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Friday, 16 February 2007

The Antarctic Ice Sheet - Page 2

Written by Isaiah Norton
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The continent of Antarctica is covered by a thick sheet of ice, averaging over 2 kilometers deep. This frozen mass holds some ninety percent of the fresh water in the world, although much of the continent is paradoxically very dry. Precipitation levels measured at single-digit centimeters per year are common, making the icy expanse in fact one of the "driest" places on Earth.


Studying the History of the Sheet

Many parts of the ice sheet are in constant motion, albeit very slowly, due to the force of gravity pushing the mass from high to lower elevations. During this motion, the weight of the ice exerts tremendous pressure against the rocky base, which grinds away gravelly sediment. Some of the sediment is incorporated into the ice and carried along with the flow. As the ice is pushed into the ocean, it eventually breaks off, forming icebergs. Bits of gravel remain frozen inside each iceberg as it is carried away from the continent by wind and currents.

As the icebergs melt, gravel is deposited in the ocean, falling slowly through the water. Over time many icebergs calve, float to sea, and melt. The sediment they deposit builds up on the sea floor, creating a record of past icebergs. Like the rings of a tree, sediment is deposited in layers on the seafloor. Sediment layers indirectly represent how many icebergs broke away during a certain time period - a proxy for what happened to the whole ice shelf during that period.

But the ocean is full of many things other than glacial sediment - plankton for instance. So each layer also contains deposits of dead plankton that have settled through the water column alongside the sediment. By studying the age of the plankton (and other once-living matter) in each layer, a history of the sediment in one particular area can be constructed. However, collecting sediment samples is difficult, so coverage of the continent is limited, and a complete historical picture has yet to be constructed and agreed upon.


In the spring of last year, I had the incredible opportunity to see part of the ice sheet up close, on a ship near the Antarctic Peninsula. The area to which we sailed had been covered by a large ice shelf, the Larsen B.  In 2002, a large section of the shelf collapsed and floated away as hundreds of icebergs.


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Last modified on Sunday, 16 December 2012

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