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Geology; March 2008; v. 36; no. 3; p. 223-226; DOI: 10.1130/G24207A.1
© 2008 Geological Society of America
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First exposure ages from the Amundsen Sea Embayment, West Antarctica: The Late Quaternary context for recent thinning of Pine Island, Smith, and Pope Glaciers

Joanne S. Johnson1, Michael J. Bentley2 and Karsten Gohl3

1 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
2 Department of Geography, Durham University, South Road, Durham DH1 3LE, UK and British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
3 Alfred-Wegener-Institut für Polar- und Meeresforschung, Postfach 120161, D-27515 Bremerhaven, Germany

Dramatic changes (acceleration, thinning, and grounding-line retreat of major ice streams) in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) have been observed during the past two decades, but the millennial-scale context for these changes is not yet known. We present the first surface exposure ages recording thinning of Pine Island, Smith, and Pope Glaciers, which all drain into the Amundsen Sea. From these we infer progressive thinning of Pine Island Glacier at an average rate of 3.8 ± 0.3 cm yr–1 for at least the past 4.7 k.y., and of Smith and Pope Glaciers at 2.3 ± 0.2 cm yr–1 over the past 14.5 k.y. These rates are more than an order of magnitude lower than the ~1.6 m yr–1 recorded by satellite altimetry for Pine Island Glacier in the period 1992–1996. Similarly low long-term rates (2.5–9 cm yr–1 since 10 ka) have been reported farther west in the Ford Ranges, Marie Byrd Land, but in that area, the same rates of thinning continue to the present day. Our data provide the first evidence that puts into context recent rates of thinning of the WAIS in the Amundsen Sea Embayment and demonstrates that these are unusually rapid. The data also provide much-needed constraints for ice sheet models, which are the primary tool for predicting the future behavior of the WAIS and its likely contribution to sea-level rise.

Key Words: West Antarctic Ice Sheet • Pine Island Glacier • exposure age • cosmogenic isotopes • glacial history






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