Cosmogenic surface exposure dating of southern Laurentide ice retreat and eastward Lake Agassiz routing: relationship to the Younger Dryas cold event

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Cosmogenic surface exposure dating of southern Laurentide ice retreat and eastward Lake Agassiz routing: relationship to the Younger Dryas cold eventAnders E. Carlson College of Earth, Ocean, and Atmospheric Sciences, Oregon State UniversityThe Younger Dryas (YD) was an abrupt climate change event during the last deglaciation that cooled the North Atlantic region 12,900-11,700 years ago. Ever since its identification over a century ago, the cause of the YD has been widely debated. YD climate cooling was likely caused by a reduction in the strength of the Gulf Stream/North Atlantic current, but what caused this weakening of ocean circulation? This project will test one of the leading hypotheses as to the cause of the YD; that eastward meltwater discharge from the Laurentide Ice Sheet (LIS) freshened the ocean surface and slowed ocean circulation. Given the heightened awareness over tipping points in the climate system, the findings of this project will provide insight into the possible effects of current and future changes in the Arctic freshwater budget. In particular, can only small changes in the Arctic freshwater discharge to the North Atlantic drive significant changes in ocean circulation leading to North Atlantic cooling? Results will also have implications for the field of archeology and geobiology as they will help assess potential underlying causes of Pleistocene large mammal extinction and adaptations of early North Americans, garnering popular attention. How have past abrupt climate change events affected humans? This project will support a postdoctoral researcher, who will be trained in the field of glacial geology, geochronology, geochemistry and paleoclimatology. Under-represented groups in the geosciences will take precedent in recruitment of the postdoctoral researcher. Because of the reduced number of permanent positions in paleoclimatology at present, support of recent PhDs is a significant broader impact. The goal of this project is to determine if the opening of eastern Lake Agassiz outlets occurred at the start of the YD, which would support the hypothesis that this routing caused the YD. The northward retreat of the southern LIS margin and opening of the eastern glacial Lake Agassiz outlets will be directly dated with boulder 10-Be surface exposure ages along the western shores of Lake Superior (the eastern outlet). If one or more of the eastern Lake Agassiz outlets opened at the start of the YD, then eastward runoff routing likely provided the freshwater forcing that drove the reduction in Atlantic meridional overturning strength. A later opening of the eastern outlets would suggest that northwest Lake Agassiz routing or another forcing was the trigger of the YD. Resolving the cause of the canonical abrupt climate event will have important implications on the underlying mechanisms that give rise to nonlinearities in the climate system and will provide a critical target for assessing climate model sensitivity to freshwater forcing. Likewise, the 10-Be chronology can be readily incorporated into glacial systems models that are simulating the last deglaciation of the LIS. Results will be disseminated to the scientific community through publications and presentations at international meetings. Carlson will continue to write popular science pieces, which will incorporate the results of this research project.