basal stratigraphy allow us to redraw the Laurentide Ice Sheet’s southwest margin in the early Holocene. We suggest that the Laurentide Ice Sheet remained in contact with Lake Superior’s southern shoreline longer than previously thought, which has implications for meltwater routing through the region.