'warm' glaciers can trap and transport silt
August 14, 2003
Edward B. Evenson, professor of earth and environmental sciences, says his team's 12-year study of the Matanuska Glacier in south-central Alaska sheds light on a riddle that has long baffled geologists - how glaciers are able to pick up and transport silt.
The Nature article, titled "Stabilizing feedbacks in glacier-bed erosion," explains how glaciers "often erode, transport and deposit sediment much more rapidly than nonglacial environments." Evenson and his students and colleagues have published 20 articles related to that topic since 1996.
When air temperatures rise and cause the ice on a glacier's surface to melt, Evenson and his group found, water penetrates through the glacier until it reaches the bottom. There, under massive pressure from the weight of the glacier above, the water becomes supercooled and its freezing point drops by a fraction of a degree, As the water flows up and out from under the glacier however, the pressure is reduced and some of the supercooled water re-freezes to form what geologists call "frazil ice" - minute crystals of ice that float atop the remaining water.
As the pressure from above continues to lessen, the frazil ice becomes larger and begins to attach to the bottom of the glacier. There, it picks up particles of silt from the dirty, silt-laden water flowing beneath the glacier, much the same as an air filter in a car removes dust and dirt from the air. As the glacier flows, grains of silt are trapped in the interstitial spaces of the ice crystals. The sediment and ice eventually separate from each other, producing layers of clean and dirty ice. This process goes on all summer as melting water penetrates down to the glacier bed.
Evenson's analysis of the "basal-stratified ice" beneath the Matanuska Glacier also found traces of tritium, an isotope of hydrogen that has been released into the atmosphere during the past 50 years by nuclear weapons testing.
"The tritium tells us that this basal-stratified ice is young," says Evenson. By contrast, he says, the ice on top of the glacier, which is not nearly so dynamic, is estimated to be several hundred years old.
The supercooling process does not occur in the "cold" glaciers of Antarctica and Greenland, where mean annual temperatures remain below freezing and prevent melting water from penetrating to the bottom of the ice sheet, says Evenson.
Among the world's "warm" glaciers, which are found in Canada, Alaska, Iceland and many mountain ranges, the supercooling process is likely to occur only in those glaciers with what Evenson calls a favorable geometry.
Glaciers located in flat areas, like the Matanuska, says Evenson, are more likely than most mountain glaciers, including those in the Alps or the Cascades, to permit the supercooling necessary for basal-stratified ice to form. As a flatland glacier advances into a basin, the glacier's shape changes, imposing the necessary pressure on the water that has penetrated to the glacier bottom.
"Because most mountain glaciers are moving downhill, the angle between the slope of the glacier's surface and the slope of its base is not right. You need a relatively flat glacier and a subglacial basin, or 'overdeepening,' and then supercooling occurs," Evenson says.
Evenson worked with researchers from the University of Buffalo, Pennsylvania State University, Michigan State University and the Cold Regions Research and Engineering Laboratory (CRREL) of Hanover, N.H., which helped fund the study. The researchers also received support from the National Science Foundation.
After studying the Matanuska Glacier, Evenson and his colleagues tried to determine if basal-stratified ice was forming in a similar manner at other warm glaciers. At the Malaspina Glacier in Alaska, they found vents full of frazil ice, indicating that the same process was occurring. In the glaciers of Iceland, they found supercooled water in over-deepened glaciers. They have concluded that this supercooling process occurs on all warm glaciers where the geometry is right, and that this same process most likely governed Ice Age glaciers. The Laurentide Ice Sheet, says Evenson, was warm along its margins and cold in its interior. As it retreated north between 18,000 and 10,000 years ago, the glacier left behind deposits of thick till, drumlins and eskers - demonstrating that it was warm at its margin.
Evenson and his students drilled hundreds of holes in glaciers, using a hose that sends boiling water through a glacier at a rate of about 1 meter per minute. By injecting a fluorescent dye and monitoring the vents through which the water comes out, the researchers were able to determine the shape and routing of the subglacial plumbing system.
Gregory Baker, a Lehigh-trained geophysicist now with the University of Buffalo, used a ground-penetrating laser to look through the glacier and measure its thickness and the amount of debris underneath it.
Editor's Note: Besides Evenson and Baker, the Nature paper was co-authored by Richard B. Alley (Penn State), Daniel E. Lawson (CRREL), and Grahame Larson (Michigan State).
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