Icebergs

Icebergs are masses of ice that have broken away from marine glaciers. They have a wide variety of shapes and sizes, depending on source and decay process and they may be floating or grounded on the seafloor. By definition, icebergs show more than 5 m of ice above sea level and are 100 m2 or more in area. The longest iceberg ever recorded was about 180 km long (Wordie and Kempe, 1933). Its mass would have been about 1012 tonnes. The median width of all Antarctic icebergs is probably less than 200 m, although tabular bergs are larger (Budd et al., 1980; Neshyba, 1980; Keys, 1984; Pyne, 1986).

Icebergs are the largest objects to have floated in the sea and can be a serious hazard to shipping. Three ships are thought to have sunk or been written off after striking Antarctic icebergs in the last 100 years (Table 4.1) and many others have been similarly threatened. Production of icebergs is the main way the Antarctic ice sheet loses mass, so it is important to understand and quantify iceberg sources, production, total mass, distribution and decay. Icebergs cause local mixing of the water through which they drift and in pack ice can create open water in their wake. Speculation persists about their possible future use as sources of fresh water (e.g., Weeks and Campbell, 1973; Lovering and Prescott, 1979; Hantke, 1986). Various aspects of Antarctic icebergs have been reviewed by Weeks and Mellor (1978), Schwerdtfeger (1979) and Kristensen (1983). This section examines the life history and some other characteristics of bergs in the Pacific Sector.

The mean annual production of Antarctic icebergs has been estimated to be 0.6-2.3 x 101S kg or 750-3,000 km3 (Orheim, 1985). This estimate is made complicated by the large size of the Antarctic, complex iceberg distribution and uncertainties in decay rates, but is equivalent to some hundreds of thousands of bergs. Production in the Pacific Sector is probably almost half of this Antarctic total. Most icebergs (perhaps 60-80% by volume) separate from the flat-topped ice shelves. Tabular icebergs are, therefore, common (e.g., Wright and Priestley, 1922). In addition, a significant proportion of bergs come from ice streams and active outlet glaciers.

Icebergs are produced (berged or calved) by different mechanisms. These are still not completely understood but are believed to be affected by: weaknesses in ice shelves or glacier tongues including surface and bottom crevasses and hingelines; lateral and vertical stresses developed during glacial thinning, flotation, vertical vibration, horizontal bending and impact of colliding icebergs; ocean swells, storm surges, tsunamis and tides; seafloor or shore topography (e.g., Wright and Priestley, 1922; Swithinbank et al., 1977: Holdsworth, 1978, 1985; Robin, 1979; Shabtaie and Bentley, 1982; Keys, 1984). Different mechanisms and different marine glaciers may produce icebergs at different rates and sizes.

Icebergs are most numerous near the coast but there is considerable spatial and temporal variation in their concentration. In the Pacific Sector, the highest concentrations of drifting bergs occur in the Amundsen and Bellingshausen Seas (Romanov, 1984) (Fig. 4.5). Average coastal concentrations vary spatially between about 5 and 9 bergs detected by radar per 1,000 km2. Maximum concentrations of drifting bergs in the western Ross Sea are at least as high as 25 bergs per 1,000 km2. However, well over 200 bergs might be visible by eye in the most concentrated areas (Deacon, 1984) near active iceberg-producing glaciers and because of icebergs drifting in clusters or collecting (grounded) on shallows (Keys, 1983, 1985). Very high concentrations, up to four bergs per square kilometre have been

0 0

Post a comment