Proposed Programme

The purpose of this project was to investigate mud-carrying bottom currents in Granite Harbour. Such currents have been suggested as the explanation for the deposits of mud flooring the deeper parts of McMurdo Sound and the basins/harbours along the southern Victoria Land Coast (Barrett et al. 1983, Kelly 1986, Macpherson 1987). There is no single obvious source or depositions! process for the mud deposits although Anderson et al. (1984) suggested impinging geostrophic currents and subglacial meitwater as potential sources.

Sediment trapping experiments in Granite Harbour show that there is a greater sediment flux near the sea floor than higher in the water column (Macpherson 1987, and Dunbar et al. 1986). The existence of a layer of sediment charged water at the sea floor (nepheloid layer) implies periodic resuspension of sea floor sediments. These sediments are a mixture of terrigenous and biogenic material. Some from budget considerations as derived from within the harbour, but some must come from the open Ross Sea (Macpherson 1987). This circumstantial evidence suggested that bottom current activity occurs in Granite Harbour.

The 1988/89 season's programme was designed to check on bottom currents originating from beneath the MacKay Glacier Tongue (MGT) in Granite Harbour. A current meter mooring with eight small (400 cm²) sediment traps was deployed in 702 m of water near the snout of the MacKay Glacier for 53 days between 15 November 1988 and 6 January 1989. The current velocities 1 m above sea floor average between 6.9 and 4.3 cm/s with intermittent speeds up to 10 cm/s. These speeds are considered capable of maintaining the suspension of fine sediment to form a near bottom nepheloid layer, and this is confirmed by much higher sediment fluxes below 520 m. The dominant flow direction at this site is towards the glacier tongue (240°-270°T) but "dally" swings occur towards the south which are most likely related to the diurnal tidal cycle.

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This season's programme (1989-90) was to study the movement of sediment entering Granite Harbour from the Ross Sea by mooring the current meter and sediment traps on the sill the two main basins of the Harbour. The mooring was to be in place for about 2 months (November - January). We also planned to service the tide gauge at Cape Roberts established in November 1988, and if possible add meteorological instruments to record local climate data for future programmes. Inspection of the tide gauge in January 1989 has raised doubts about the permanence of the transducer installation which may not have survived ice foot breakout.

A further part of this season's work was related to a future proposal aimed at studying the Holocene Marine Glacial History in this area of the Ross Sea. We intended to carry out a reconnaissance survey of both coastal features in Granite Harbour and nunataks on the south side of the MacKay Glacier to find evidence of Holocene glacier expansion both at sea level and well above the present day MGT grounding line.