How the Ice Sheets of North America rerouted rivers, carved valleys, and changed global climate

Andrew Wickert, Assistant Professor of Earth Sciences and St. Anthony Falls Laboratory, University of Minnesota

Over the last glacial cycle, ice sheets and the resultant glacial isostatic adjustment (GIA) rearranged river systems. As these riverine threads that tied the ice sheets to the sea were stretched, severed, and restructured, they also shrank and swelled with the pulse of meltwater inputs and time-varying drainage basin areas. I will present reconstructed drainage basins and river discharge paleohydrographs across North America from the Last Glacial Maximum to present, and use these to evaluate leading models of the massive North American ice-sheet complex against terrestrial and marine geologic data. The particularly dynamic Mississippi River basin inspires a case study of its geomorphic history, where new data show that a buried 50-60-meter high waterfall heads a reach where the Mississippi has deeply incised bedrock that was once uplifted in the glacial forebulge. This event that was likely precipitated by an early ice advance that dammed the the paleo-Upper-Mississippi in Wisconsin and caused it to overtop its drainage divide and join the modern Mississippi in Iowa. Returning to the more recent geologic past, I will present a new compilation of isotopic records from the Gulf of Mexico that can help to revise the Great Lakes glacial chronology. I will close by presenting new model results of how freshwater discharge to the ocean during Meltwater Pulse 1A, the most rapid period of deglacial sea-level rise, dramatically affected the Atlantic Meridional Overturning Circulation and past climate. To obtain these results, one must link the ice sheets, terrestrial hydrology, and the oceans. By probing this link, we can link multiple data sources to critically evaluate and improve our knowledge of the late Quaternary, connect geomorphic change with climate and ice-sheet drivers, and more rigorously couple ocean circulation and climate feedbacks to deglaciation.