Some Antarctic ice is building up from bottom

March 4th, 2011 - 3:11 pm ICT by ANI  

Washington, Mar 4 (ANI): New findings by scientists working in the remotest part of Antarctica could reshape our understanding of how the ice sheet expands and moves, and how it might react to climate change.

Researchers-studying the invisible Gamburtsev Mountains buried under as much as two miles of ice- have discovered that liquid water locked deep under the continent’s coat of ice regularly thaws and refreezes to the bottom, creating as much as half the thickness of the ice in places, and actively modifying its structure.

Ice sheets are well known to grow from the top as snow falls and builds up annual layers over thousands of years, but scientists until recently have known little about the processes going on far below.

In 2006, researchers in the current study showed that lakes of liquid water underlie widespread parts of Antarctica. In 2008-2009, they mounted an expedition using geophysical instruments to create 3-D images of the Gamburtsevs, a range larger than the European Alps.

The expedition also made detailed images of the overlying ice, and subglacial water.

“We usually think of ice sheets like cakes-one layer at a time added from the top. This is like someone injected a layer of frosting at the bottom-a really thick layer,” said Robin Bell, of the Columbia University’s Lamont-Doherty Earth Observatory and a project co-leader.

“Water has always been known to be important to ice sheet dynamics, but mostly as a lubricant. As ice sheets change, we want to predict how they will change. Our results show that models must include water beneath,” he said.

The scientists found that refrozen ice makes up 24 percent of the ice sheet base around Dome A, a 13,800-foot-high plateau that forms the high point of the East Antarctic ice sheet, at 3.8 million square miles roughly the size of the continental United States.

In places, slightly more than half the ice thickness appears to have originated from the bottom, not the top. Here, rates of refreezing are greater than surface accumulation rates.

The researchers suggested that such refreezing has been going on since East Antarctica became encased in a large ice sheet some 32 million years ago. They may never know for sure: the ice is always moving from the deep interior toward the coast, so ice formed millions of years ago, and the evidence it would carry, is long gone.

Deeply buried ice may melt because overlying layers insulate the base, hemming in heat created there by friction, or radiating naturally from underlying rock. When the ice melts, refreezing may take place in multiple ways, the researchers say.

If it collects along mountain ridges and heads of valleys, where the ice is thinner, low temperatures penetrating from the surface may refreeze it. In other cases, water gets squeezed up valley walls, and changes pressure rapidly.

In the depths, water remains liquid even when it is below the normal freezing point, due to pressure exerted on it. But once moved up to an area of less pressure, such supercooled water can freeze almost instantly. Images produced by the researchers show that the refreezing deforms the ice sheet upward.

“When we first saw these structures in the field, we thought they looked like beehives and were worried they were an error in the data. As they were seen on many lines, it became clear that they were real. We did not think that water moving through ancient river valleys beneath more than one mile of ice would change the basic structure of the ice sheet,” said Bell.

Because the ice is in motion, understanding how it forms and deforms at the base is critical to understanding how the sheets will move, particularly in response to climate changes, said researchers.

The study appeared in the early online edition of the leading journal Science. (ANI)

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