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Victoria University Antarctic Research Expedition Science and Logistics Reports 1992-93: VUWAE 37

Proposed programme

Proposed programme

The project is to determine the timing and rate of retreat of the edge of the Antarctic icesheet across the Ross continental shelf since the last glacial maximum 20,000 years ago to resolve the present substantial differences of opinion. This involves coring from fast sea ice off the Victoria Land coast in Granite Harbour and northwards, and from a ship in the central Ross Sea.

The cores will penetrate the recent layer of mud, deposited under sea ice/open water conditions like today's, and into diamictite beneath, deposited when the shelf was covered by the extended ice sheet. The corer has been designed to penetrate and recover up to 6 m of both soft mud and diamictite in water depths to 1000 m. The timing of glacial retreat is obtained from carbon dating contemporaneous shell material and organic carbon in organic rich sediment just above the diamictite. Thorium 230 dating by mass spectrometry on suitable carbonate materials may also be attempted.

The first stage is to check the local retreat history from cores with that developed from terrestrial ages in the McMurdo Sound area. The second stage of this programme extends the study north of Granite Harbour and out into the Ross Sea using a ship and the sea ice where appropriate.

The volume and extent of the Antarctic ice sheet at the height of the last glaciation is still virtually unknown, as is the history of the retreat of its margin to its present position.The question is of interest to those studying the structure of the earth (Lambeck 1990) as well as those concerned with the Antarctic contribution to past and future sea level change (SCAR1989). There is a widely held view that the Antarctic ice sheet expanded to the edge of the continental shelf, increasing in volume by some 30% (Kellogg et al. 1979, Denton et al. 1989, 1991). This is based partly on numerical modelling and partly on the wide extent of over-compacted diamictite beneath a metre or so of mud on the continental shelf (Anderson and Molnia 1988).Others cite evidence of the old beaches on the Victoria Land coast and in Prudz Bay which are raised only a few metres to indicate past ice volumes were only a few percent larger than today (Calhoun et al. 1992).

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The only attempt that we know which uses this approach to establish the retreat history of the ice sheet anywhere on the Antarctic continental shelf has been by Domack et al. (1990, 1991) from the Oates Coast. They obtained ages from piston cores for the beginning of post-glacial mud accumulation there ranging from 9000 to 3000 years ago. However, there were problems with either the dating procedure or the coring as the pattern of ages with core depth was irregular and inconsistent. Piston corers are known to blow away the top few centimetres (and more at times) of surface sediment, and occasionally disturb sediment as it pierces the sea floor in a few seconds. Our vibracoring system has been designed to take undisturbed core by vibrating its way down over a period of 15 minutes. Thus we should from the outset have better quality core material to date.

The Ross continental shelf is an area of particular interest because of concern over the behaviour of the West Antarctic Ice Sheet (Bindschadler 1990). West Antarctic ice may have reached the edge of the continental shelf 20,000 years ago, though Drewry (1979) has argued for much more limited extent of the ice in the Ross Sea at that time, which he considers is more in keeping with the relatively small negative regional gravity anomaly over the Ross continental shelf.

In the last few years research by VUW personnel (Barrett et al. 1983; Ward, 1984; Ward et al. 1987; Macpherson 1987; Pyne et al. 1991.) has substantially improved our understanding of sedimentary processes along the Victoria Land coast, and has led to the development of techniques and expertise for sampling the sea floor from the fast ice. We are now interested in using these skills to acquire data on the retreat history of the ice margin since the last glacial maximum.

This season's programme was to plot the retreat of grounded ice, firstly by coring from the sea ice in the area of Granite Harbour and secondly by coring from an ice breaker in McMurdo Sound and northward in the Victoria Land basin. It became clear during the year that the ice breaker support could not be guaranteed. At best we could only expect 1-2 days ship time during tanker refuelling and the details of the ship's coring and echo-sounding equipment were not forthcoming from NSF via NZAP for our planning. Consequently we cancelled the ship based programme. However an opportunity arose at short notice to use the ice breaker in early February 1993. Our revised programme objectives were to collect 3.5 kHz data (with the ice breaker's echosounder) in Granite Harbour and around the Nordenskjold Ice Tongue to determine the bathymetry and thickness of the "post glacial mud blanket". The second objective was to collect 3.5 Khz data offshore of Cape Roberts to determine detailed bathymetry in the area of proposed drill sites for the Cape Roberts Project.

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