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Friday, 16 February 2007

The Antarctic Ice Sheet

Written by Isaiah Norton
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ice shelf

Up-close view of a rough patch of ice shelf

Antarctica's Ice Sheet

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.

An ice sheet consists of a large amount of ice that stays in one place and does not appreciably melt or otherwise disappear - at least on the brief timescale of civilization. Antarctica's ice sheet is divided in two different sections: the Eastern- and Western- Antarctic Ice Sheet. The Eastern Antarctic Ice Sheet is grounded on a continental shelf above sea level, just like most glacial fields in the Northern Hemisphere. However, the Western Antarctic Ice Sheet rests on the sea floor, stuck in place under the massive weight of ice.


The basic principle of ice sheet formation is as follows. Ice sheets form over thousands to millions of years when the amount of snow falling during cold periods exceeds the amount of water escaping during summer. Over time, layer builds upon layer, re-freezing into glacial ice — like the formation of an ice dam on the eves of a roof. In some places, water melts and escapes directly as runoff, but in most of Antarctica the temperature is consistently below freezing, so ice is primarily lost at the margins where the shelf runs into the sea.



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.



The purpose of the voyage was to investigate the remaining shelf, and the surrounding sea floor on which the Larsen B used to rest. We took samples of sediment to be used for dating its history as described above. In addition, the ship's sonar systems created a detailed map of the topography of basins formerly covered by ice, useful for understanding the flow patterns. This data may provide clues as to the reason for the immense collapse,  yet to be determined.

I have included a few pictures to give an inPerson taste of the area.

ice shelf

View along a section of the remaining shelf











An iceberg. The brown patch in the middle, center-left of the picture is
probably sediment carried out to sea.









All pictures and text ©Isaiah Norton

Last modified on Sunday, 16 December 2012

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