A big surprise beneath the ice

March 5, 2011 - 0:0

A scientific study summarized online on Thursday in the journal Science shows that ice melts and re-freezes extensively at the base of the Antarctic ice sheet. This process apparently creates huge chunks of fresh ice that disrupt the usual layer-cake structure of the ice sheet, and the chunks can grow big enough that they change the shape and elevation of the ice sheet at the surface, the researchers reported.

The findings are a surprise. While scientists have long known that ice can melt at the base of miles-thick ice sheets from a combination of friction and pressure, their working assumption has been that this melt water functioned mainly as a layer of lubricant, sometimes speeding movement of glaciers.
The new findings suggest it is far more important than that. They imply that, at least in some places, melt water is a prime mechanism for contributing to the overall topography of the ice sheets, and may also govern important aspects of the behavior of the glaciers that ultimately empty ice into the sea.
Assuming that the findings stand up to scrutiny, glaciologists are now confronted with a new task: mapping the re-frozen chunks beneath thousands of square miles of ice and figuring out how they, and the process that created them, might alter the behavior of the ice sheets as greenhouse gases warm the planet.
The new paper covers only a section of Antarctica but it is setting off a rush to find the same kind of ice chunks in the Greenland ice sheet.
Robin E. Bell, a scientist at Columbia University’s Lamont-Doherty Earth Observatory who led the research, said the results were so unexpected that at first, her team could not believe it. The team, from seven nations, flew planes over a section of the ice sheet in eastern Antarctica in 2008 and 2009, shooting radar waves into the ice to map the hidden structure. This sort of thing had been done before, but improved equipment allowed the scientists to see more detail than in the past.
Ice sheets generally grow in layers year by year, as snow falls and gradually gets compressed by the weight of new snow on top. The layers can be seen clearly in radar images, draped over the mountains that underlie parts of eastern Antarctica, “kind of like if you dropped a giant tortilla onto mountain ranges,” Dr. Bell said.
Yet some of the detailed new radar images obtained by the team showed huge chunks of ice that appeared to have melted and re-frozen, looking nothing like the usual layers. When the images began to come in, “at first we thought there were errors in the data,” Dr. Bell said. “Or that the people on the planes were so tired they were seeing things.”
But the results proved to be solid, resulting in the paper that was released Thursday and is scheduled for future publication in the journal Science.
“In the last 15 years, we’ve gone from thinking there’s a little water under ice sheets, to thinking there are lakes the size of New Jersey under ice sheets, to thinking water can move around underneath ice sheets,” Dr. Bell said. “Now, we know that water can modify the basic structure of the ice sheets.”
The work was supported by the National Science Foundation, a leading American agency financed by taxpayers, and by other governments. Scientists from the United States, Britain, Germany, China, Australia, Japan and Canada took part in the work. Scientists flew geophysical instruments over a section part of the East Antarctic ice sheet to image what lay below.
(Source: The NYT)