REGIONAL SETTING OF PRYDZ BAY

Bathymetry

Prydz Bay is an embayment along the Antarctic margin between 66E and 79E. It is bounded on the southwestern side by the Amery Ice Shelf, on the southeast by the Ingrid Christensen Coast, and by Mac. Robertson Land to the west, ending in Cape Darnley (Fig. F3). The eastern side of the bay has water depths from 200 to 300 m near the shelf edge forming Four Ladies Bank (Fig. F3). The bank surface slopes gently inshore and to the west. Along the Ingrid Christensen Coast, water depths reach 1000 m in the Svenner Channel. In front of the Amery Ice Shelf, the Amery Depression is mostly 600-700 m deep but reaches 1400 m in several closed depressions in the southwestern corner of the bay (the Lambert and Nanok Deeps) (Fig. F3). The western side of the bay is crossed by Prydz Channel, which runs from the Amery Depression to the shelf edge at 600 m below sea level. Prydz Channel separates Four Ladies Bank from Fram Bank, the shallow area around Cape Darnley (Fig. F3).

The steep continental slope on the eastern half of Prydz Bay is cut by submarine canyon tributaries and is overlain by slump deposits (O'Brien and Leitchenkov, 1997). On the western side of Prydz Bay, contours bulge seaward in the Prydz Channel Fan (Fig. F3), which slopes smoothly from the shelf edge to ~2700 m water depth. The head of Wilkins Canyon (Vanney and Johnson, 1985) is situated just west of Prydz Channel Fan and north of Fram Bank (Fig. F3). Wilkins Canyon runs north from the shelf edge before swinging northeast at ~65S. To the west of Wilkins Canyon is a ridge of drift sediment separating it from Wild Canyon, which has its head on the continental slope off Mac. Robertson Land (Kuvaas and Leitchenkov, 1992).

Circulation in Prydz Bay

Surface circulation in Prydz Bay is characterized by a closed cyclonic gyre adjacent to the Amery Ice Shelf (Fig. F4) (Smith et al., 1984; Wong, 1994). There is inflow of cold water from the east near the West Ice Shelf and outflow near Cape Darnley. In contrast to the Ross and Weddell Sea basins, Prydz Bay holds a relatively small volume of highly saline deep water. It has been suggested that this is related to the geography and bathymetry of Prydz Bay (Smith et al., 1984). Because of its closed circulation and lack of significant bottom-water production, water masses in Prydz Bay play a limited role in current activity beyond the shelf.

Circulation on the Continental Rise

Kuvaas and Leitchenkov (1992) interpret the deposits on the continental rise offshore from Prydz Bay to be the result of contour-current activity. These currents can be attributed to the activity of large Antarctic deep-water masses. Site 1165 is located within or near a large cyclonic gyre, known as the Antarctic Divergence (AD), between 60E and 100E (Fig. F4). Here, the eastward-moving ACC, driven by the prevailing westerlies, meets the westward moving Polar Current (PC). A study of deep-water circulation showed eastward flow north of 63S and a band of westward-flowing water between the AD and the continental rise (Smith et al., 1984). Because of the proximity to the AD, sedimentation in this region may, at various times, have been subject to transportation via the ACC or PC, or it may have circulated around the region in accordance with the gyre. The position of the AD may be a key control on the nature of sedimentation at this site.

Glaciology

The major glacial drainage system in the region is the Lambert Glacier-Amery Ice Shelf system. The catchment is ~1.09 million km2, representing ~20% of the East Antarctic Ice Sheet (Fig. F1) (Allison, 1979). The three largest glaciers are the Lambert, Fisher, and Mellor Glaciers that amalgamate in the southern Prince Charles Mountains to form the main Lambert-Amery ice stream (Fig. F1). They are joined by other glaciers of which the Charybdis Glacier, flowing from the western side of the Prince Charles Mountains (Fig. F1), is the largest. Most glaciers flowing into the Lambert-Amery system originate more than 200 km from the present coast, although a few small glaciers join the Amery Ice Shelf from the western side nearer the coast.

The ice-shelf grounding zone was thought to be a sinuous line running approximately east-west; however, more recent Global Positioning System and satellite image analysis has shown it to be in the southern Prince Charles Mountains, ~500 km upstream from the present seaward edge of the Amery Ice Shelf (Fig. F1). The ice reaches thicknesses of 2500 m in the Southern Prince Charles Mountains. This thins to ~400 m at the seaward edge of the Amery Ice Shelf, of which ~40% is snow that accumulates on the ice shelf and seawater ice that freezes onto the base (Budd et al., 1982).

Maximum ice velocities of 231-347 m/yr have been measured in the Prince Charles Mountains, whereas velocities up to 1200 m/yr have been measured for the centerline of the Amery Ice Shelf (Budd et al., 1982). The seaward edge of the Amery Ice Shelf is presently moving northward, but this is the result of spreading under its own weight rather than advance caused by an increase in mass balance (Budd, 1966). This spreading produces a major iceberg calving event every ~50 years, the last in 1963 (Budd, 1966).

The other source of glacial ice flowing into Prydz Bay is the Ingrid Christensen Coast, where ice cliffs and numerous relatively small glaciers enter the bay. The largest of these are the Sorsedal Glacier that flows south of the Vestfold Hills and the glaciers that contribute to the Publication Ice Shelf. The Svenner Channel consists of a series of swales seaward of these larger glaciers, suggesting that the swales formed at the glaciers' confluence with the Lambert Glacier during periods when grounded ice filled the bay (O'Brien and Harris, 1996). The western side of Prydz Bay has only a few small glaciers that flow into the Amery Ice Shelf. The coast between the ice shelf and Cape Darnley provides only a small amount of ice because the ice divide is close to the coast.

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