A NEW SURFACE EXPOSURE CHRONOLOGY FOR THE MOST RECENT DEGLACIATION OF SOUTHERN LAKE SUPERIOR AND THE IMPLICATIONS FOR PROGLACIAL LAKE EVOLUTION AND CONTINENTAL-SCALE MELTWATER ROUTING

The Lake Superior basin sits at the heart of the North American continent. As such, the most recent deglaciation of Lake Superior allowed for a complex evolution of continental-scale meltwater routing associated with a variety of alternative drainage channels. Similarly, the opening of the western Superior basin, along with the Laurentide Ice Sheet extent controlling the availability of various eastern outlets, gave rise to multiple proglacial lakes, most notably Glacial Lake Duluth with its distinct wave-cut beaches as high as 150 m above modern lake elevation. The existing chronology suggests that the deglaciation of Lake Superior nearly coincides with the Younger Dryas cold interval (12.8 - 11.5 ka), but there is current disagreement over the direction of meltwater routing that led to the associated reduction in North Atlantic overturning. Determining the availability of an eastward meltwater routing during the Younger Dryas requires a precise chronology from the southern shore of Lake Superior. However, the timing of deglaciation from the southern shore remains poorly constrained in part due to a reliance on minimum-limiting radiocarbon dates. Here, we present a new ¹⁰Be exposure age chronology from large erratic boulders at two locations along the southern shore. First, we present an exposure chronology for one of the wave-cut beaches of Glacial Lake Duluth on the Bayfield Peninsula along the southwestern shore of Lake Superior. We interpret these exposure ages to provide a timing of lake level fall for Glacial Lake Duluth, with a mean age of 12.9 ± 0.3 ka (n=5 samples), indicating the drainage of Glacial Lake Duluth at the start of the Younger Dryas. A second site from topographic highs in the Huron Mountains near Marquette, MI, provides a timing for final deglaciation of the eastern Lake Superior basin at 11.3 ± 0.2 ka (n=11 samples), which is consistent with the timing of the Marquette readvance closing off the eastern outlet yet again, in response to the Younger Dryas. We explore the implications of this new chronology on the evolution of continental meltwater routing through the Lake Superior basin within the context of existing deglacial chronologies and meltwater proxies.