It takes two: New constraints on Cordilleran Ice Sheet extent and retreat from 10Be and radiocarbon dating
Abstract
Precisely constraining the timing of marine ice sheet advance and retreat provides important insight into the processes that drive change on these sensitive margins. Here, we reconstruct the latest Pleistocene activity of the Cordilleran Ice Sheet (CIS) in southeastern (SE) Alaska. This marine-terminating region of the CIS, which spanned 500 km of the north Pacific coast, is analogous to marine margins in Greenland and Antarctica and may have possessed coastal ice-free areas (refugia) that served as "stepping stones" for early human migration to the Americas. Yet despite the region's potential role in the peopling of the Americas and its similarity to modern ice margins, well-dated records of its late Pleistocene glacial history are lacking. To address this matter, 13 bedrock and boulder samples were collected from potential refugia SE Alaska for 10Be exposure dating. The eight 10Be ages that have been obtained thus far reveal that the outer islands of SE Alaska were covered by the CIS until 17 ka, when sea surface temperatures along the north Pacific coast were rapidly increasing. Additionally, we have calibrated for the first time 134 previously published radiocarbon ages from SE Alaskan cave fossils, and the results reveal a nearly continuous fossil record that spans from the limit of the radiocarbon dating method to the present. However, none of the cave fossils yielded ages between 19.8 and 17.2 cal kyr BP. Taken together, the results of our 10Be and radiocarbon dating suggest that (1) in contrast to previous reconstructions, the CIS occupied the outer islands of SE Alaska from 20-17 ka and (2) the retreat of its marine margin was triggered by ocean warming. Although we have yet to find evidence of refugia, SE Alaska deglaciated 1 kyr prior to the first pulse of human migration to the Americas at 16 ka, and therefore the region may have indeed served as a "stepping stone" to the New World. This project is ongoing, and future work will focus on generating a high-resolution chronology of CIS retreat in SE Alaska, which will help to improve numerical ice sheet models that aim to predict the response of marine-terminating ice sheets to climate change.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP11E..08L
- Keywords:
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- 0728 Ice shelves;
- CRYOSPHEREDE: 1105 Quaternary geochronology;
- GEOCHRONOLOGYDE: 4207 Arctic and Antarctic oceanography;
- OCEANOGRAPHY: GENERALDE: 4901 Abrupt/rapid climate change;
- PALEOCEANOGRAPHY