Evidence of solid Earth influence on stability of the marine-terminating
Puget Lobe of the Cordilleran Ice Sheet
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 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.