A new 10Be chronology of the LGM in the Brooks Range, Arctic Alaska
Abstract
Unlike most of the Arctic, Alaska was glaciated by mountain glaciers rather than an ice sheet during the Last Glacial Maximum (LGM). Reconstructing past mountain-glacier fluctuations in the Brooks Range can shed light on past climatic change on both regional and global scales, such as the history of temperature and moisture availability as well as the synchronicity of glaciation throughout the Arctic. The existing LGM chronology in the Brooks Range is based on 14C ages in outwash sequences, which bracket the timing of glacier advance and retreat. Interestingly, the existing chronology suggests that the local LGM occurred ~29-27 ka in the Brooks Range, which is prior to the global LGM (~26-19 ka). We aim to test the outwash radiocarbon chronology by directly dating LGM moraines using cosmogenic 10Be exposure dating. We collected nine samples from large moraine boulders resting on an LGM (locally termed the Itkillik II glaciation) terminal moraine sequence in Alapah Valley in the north-central Brooks Range. In addition, we collected five boulders from a second moraine upvalley associated with Itkillik III glacial phase, thought by previous workers to be Late Glacial in age. The samples from the Itkillik II moraine are awaiting AMS analysis at the time of this writing. The 10Be ages from the Itkillik III moraine average 17.2×1.0 ka. The Itkillik III moraine is 70 km upvalley from Itkillik II moraines, implying considerable retreat between the LGM terminal moraine and the 17 ka moraine. This contrasts with lower latitudes where glaciers remained near their LGM position until ~17 ka or even later. The 10Be ages we have obtained from the Itkillik III moraine correlate with the established glacial record based on radiocarbon-dated outwash deposits. If the existing 14C chronology in outwash is correct, it implies a relatively early local LGM followed by significant glacier recession between ~27 and ~17 ka, during the global LGM. This implies relative warmth or reduced moisture availability in Arctic Alaska during the global LGM.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMPP21C1930C
- Keywords:
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- 1150 GEOCHRONOLOGY Cosmogenic-nuclide exposure dating;
- 4914 PALEOCEANOGRAPHY Continental climate records