Outcrop Perspectives on Spatially Variable Retreat of the Marine-terminating southern Cordilleran Ice Sheet

Understanding the drivers of retreat for marine-terminating ice sheets is important for constraining ice-sheet contributions to global sea level rise. In part, the stability of marine-terminating ice systems is influenced by solid-Earth conditions, such as topography, at the grounded-ice margin. While the Cordilleran Ice Sheet (CIS) is thought to have contributed significantly to global mean sea level during its post-Last Glacial Maximum (LGM) collapse, the drivers and spatial patterns of retreat are not well constrained. The sedimentology and stratigraphy of coastal outcrops in the deglaciated Puget Lowland of Washington state, largely below sea level during glacial maxima and subsequently uplifted above sea level due to glacial isostatic adjustment (GIA), record the late Pleistocene history of the CIS. The preservation of LGM till and post-LGM deglacial sediments provide a unique opportunity to assess spatial variability in marine ice-sheet behavior of the southernmost CIS. Based on physical and chemical sedimentology and centimeter-scale stratigraphy, we assess outcrops that record gradual and abrupt transitions from subglacial till to glacimarine sediments, as well as outcrops that lack glacimarine sediments and instead are comprised of subglacial to terrestrial (i.e., subaerial) sediment transitions. Additionally, paired stratigraphic and geochronological work will allow for comparison between outcrops and, potentially, durations of glacimarine sedimentation and timing of (de)glacial events. We characterize these deglacial sequences in order to better understand the controls, including the potential role of topography and GIA, on spatially variable retreat of the southern CIS. Ultimately, this work leads to a better understanding of shallow marine and coastal ice sheet retreat that is relevant to sectors of the contemporary Antarctic and Greenland ice sheets and marine-terminating outlet glaciers elsewhere.