Antarctica is comprised of two main grounded ice sheets; the West Antarctic and the East Antarctic Ice Sheets separated by the Transantarctic Mountains. Another significant region of grounded ice exists in the Antarctic Peninsula where a series of ice caps and glaciers are located (Fig. 12.1). The present volume of ice in Antarctica is around 25.4 x 106km3 (equivalent to 57 m of global sea level) of which 91% is within the East Antarctic Ice Sheet (Lythe et al., 2001). The maximum ice sheet surface elevation of 4093 m occurs at Dome Argus which, along with several other subsidiary ice domes connected by ridges, forms a major ice divide through the centre of East Antarctica. Ice thickness in the central regions of the continent varies between 2800 and 4500 m mainly as a consequence of bedrock topography which is known to vary spatially by more than a vertical kilometre over just a few km. If the ice were to be removed from East Antarctica, the bedrock surface would be largely above sea level. However, if the same were to happen over West Antarctica, even accounting for isostatic rebound, the bedrock would remain below the modern sea level. Thus, the West Antarctic Ice Sheet is often referred to as a marine-based ice sheet.

Ice drains from the interior domes via fast flowing rivers of ice known as 'ice streams', where the dominant method of flow is by sliding or basal sediment deformation. These contribute grounded ice to numerous floating 'ice shelves' that surround the grounded ice continent. The Ross and the Filchner-Ronne ice shelves are Antarctica's largest, with areas of 472,000 and 430,000 km2, respectively. Icebergs, usually of tabular form, calve from the marine margins of ice shelves, where ice thickness is usually 250-300 m. This process represents an important mechanism by which ice is lost from the ice-sheet system, accounting for 75-85% of all ice lost from Antarctica (Jacobs et al., 1992). A significant amount is also lost due to bottom melting near the grounding lines of some outlet glaciers, particularly those where the grounding line is well below sea level (Rignot and Jacobs, 2002).

Figure 12.1: Location map and surface elevation of Antarctica with place names mentioned in the text. Contours are at 500 m intervals.
0 0

Post a comment