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 late Pleistocene history of the CIS. The preservation of LGM glacial and post-LGM deglacial sediments provides a unique opportunity to assess 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 about 12,000 years before present, placing glacial ice in the region for about 3,000 years longer than previously thought. Stand-still of marine-terminating ice for a millenia, 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; relevant to sectors of the contemporary Antarctic and Greenland ice sheets and marine-terminating outlet glaciers.

Ice streams deposit sediment at their grounding lines, where ice reaches flotation. Grounding Zone Wedge (GZW) deposits indicate standstills in past grounding-line retreat, and are thought to stabilize grounding lines by reducing local water depth, restricting ice flow. However, the mechanisms of GZW growth are uncertain, as are the effects of sedimentation on a retreating grounding-line prior to GZW formation. We develop a 1-D coupled model of ice flow and sediment transport, considering both subglacial deposition of deforming sediments, and proglacial melt-out of entrained sediments from ice shelves. A refined grid near the grounding line resolves small sediment features and their effect on ice dynamics. The model simulates the growth of low-profile, prograding, asymmetric features consistent with observed GZWs. We find that the characteristic shape of GZWs arises from the coupling of sedimentation and ice dynamics. This mechanism is consistent with deposition from either deforming or entrained sediments, and does not require a low-profile ice shelf to limit vertical GZW growth. We also find that during grounding-line retreat, sedimentation provides a stabilizing feedback when other factors initially slow retreat. This may turn a slowdown in retreat into a long standstill, even when ice dynamics are far out of equilibrium. The feedback depends on total sediment flux and its spatial pattern of deposition, making these priorities for future study. Our study suggests that sedimentation might significantly extend pauses in deglaciation, and the model provides a new tool for exploring links between ice-stream dynamics and submarine landforms.