The Antarctic Ice Sheet, covering 98% of the continent, is the largest ice sheet on Earth. The ice sheet averages ~1.6 km thick, but it is over 4 km thick where it overlies deep subglacial basins. It is divided in two by the Transantarctic Mountains, with the smaller West Antarctic Ice Sheet on one side and the larger East Antarctic Ice Sheet on the other. The continent contains about 90% of the world's glacier ice, which equates to about 70% of the entire world's fresh water. If the ice sheet melted, sea levels would rise globally by about 60 m. In most of the interior of the continent precipitation is very low, often as little as 20 mm per year, although precipitation rates rise towards the coast, where the air contains more moisture. Of the two components of the ice sheet, the West Antarctic Ice Sheet has received most scientific attention because of the possibility that it could collapse, or disintegrate rapidly. The reason for this potential instability stems from the fact that the ice sheet overlies a basin with a mean elevation below contemporary sea level. If the West Antarctic Ice Sheet were to collapse, global sea levels could rise by up to 6 m in a matter of centuries. As a whole the Antarctic Ice Sheet is a complex system, but we can identify six main components.
Glacial Geology: Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser © 2009 John Wiley & Sons, Ltd
1. A high-elevation plateau or 'Polar plateau'. Here there is little moisture and so snow accumulates very slowly such that it is measured in millimetres to centimetres per year. It has been suggested that peripheral thinning of the ice sheet, which has been observed recently, is balanced by interior thickening on the Polar plateau, but the absence of detailed mass balance studies of the ice sheet makes this difficult to establish with any degree of certainty. As a result, we do not know if the ice sheet is in negative or positive mass balance (see Section 3.1). The longest continuous deep ice cores have been drilled on the Polar plateau, providing information about the climatic and atmospheric records in Antarctica over eight glacial cycles spanning the past 740 000 years.
2. Peripheral ice streams. These ice streams are key components of the glacial system because they discharge the majority of the ice and sediment associated with the Antarctic Ice Sheet; for example, ice streams account for over 90% of the overall discharge from the ice sheet (Figure 2.2). Ice streams are narrow corridors of fast flowing ice with velocities in the range of 0.5-1 km per year, over two orders of magnitude faster than adjacent ice. Consequently, ice streams are typically heavily crevassed, with abrupt lateral margins (shear margins) between the ice streams and surrounding ice. The variables that control the location, dynamics and behaviour of these ice streams are uncertain, but in some cases they appear to overlie sedimentary basins of soft-sediment that may deform beneath the ice thereby assisting forward flow (see Section 3.3). To add to this complexity it is now known that ice streams can switch on or switch off through
2.1 The Antarctic Ice Sheet 9
time. For example, the Kamb Ice Stream, formerly known as Ice Stream C, in West Antarctica switched off around 150 years ago and has been stagnant ever since.
3. Fringing ice shelves. These are the floating continuation of outlet glaciers and surround nearly half of the Antarctic continent. Some of these ice shelves are very large, such as the Ross Ice Shelf, which is about the same area as France. Ice shelves perform a vital role in influencing the dynamics, and therefore the system response time, of upstream ice from inland Antarctica because they hold back or buttress ice flow from the glaciers or ice streams that feed them (Box 2.1). Calving of icebergs from ice-shelf termini accounts for approximately 90% of Antarctic ice loss. There is also rapid heat exchange between the ice shelves and the ocean beneath, meaning that they add considerable quantities of fresh water to the oceans. Finally, ice shelves are capable of entraining, transporting and depositing large quantities of glacial sediment (Figure 2.3) and dispersing it more widely as calved icebergs drift out to sea.
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