Amery Ice Shelf: home of the Troglodytes
The Amery Ice Shelf – Ocean Research (AMISOR) project recently completed its third field season. This summer the project team drilled the ice shelf at the same geographic location as had the 1968 four man wintering party of Max Corry, Neville Collins, Alan Nichols, and Julian Sansom. These four have come to be known as the Troglodytes as they had to establish an underground tunnel system to connect various parts of the camp after heavy autumn snowfalls buried their fibreglass caravans.
The objective of this year's activity is to investigate melt and refreezing processes at the base of the ice shelf, processes which affect both the Antarctic ice sheet and the surrounding ocean. The 2001-2002 drill site is in a region of basal freezing, first identified in samples obtained to a depth of 315 m by the 1968 party, and subsequently confirmed from field and remote sensing studies.
Accumulating marine ice at the base of the shelf presented new challenges for the hot water drilling team. Deployment was delayed by inclement weather in December, when Davis had record levels of cloud cover and snowfall days, but after that, lessons learned from previous seasons resulted in a routine and smooth operation in generally fine conditions.
A 400-500 mm diameter borehole was melted through the ice shelf – at this position some 479 m thick – in less than 48 hours. There was some confusion when the drill head came into contact with seawater only 376 m down, rendering the water level sensor in the subsurface reservoir useless as an indicator of actual breakthrough. Drilling continued until only the last few wraps of hose remained on the winch, by which time it was believed that the drill head (at 490 m depth) was in the ocean cavity beneath the shelf. A borehole caliper tool then confirmed the actual depth of the base of the shelf at 479 m.
Repeat measurements of parameters including ocean salinity, temperature and current velocity were made in the cavity, and seawater samples taken. A new hot water coring head was employed to obtain small samples of the ice at four depths: at 240 m in the body of continental ice, at 290 m near the top of the refrozen marine layer, at 360 m immediately above where the hydraulic connection was achieved, and at 390 m. Ice at 390 m showed evidence of brine channels within, offering a likely explanation for the seawater connection at 376 m and prompting the label honeycomb ice. Detailed analysis of all core samples at the Antarctic CRC will enable a better picture of the fabric of the ice at different depths.
The AMISOR project, and in particular the 2001-02 field team of Russell Brand, Alan Elcheikh, Adam Drinkell, Shavawn Donoghue, Doug Thost and Mike Craven, owe a great deal to the willing assistance received from many AAD and Antarctic CRC people, as well as ANARE expeditioners (especially aircrews) and ships' crew. The current program is heavily indebted to all who have gone before, particularly the four intrepid Troglodytes. We gained but a mere inkling of what conditions during that 1968 winter must have been like for them.
Mike Craven, Glaciology Program, AAD