Spatial variability of marine-terminating ice sheet retreat in the Puget Lowland

Abstract

Abstract. Understanding drivers of marine-terminating ice sheet behavior is important for constraining ice contributions to global sea level rise. In part, the stability of marine-terminating ice is influenced by solid Earth conditions at the grounded-ice margin. While the Cordilleran Ice Sheet (CIS) contributed significantly to global mean sea level during its final post-Last-Glacial-Maximum (LGM) collapse, the drivers and patterns of retreat are not well constrained. Coastal outcrops in the deglaciated Puget Lowland of Washington State – largely below sea level during glacial maxima, then uplifted above sea level via glacial isostatic adjustment (GIA) – record the late Pleistocene history of the CIS. The preservation of LGM glacial and post-LGM deglacial sediments provides a unique opportunity to assess the variability in marine ice sheet behavior of the southernmost CIS. Based on paired stratigraphic and geochronological work, with a newly developed marine reservoir correction for this region, we identify that the late-stage CIS experienced stepwise retreat into a marine environment between 15 000 and 14 000 years before present, consistent with timing of marine incursion into the region reported in earlier works. Standstill of marine-terminating ice for at least 500 years, paired with rapid vertical landscape evolution, was followed by continued retreat of ice in a subaerial environment. These results suggest rapid rates of solid Earth uplift and topographic support (e.g., grounding zone wedges) stabilized the ice margin, supporting final subaerial ice retreat. This work leads to a better understanding of shallow-marine and coastal-ice-sheet retreat and is relevant to sectors of the contemporary Antarctic and Greenland ice sheets and marine-terminating outlet glaciers.