RETREAT OF ICE IN WISCONSIN AND NORTH ATLANTIC CLIMATE CHANGE OF THE LAST DEGLACIATION

The state of Wisconsin resides at an important juncture between Lakes Michigan, Superior and the more eastern Great Lakes. When even minimally glaciated by the Laurentide Ice Sheet, runoff from western Canada was routed southwards through the Mississippi River into the Gulf of Mexico. If Laurentide ice advanced further south into Wisconsin covering the Straits of Mackinac, local meltwater and Lake Michigan runoff were also routed into the Gulf of Mexico. With much of the Laurentide runoff draining into the Gulf of Mexico, convection can occur in the North Atlantic, allowing North Atlantic climate to warm. Retreat and readvance of Laurentide ice in and out of Wisconsin therefore plays an important role in the evolution of climate during the last deglaciation. Here I review the deglacial history of Wisconsin that was mainly constructed through the work of Dave Mickelson, its relationship to the routing of southern Laurentide runoff between the Gulf of Mexico and the North Atlantic, and deglacial climate change. Retreat of ice out of Lake Michigan and the Green Bay lowlands before ~17 ka during the Mackinaw interstade routed Laurentide runoff eastward where it likely sustained reduced convection during the Oldest Dryas cold event. Readvance of ice into Wisconsin during the Port Huron stade after ~15.2 ka routed runoff back to the Gulf of Mexico, and North Atlantic climate subsequently warmed into the Bølling. Ice remained in Wisconsin during the Bølling, and therefore did not constitute a significant contribution to the concurrent abrupt rise in sea level called Meltwater Pulse 1A. Retreat during the Two Creeks interstade ~14 ka again routed runoff eastward and climate cooled into the Older Dryas, with the following Greatlakean readvance allowing warming into the Allerød. Subsequent retreat by ~13 ka opened Lake Superior and all of western Canadian runoff drained eastward, causing the Younger Dryas cold event. The final Marquette readvance into Wisconsin ~11.5 ka rerouted western Canadian runoff and ended the Younger Dryas cold event.