REGIONAL SETTING OF PRYDZ BAY

The continental slope on the eastern half of Prydz Bay is steep with submarine canyon tributaries and slump deposits (O'Brien and Leitchenkov, 1997). On the western side, contours bulge seaward in the Prydz Channel Fan (Fig. 3), which slopes smoothly from the shelf edge to its edge at ~2700 m water depth. The head of a submarine canyon (Wilkins Canyon) (Vanney and Johnson, 1985) is situated just west of the Prydz Channel Fan, north of Fram Bank (Fig. 3). It runs north from the shelf edge and then bends northeast from ~65%S. To the west of Wilkins Canyon is a ridge of drift sediment that separates it from Wild Canyon, which has its head on the continental slope off Mac. Robertson Land (Kuvass and Leitchenkov, 1992).

Circulation and Water Masses in Prydz Bay
The circulation in Prydz Bay is characterized by a closed cyclonic gyre adjacent to the Amery Ice Shelf (Fig. 4) (Smith et al., 1984; Wong, 1994). There is inflow of some 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 as 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, between 60°E and 100°E (known as the Antarctic Divergence [AD]) (Fig. 4). Here, the eastward-moving Antarctic Circumpolar Current (ACC), driven by prevailing westerlies, meets the westward moving Polar Current (PC). A study of deep-water circulation showed eastward flow north of 63°S 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 bottom-current activity at this site.

Water Masses in the Prydz Bay Region
Circumpolar Deep Water (CDW) (0° to 2°C; 34.50 to 34.75 salinity), a large mass of cold, moderately saline water along with colder and more saline Antarctic Bottom Water (AABW) (0°C; 34.60 to 34.72 salinity), are the major bodies of deep water carried by the ACC (Smith et al., 1984). AABW is not actively formed in Prydz Bay, where waters are only moderately saline. This large water mass is primarily formed during winter on the margins of the Weddell and Ross Seas. Additionally, large ice shelves are undercut by inflowing water, and brines are winnowed out, making this Ice Shelf Water (ISW) a cold and highly saline component of AABW (e.g., Grobe and Mackensen, 1992). The overlying CDW also comprises a large portion of the water mass and its composition is influenced by North Atlantic Deep Water (NADW) flowing in from the north. Since NADW is a warmer water mass, changes in contribution to CDW have an impact on the development of sea ice in Antarctica.

Glaciology
The major glacial drainage system in the region is the Lambert Glacier-Amery Ice Shelf system. The system drains ~1.09 million km² representing ~20% of the East Antarctic Ice Sheet (Fig. 1) (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. 1). They are joined by other glaciers of which the Charybdis Glacier is the largest. It flows from the western side of the northern Prince Charles Mountains (Fig. 1). 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.

The grounding zone was thought to be a sinuous line running approximately east-west; however, more recent Global Positioning System (GPS) 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. 1). 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 an advance caused by an increase in mass balance (Budd, 1966). This spreading produces a major iceberg calving event approximately every 50 years; the last one being 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 are the Sorsedal that flows south of the Vestfold Hills and the glaciers that contribute to the Publication Ice Shelf. The bedrock swales of the Svenner Channel are seaward of these larger glaciers suggesting that they formed at their confluence with the Lambert during periods when the bay was occupied by grounded ice (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.