wo decades, and raise concerns that a major retreat of the West Antarctic Ice Sheet is in progress. Until now, however, there has been little evidence that would allow a comparison of recent rates of change with those that have arisen naturally since the end of the last glacial period. Johnson et al. present the first surface exposure ages from rock outcrops in the Amundsen Sea region of the West Antarctic Ice Sheet. The ages indicate the mean rates of thinning of Pine Island, Smith, and Pope Glaciers since 14.5 thousand years ago. The average rate of thinning varies from 2.3 to 3.8 centimeters per year; this is an order of magnitude slower than the average rate of thinning of Pine Island Glacier (1.6 meters per year) seen in satellite altimetry data between 1992 and 1996. The new data also record a general trend of progressive thinning throughout the past 14.5 thousand years, suggesting that currently observed changes are not simply a long-delayed response to conditions at the end of the glacial period. Further data are required to determine whether there have been short-lived rapid thinning events since the end of the last glacial period, but the data presented in this paper provide the first long-term evidence that current rates of ice sheet thinning in this part of the West Antarctic Ice Sheet may be unusually rapid.
Upheaval Dome, Utah, USA: Impact origin confirmed
Elmar Buchner, Institut fr Geologie, Universitt Stuttgart, Herdweg 51, 70174 Stuttgart, Germany; Thomas Kenkmann, Humboldt-Universitt zu Berlin, Museum fr Naturkunde, Institut fr Mineralogie, Berlin D-10115, Germany. Pages 227-230.
The origin of Upheaval Dome in Utah has been discussed for decades; it has been interpreted as a crypto volcanic feature, a salt diapir, a pinched-off salt diapir, and as an eroded impact crater. Buchner and Kenkmann present new and unambiguous evidence for the impact origin of Upheaval Dome.
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