Advances in understanding the southern terrestrial margin of the Laurentide Ice Sheet during the Last Glacial Maximum and last deglaciation

Abstract:

In the late Pleistocene, the Laurentide Ice Sheet (LIS) was the largest ice sheet on earth, responsible for 70-75 m of global sea level that rapidly returned to the ocean during deglaciation. It altered atmospheric circulation, deflected the crust causing the rerouting of major rivers, and is implicated in massive discharges of meltwater and icebergs that may have affected climate. Despite its importance, knowledge of the temporal evolution of the LIS remains surprisingly poor in certain sectors, particularly for the Last Glacial Maximum (LGM) and last deglaciation. Because of its size, models are necessary to bridge geologic data of ice-sheet evolution with sea-level and climate changes, and recent model improvement has led to the community trying to extract increasingly complex insights from model output. Yet, the underpinning LGM and deglacial chronology of the LIS, essential model input, has not kept pace. Here, we present an effort to advance the understanding of LIS extents and fluctuations in the Great Lakes and Western Lobes sectors, which together are the terminal area for much of the terrestrial, southern ice-sheet margin. We integrate chronological data with detailed geomorphic mapping to go beyond the traditional isochron approach of representing deglaciation patterns. The chronological data include a large compilation of radiocarbon ages (n=291) from the Great Lakes sector (e.g., Erie Lobe) that indicate a complex LGM, with advances at ~25.8, 23.4 and 21.5 ka, and subsequent recession that slowed from ~19.0 to 16.8 ka. Radiocarbon and cosmogenic nuclide exposure ages from the Western Lobes sector (i.e., James and Des Moines Lobes) show similarly timed maximum extents at ~16.1 ka, followed by rapid recession. These large-scale LIS fluctuations document the complex interaction of climate and ice-sheet dynamics and may have influenced deglacial sea-level changes.