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Ancient 'missing' ice may have formed on West Antarctica

26.08.2009
West Antarctica was higher and larger 34 million years ago than previously thought, making it a possible site for ice that seemed to be missing during a key climate transition, scientists report. This finding, which has important implications for climate change, is published today in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

"Using data from prior geological studies, we have constructed a model for the topography of West Antarctic bedrock at the time of the start of the global climate transition from warm 'greenhouse' Earth to the current cool 'icehouse' Earth some 34 million years ago," explains Douglas S. Wilson of the University of California at Santa Barbara (UCSB), first author of the new work.

He and his UCSB co-author Bruce Luyendyk conclude that, contrary to most current models for bedrock elevations of West Antarctica, the region's bedrock in the past was at much higher elevation and covered a much larger area than today. Those models assume that an archipelago of large islands existed under the ice at the start of the climate transition, similar to today, but Wilson and Luyendyk find that does not fit their new model. In fact, the authors state that the land area above sea level of West Antarctica was about 25 percent greater in the past.

In the existing theory, the low elevation of West Antarctica relegates it to a minor role in the ice accumulation that began 34 million years ago; ice sheets grew on the higher and larger East Antarctic subcontinent. West Antarctica only joined the process around 14 million years ago.

"But a problem exists with leaving West Antarctica out of the early ice history," says Wilson. "From other evidence, it is believed that the amount of ice that grew on Earth at the 34 million year climate transition was too large to be accounted for by formation on East Antarctica alone, the most obvious location for ice sheet growth. Another site is needed to host the extra missing ice."

Evidence for that large mass of ice comes from two sources: One is geologic records of lowered sea level at the time, which indicate how much ice formed on land to produce the sea-level drop. The other is shells of marine microfossils which have chemical and isotopic compositions that are sensitive to ocean temperatures and to the amount of ice on land.

By showing that West Antarctica had a higher elevation 34 million years ago than previously thought, the new study reveals a possible site for the accumulation of the early ice that had been unaccounted for. Moreover, "preliminary climate modeling by researchers at Pennsylvania State University demonstrates that this new model of higher elevation West Antarctica bedrock topography can indeed host the missing ice," says Luyendyk.

"Our results, therefore, have opened up a new paradigm for the history of the growth of the great global ice sheets. Both East and West Antarctica hosted the growing ice," he adds.

The new hypothesis may solve another conflict among climate scientists. Given that more ice grew than could be hosted on East Antarctica alone, some researchers have proposed that the missing ice formed in the northern hemisphere. This would have been many millions of years before the well-known documentation of ice growth there, which started about 3 million years ago; evidence for ice sheets in the northern hemisphere prior to that time is not established. The new bedrock model shows it is not necessary to have ice hosted in the northern polar regions at the start of global climate transition; West Antarctica could have accommodated the extra ice.

The National Science Foundation's Office of Polar Programs funded this research.

Title:
"West Antarctic paleotopography estimated at the Eocene-Oligocene climate transition"
Citation:
Wilson, D. S., and B. P. Luyendyk (2009),West Antarctic paleotopography estimated at the Eocene-Oligocene climate transition, Geophys. Res. Lett., 36, L16302, doi:10.1029/2009GL039297.
Authors:
Douglas S. Wilson: Marine Science Institute and Department of Earth Science, University of California, Santa Barbara, California, USA.

Bruce P. Luyendyk: Department of Earth Science and Institute for Crustal Studies, University of California, Santa Barbara, California, USA.

Contact information for authors:
Douglas Wilson, Associate Research Geophysicist, Department of Earth Science and Marine Science Institute, Tel: +1 (805) 450-0025, email:

dwilson@geol.ucsb.edu

Bruce Luyendyk, Professor, Department of Earth Science, Tel: +1 (805) 893-405,
email: luyendyk@geol.ucsb.edu

Peter Weiss | American Geophysical Union
Further information:
http://www.agu.org

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