Australia: The Land Where Time Began
Antarctica - Ice Shelves
Ice streams are the most dynamic component of an ice sheet (Hughes, 1973, 1977, 1981; Bindschadler et al., 1987Denton et al., 1991; Goldstein et al., 1993Williams et al, 1993), with almost 90 % of ice flow from West Antarctica converging into ice streams. These ice streams eventually form ice shelves and glacier tongues following their separation from the bed at the grounding line and float, and slowly deform under their own weight. At the present ice shelves are grounded at depths of up to 1,300 m (Keys, 1990).
Antarctic ice shelves of the present have been divided into 2 glacial drainage categories, most being included into one or other of these categories:
1. Ice shelves that are extensions of marine ice sheets, such as the Ross Ice Shelf and the Ronne-Filchner Ice Shelf.
2. Those fed entirely by direct accretion of precipitation, basal freezing and ice flowing from sources that are predominantly of terrestrial origin, such as the George VI Ice Shelf and the Larsen Ice Shelf, the latter ice shelves being typically confined to coastal embayments. Ice shelves, such as the Ross Ice Shelf and the Ronne-Filchner Ice Shelf, that are larger, are stabilised by points of basal pinning, such as islands and other topographic highs, that buttress the adjacent marine ice sheet (Hughes, 1973, 1977, 1981).
The large convergent ice streams of West Antarctic flow into the Weddell Sea to form vast floating ice shelves, the Ronne-Filchner Ice Shelf and into the Ross Sea to form the and the Ross Ice Shelf. In East Antarctica there is a single large ice shelf, the Amery Ice Shelf. It has been argued (Hughes, 1977) ice shelves restrain the velocities of ice streams, thereby playing a key role in the the stabilisation of marine ice sheets, though this buttressing of ice streams by ice shelves has been debated by glaciologists.
The Ross Ice Shelf has an area of 536,000 km2, which is slightly larger than the Ronne-Filchner Ice Shelf, which is 532,000 km2. The drainage basin of these 2 ice shelves has a combined area that is about 62% of the total surface area of Antarctica. About 53 % of the total ice drained from Antarctica goes to these ice shelves, but they occupy only 10 % of the coastline.
According to the author1 calving of icebergs is the dominant process involved in the ablation of ice shelves, which has been suggested to possibly account for hundreds of metres of ice loss from the ice front per year. For the Ross and Ronne-Filchner ice shelves the only portion of them where bottom melting is significant is near the ice front, melt rates reaching 10 m/yr (Robin, 1979; Doake, 1985). In the case of the Amery Ice Shelf the base is frozen to within 70 km of the front of the ice shelf (Budd, Corry & Jacka, 1982). The margins of the ice shelves are constantly in a state of flux as a result of the continual forward movement of the shelf and the removal of ice in icebergs that calve from the ice front (Keys, 1990).
The Larsen, Shackleton, West, Fimbul, Riiser-Larsen, and Lazarev Ice Shelves, that are fringing ice shelves, are also large features along the coast of Antarctica, that occupy a significant portion of the Antarctic coast, East and West. A key role is played by buoyancy of the ice shelf, and the protection by the sea ice, in maintaining the fringing ice shelves, as there is a lack of confining topography,
Ice shelves that are confined are smaller than fringing ice shelves, and are maintained within large valleys, bays, and archipelagos by physiographic restraint. There are several confined ice shelves that are present on the coasts of Marie Byrd Land and Palmer Land, including the George VI Ice Shelf, the Abbot Ice Shelf and the Getz Ice Shelf.
It has been argued that, as with the situation at the present, ice shelves can only exist in polar and subpolar settings (Robin & Adie, 1964). The author1 suggests the equilibrium line altitude - the level at which accumulation and ablation are balanced - occurs below sea level (Robin, 1988) is another requirement for the existence of ice shelves. The region of the Antarctic Peninsula is the location of the warmest Atmospheric conditions where ice shelves can be present to the south of -7oC mean annual isotherm, with ice shelves present along the western side of the Antarctic Peninsula in protected regions where flow is confined by valley walls. The -7oC mean annual isotherm is situated almost 350 km further north on the eastern side of the Antarctica Peninsula than on the western side, and it is on the eastern side where the vast Larsen Ice Shelf is situated.
The author1 suggests the ice shelf is therefore sensitive to changes in the glacial regime of these outlet glaciers, as well as the ice caps that feed them. The western Weddell Sea has a perennial covering of sea ice which protects the ice shelf from wave erosion, though the calving wall of the northern Larsen Ice Shelf is retreating at 1 km/yr, this retreat being believed to be the result of a recent warming trend in the region (Doake, 1982). It is not yet certain if the stability of the outer glaciers that flow into the Larsen Ice Shelf will be influenced by the retreat of the ice shelf within the next few decades.
The George VI Ice Shelf, which drains ice caps flowing from Palmer Land, is the largest in the region of the western Antarctic Peninsula is an example of an ice shelf that is confined by valley walls. It comprises more than 90% of ice that flows into Marguerite Bay, the remainder draining from Alexander Island. The Wordie Ice Shelf was another ice shelf flowing into Marguerite bay, but this ice shelf collapsed within the last 2 decades (Doake & Vaughan, 1991).
The George VI Ice Shelf base is melting at an average rate of 2 m/yr and retreating at 1 km/yr (Potter & Paren, 1985). The cause of this melting is believed to be the circulation of water masses beneath the ice shelf, that are relatively warm, and this illustrates the importance of oceanographic circulation in the stability of ice shelves (Potter & Paren, 1985; Jenkins & Doake, 1991; Jacobs et al., 1992). It has been suggested (Hughes, 1981) that ice shelf thinning could reduce the back-stress exerted by the ice shelf, ultimately leading to grounding line retreat. Parts of the George VI Ice Shelf have been found to be thickening locally, and this is believed to be a response to increased rates of precipitation several hundred years ago. Uncertainty regarding the present mass balance of this glacial system is the result of the interplay between thickening of the ice shelf and melting of the ice shelf base (Bishop & Walton, 1981; Potter & Paren, 1985; Jacobs et al., 1996).
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|