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Victoria University Antarctic Research Expedition Science and Logistics Reports 2002-03: VUWAE 47
1. Popular Summary of Scientific Work Achieved
1. Popular Summary of Scientific Work Achieved
SUMMARY
This project investigated water flow and sedimentation beneath the McMurdo Ice Shelf south of Ross Island. Its purpose is to help understand the past (and future) behaviour of the huge Ross Ice Shelf, which lies immediately to the south. Team members from the Alfred Wegener Institute used a hot water drilling system (Nixdorf et al., 1994) to melt 0.6-m-wide access holes through the ice shelf in two locations 5 and 12 km east of Scott Base, thus allowing measurements and samples to be taken from the water column and sea floor beneath. The sites are relatively remote, being covered by ice 70 and 143 m thick and with sea floor 926 and 923 m below sea level respectively.
Water column measurements from the two locations show that the main current direction is to the east from McMurdo Sound through to the Ross Ice Shelf, with speeds averaging 5 to 7 cm/sec but at times reaching 17 cm/sec. Flows at a third and shallower location at the ice shelf edge below the sea ice off Scott Base were much stronger – up to 60 cm/sec. Water column profiles of salinity and temperature are similar to those found 25 years ago at the first ever hole drilled through the Ross Ice Shelf 400 km south at J9. These waters are some of the coldest and densest in the world, helping drive deep ocean circulation as they flow north into the Pacific Ocean.
The sea floor at both sites is soft mud with siliceous algae and calcareous microfossils. The former are well known in open Ross Sea surface waters, being swept in by currents. The latter live in mud and were not expected because of the cold and deep corrosive waters. One of the cores passed down into a stony glacial deposit believed to represent sediment released from basal ice grounded nearby more than 10,000 years ago when ice filled most of the Ross Sea after the last glaciation. However penetration of low energy (3.5 kHz) seismic waves 300 m into the soft sea floor shows that the sites are likely to have been too deep to have been compacted or eroded by grounded ice. This observation, together with the fine-grained nature of the sediment confirms the basin's suitability for the planned 1000-m-deep ANDRILL hole for recovering a long and sensitive record of Ross Ice Shelf history. Current speeds in the water column also indicate its technical feasibility.
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Fig.1 Hut Point Peninsula and the McMurdo Ice Shelf to the SE, showing the bathymetry of the broad channel connecting the waters of McMurdo Sound to the NW with those beneath the Ross Ice Shelf to the east. The intersecting lines mark seismic surveys run by the Institute of Geological & Nuclear Sciences (Melhuish et al., 1995; Bannister and Naish, 2002; Horgan et al., 2003) for imaging the geometry of the strata filling the basin (see fig. 2). The red square is the current monitoring site at the shelf edge.
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Fig. 2 The Ross Embayment (image from NASA) and a composite seismic section beneath the ice shelf south of Ross Island, showing the setting for the K-042 operation. Seismic data are from Bannister and Naish, 2002, on the left and Melhuish et al., 1995 on the right.