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ground is no surprise at Earths poles. In the Arctic, permafrost consists
mostly of frozen soil and rock, up to several meters thick, with some ice layers.
In the Dry Valleys of the Antarctic, however, permafrost often seems to be thick
layers of ice, covered by layers of thin till that are generally only several
centimeters thick. These tongues of ice seem to be remnants of glaciers from
neighboring valleys. Still, ice in the Dry Valleys appears not to have moved
for a long time, and researchers are trying to determine exactly how dynamic
the ice system may be.
Beacon Valley is one of the Antarctics so-called Dry Valleys, which have very little precipitation and are some of the only ice-free regions on the continent. But the ice beneath the glacial till that carpets these valleys may be more dynamic than it looks. Photo by Ron Sletten.
Very little precipitation falls in these valleys, in a region that is so dry that the ice is constantly sublimating basically evaporating molecule by molecule, without turning to water first. Also, the valleys are patterned by polygon-shaped stress cracks, which form to relieve stress as temperatures fall and the ground contracts, in much the same way columnar basalt or mud cracks form elsewhere.
These cracks commonly fill with water in spring in the Arctic, or ice wedges in winter, but in the Antarctic, they fill with windblown silt and sand, says Ron Sletten of the University of Washington, Seattle, a co-author of new research looking at the activity of ice in the Dry Valleys. Sletten and colleagues worked with modeler and mathematician Felix Ng of MIT to redate and model formation of the till overlying the ice in Beacon Valley. By analyzing previously published cosmogenic helium isotope measurements to figure out how long the rocks have been exposed to cosmic rays, they found that the till had formed in the past 310,000 to 43,000 years, the team reports in the February Geology.
During freezing and thawing cycles in the Antarctic, permafrost forms pentagon-shaped cracks where windblown silt deposits. Photo by Ron Sletten.
As the ice sublimates, the rocks and clasts below are brought toward the surface, forming layers below the till already in place. Their model shows that the rate of ice sublimation is greater than 7 meters every 1 million years. At that rate, the ice beneath the rocks and till is probably younger than 1 million years old, showing a relatively active system for the Dry Valleys.
That rate of sublimation, however, is too fast to fit some previous research, in which scientists found that one Dry Valley till deposit contained a layer of ash that dated to 8 million years old. To have ash that old in place would be evidence for extraordinarily ancient and stable ice beneath it, potentially containing ancient climate data. But Ngs model and the teams redated rocks measurements that match most of the rock exposures dated in the valleys show that hundreds of meters of ice would have sublimated away over 8 million years, making it difficult to argue that the ash may still be in place, they say.
Bernard Hallet, a co-author of the new results, says that hundreds of meters of ablation would totally disrupt the till, mixing the surface layers. Although the landscape is relatively stable, he says, even with lower rates of sublimation, there is enough activity cracks opening and closing on a more active surface where it becomes more difficult to preserve anything for millions of years.
But David Marchant of Boston University, who published some of the past research with colleagues on the ash layer, says that the new dates should be taken as a minimum possible age of the ancient valley ice, leaving open the possibility that the ice beneath is much older. He also says that the ash appeared to be unperturbed in its till layer: The fact that we can still find areas undisturbed tells us about the longevity of polygon cracks, he says, and that ice has been on the continent for an awfully long time.
Kurt Cuffey of the University of California at Berkeley calls Ng and coworkers results a very solid analysis that clearly shows that its not likely that the ice underneath that till can be 8 million years old. The amount of sublimation necessary for the ice to be that old may be more likely to occur on other planets, such as Mars, but probably not on Earth, he says. The larger point remains, however, that old ice still sits in the Antarctic Dry Valleys, as well as ash layers that have not been transported far, Kuffey says, which is good evidence that the Antarctic ice sheet has not overwhelmed this region in a very long time.
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