March GEOLOGY and GSA TODAY media highlights

Dramatic acceleration and thinning of major ice streams in the Amundsen Sea sector of the West Antarctic Ice Sheet have been observed during the last two 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 für Geologie, Universität Stuttgart, Herdweg 51, 70174 Stuttgart, Germany; Thomas Kenkmann, Humboldt-Universität zu Berlin, Museum für Naturkunde, Institut für 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.

Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic

Anthony Cohen et al., Department of Earth and Environmental Sciences, The Open University, Milton Keynes MK7 6AA, UK. Pages 231-234.

What happens to the Earth during and after abrupt global warming, over time scales of hundreds or thousands of years? One way of addressing this question is to use computer models to try to predict the course of future climate and environmental conditions. However, the uncertainty of longer-term predictions may be substantial because of our relatively poor understanding of the great complexity of the Earth’s behavior. Adopting a complementary approach in their study, Pearce et al. examine and de-code the geological record of extreme, but infrequent, events that occurred in the distant past. These records have the potential to provide quantifiable information about precisely how Earth has actually responded to severe environmental change in the longer term. Pearce et al. show that there was widespread reduction in the oxygen content of the oceans during an abrupt period of global warming 183 million years ago in the Early Jurassic period, and that these conditions persisted for approximately 200,000 years. The authors also demonstrate that the changes in seawater oxygenation at that time were periodic and were coupled with regular, large-scale fluctuations in the global carbon cycle. The precise relationships between the various expressions of this environmental crisis in the Early Jurassic, which also involved a significant mass extinction of marine and terrestrial species, may be able to provide valuable constraints that could help to validate our predictions about environmental change in the future.