LATEST PLEISTOCENE DEGLACIATION OF THE CORDILLERAN ICE SHEET IN SOUTHEASTERN ALASKA

The most significant contribution to major sea level rise in the next few centuries may be from the potential collapse of marine-terminating portions of ice sheets. Marine-terminating ice sheet retreat is driven by processes that can lead to catastrophic collapse, but these processes are not fully understood. Precisely constraining the timing of past marine ice sheet retreat provides important insight into the processes that drive change on these margins. Here, we reconstruct the latest Pleistocene retreat of the Cordilleran Ice Sheet (CIS) in southeastern Alaska. This marine-terminating sector of the CIS is analogous to regions of the present-day Greenland and Antarctic ice sheets, yet well-dated records of its latest Pleistocene glacial history are lacking. To address this matter, we collected bedrock and boulder samples for ¹⁰Be exposure dating along west-to-east transects from the outer southeastern Alaskan coast to the Coast Mountains. The nine ¹⁰Be ages that have been obtained thus far reveal that the CIS began retreating from the outer coast at 17.0 ± 0.7 ka, when sea surface temperatures in the North Pacific were rapidly increasing. We will present additional ¹⁰Be ages from the inland portions of the transects, which will further constrain the timing of CIS retreat and allow us to determine if this sector of the CIS retreated catastrophically or gradually during deglaciation. The results presented here provide critical information about the sensitivity of a marine-terminating ice sheet to rapid climate change during the last deglaciation. This adds to our collective ability to better predict the behavior of modern ice sheets in a warming world, which will ultimately reduce uncertainties in estimates of sea level rise.