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Victoria University Antarctic Research Expedition Science and Logistics Reports 1978-79: VUWAE 23

Sediment on ice-bergs and sea-ice in western McMurdo Sound (Peter Barrett and Phil Bentley)

Sediment on ice-bergs and sea-ice in western McMurdo Sound (Peter Barrett and Phil Bentley)

Sediment on the sea floor in polar regions has three main origins - floating ice, wind and biogenic production. Part of the VUWAE programme this year was to investigate sediment in transport today by wind and ice in western McMurdo sound.

Ice-bergs are a common feature along the western shore of the Sound. Most are floating and are trapped in the sea-ice, but several large bergs, many to 80 m above sea level, have grounded for some years near the site of DVDP15 (Barrett and Treves, et al. 1976, Fig. 2). Most of the bergs have no visible trace of glacial debris, although patches of wind-blown sand are quite common. This year, however, one small and two large ice-bergs were found in the New Harbour area with a continuous thin (0.3 m) cover of rock debris clearly destined for the sea floor. The largest (Plates XII and XIII) reached a height of about 20 m above sea level and was about 1 km long.

From our examination of the debris we concluded that it accumulated on the surface of a glacier that reached the coast and calved. There are a few abraded and striated pebbles, which must have had a basal glacial origin, but most are angular, which, together with the lack of mud-sized material, indicates a superglacial origin for most of the debris. The ice beneath the debris layer appears to contain no debris itself and therefore can not be the source.

A striking feature of the debris is the wide range of rock types - granite (several types), marble, schist, porphyry, dolerite, quartzarenite, and basalt. This range can at present be found only between the Koettlitz and Mackay Glaciers, and the berg most probably came from the Koettlitz Glacier area.

Our sea floor sampling (see earlier) showed that present day sea floor sediment in western McMurdo sound contains less than 1% gravel, which is the dominant size fraction in the ice berg debris, indicating that ice bergs of this type are not a major sediment source.

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The texture of sea floor sediment samples and of cores obtained from DVDP 15 several kilometres north of the MSSTS 1 site suggests that wind-blown sand, in contrast to ice-rafted debris, is a major source of sediment in western McMurdo Sound. This year we carried out a small sediment sampling programme to estimate annual sediment flux from this source and to document the texture and mineralogy of the sediment.

Sampling of the sediment was concentrated in New Harbour, but several samples were also collected from near MSSTS 1. Bathymetric stations in New Harbour were used as the main sample localities with subsidiary localities further seaward and nearer the coast. Sampling was mainly by digging a snow pit (Plate XV) of uniform size (20 × 20 cm) and sediment layers within the 'block' being retained.

In New Harbour the sediments are well sorted medium quartzo-feldspathic sand and silts, and there is a marked decrease in grain size seawards. Sediments near MSSTS 1 are of basaltic composition, and generally of fine sand and silt size.

Sediments are generally blown within a mobile snow layer up to 15 cm above the sea-ice surface, although wind velocity is an important variable. The main form of movement is by saltation, debris tending to accumulate either within fresh snow dunes, or on higher relief sastrugi and old upthrust ice blocks. Sediment accumulation is more concentrated near the coast (e.g. New Harbour) and around ice bergs, (Plate XIV) which serve as effective wind-breaks. Concentrations of sediment are often found in shallow pits in the ice, sediment melting and consequent concentration on and within older, harder layers of ice.

Initial estimates on sediment flux indicate 1 × 104 tonnes of sediment was deposited by the wind on the sea ice in New Harbour in 1978, an average rate of 0.5 mm/year The sand on the sea ice is transported and deposited over large areas of the western Ross Sea when the ice breaks out and is blown north. However the accumulation rate of sediment on the sea ice can be used to infer the flux of wind-blown sediment under open water conditions, which are believed to occur for almost 1/5 of the time. On this assumption the sedimentation rate for wind-blown sand in New Harbour averages 0.1 mm/year.

References

BARRETT, P.J., TREVES, S.B. et al. 1976. Initial report of DVDP15, western McMurdo Sound, Antarctica. Dry Valley Drilling Project Bull. No. 7, Northern Illinois Univ., DeKalb, 1-100.

NORTHEY, D.J., BROWN, C., CHRISTOFFEL, D.A., WONG, K., BARRETT, P.J. 1975. A continuous seismic profiling survey of McMurdo Sound, Antarctica - 1975. Dry Valley Drilling Project Bull. No. 5, Northern Illinois Univ., DeKalb, 150-166.