We have been watching surface melting of the Greenland Ice Sheet (see Greenland Ice Sheet Today) as one sign of climate change. Covering some 656,000 square miles and ranging from 1 to 2 miles thick, this mass of ice is often considered a bellwether for change in the Arctic. But there is still much to know about what goes on beneath its surface. Recently, scientists were surprised to find evidence of soil underneath the Greenland Ice Sheet. Why is it there? And what does it reveal about the history of the ice sheet?
Ice cores and elements
When glaciers and ice sheets flow over the Earth, they typically bulldoze the surface free of anything that lies in their path. The force and weight of flowing ice scrapes up everything from trees and large boulders to the tiniest specks of soil.
Paul Bierman, a geomorphologist at the University of Vermont, was studying surface erosion in Greenland long after a team drilling ice cores hit bedrock at Summit, a research base atop one of the thickest parts of the ice sheet. Most scientists are interested in the long stretches of ice within the core: by examining the ancient atmospheric gases and particles trapped in the core’s ice layers, they can reconstruct past climates. But Bierman and his colleagues were curious about the bottom few meters of the 3,000 meter-long core. The researchers were looking for evidence of the typical bulldozer-like erosion, expecting to find little more than bedrock under the ice sheet.
Bierman was looking specifically for a chemical element that forms when cosmic rays hit the Earth’s surface. Since Greenland had been covered by ice for significant portions of the past few million years, Bierman didn’t expect to find much of this element. He said, “We thought that the ice sheet had been erosive enough to remove any material that predated the ice on Greenland.” Instead, he and his team were surprised to find plenty of it in the core samples. “We realized that in fact there was preservation of material from a long time ago,” Bierman said.
That meant the massive ice sheet didn’t bulldoze the tundra off Greenland’s surface, but simply froze it in place. How does a thick, moving sheet of ice leave an ancient landscape intact?
Warm-based or cold-based ice
The answer lies in whether the base of the ice is warm or cold. A warm-based glacier or ice sheet will have liquid water at its base. Water and ice together are highly erosive, and the combination often scours landscapes. A cold-based ice sheet, however, will simply freeze to the bed, preserving whatever lies under the ice, even as the upper layers of ice flow over it.
“We know that the ice at Summit is frozen to the bed today,” Bierman said, “And we infer that it must have been frozen to the bed in the past, because if it weren’t, the ice would be quite erosive and would have long ago removed the soil material.”
In addition to the element Bierman found, the base of the core samples also contained levels of carbon and nitrogen that indicate there is an ancient Arctic or tundra soil underlying this portion of the Greenland Ice Sheet. None of these would exist had the ice been warm-based and scoured the landscape bare.
Although this finding doesn’t hint at the distant origins of the ice sheet, it sheds light on its more recent geologic past. “It tells us that over the last 2.5 to 3 million years, the ice sheet has most likely been there,” Bierman said. This time period means that at least near Summit, the Greenland Ice Sheet has existed throughout the current ice age, which started about 2.5 million years ago, and survived a series of more temperate interglacial periods during that time. Bierman and his team hope to investigate the base of ice cores elsewhere in Greenland to see whether other parts of the ice sheet likewise survived, or melted and reformed. Even if some of the ice sheet retreated or melted away completely at some point during this time, the ice over Summit, at least, has remained stable.
Bierman, P. R., L. B. Corbett, J. A. Graly, T. A. Neumann, A. Lini, B. T. Crosby, and D. H. Hood. 2014. Preservation of a preglacial landscape under the center of the Greenland Ice Sheet. Science 344, doi:10.1126/science.1249047.
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