The Mackay Glacier / Granite Harbour System (Ross Dependency, Antarctica) - a Study in Nearshore Glacial Marine Sedimentation

Abstract

Granite Harbour (77° south) is a glacially scoured embayment twenty kilometres across and up to 900 metres deep on the Victoria Land coast of Antarctica. Most of the ice entering the Harbour comes from the Mackay Glacier, a 500 metre thick outlet glacier from the East Antarctic Ice Sheet. The Mackay is largely wet-based, flowing at a rate of 214 m yr-1, and terminates as a floating ice tongue more than four kilometres long. Granite Harbour is of normal marine salinity as it receives virtually no input from meltwater streams, and as it is ice covered and microtidal, currents and waves have little influence on sediments. Sediment entering and being deposited within the Harbour was sampled during the austral summers of 1983 to 1985, and rates of transport and deposition determined. More than 60 textural analyses from 18 shallow penetration gravity cores show that the sea floor sediment is dominantly sandy mud with a few scattered clasts. The sea floor sediment is divided into seven lithofacies which correspond to divisions of glacial marine sedimentation made by Anderson et al. (1980) and Powell (1984). A bathymetric map of the Harbour has been compiled and is used as a base for mapping sediment patterns. From a number of debris-rich bergs of overturned Mackay basal ice, it has been calculated that about 295 000 tonnes yr-1 of sediment enters the Harbour as basal debris, but melts out beneath the Mackay Tongue within one to two kilometres of the grounding line. Textural studies indicate that about one third of the basal sediment is carried into the deeper parts of the Harbour, probably by lateral advection, in low velocity bottom-hugging currents from beneath the Mackay Glacier Tongue. The remainder is inferred to form a prograding wedge of sediment seaward of the grounding line. Lateral advection is also inferred for transporting up to 45 000 tonnes yr-1 of biogenic-rich mud from shallow areas or from outside the Harbour and depositing it within the Harbour basins. Less important processes that introduce quantifiable amounts of sediment are the deposition by dominantly katabatic winds of about 1 810 tonnes yr-1 of coarse silt/fine sand onto the Mackay Glacier surface and the sea ice, and the settling from free suspension in the water column of about 340 tonnes yr-1 of biogenic rich debris. Sediment accumulation rates have been determined using the decay profiles of unsupported 210Pb within two sea floor cores from the deep areas of Granite Harbour. The rate has averaged 2.48 mm yr-1 over the past 100 years. This rate is used to provide as estimate of sediment depostion of 150 000 tonnes yr-1 for the area of the Harbour deeper than 400 m (about 50%). Ice-rafted detritus is present within sea floor sediments from the Harbour, but most has characteristics associated with supraglacial debris. An implication of this observation is that coarse debris, within the recent veneer of Antarctic continental shelf sediment, probably represent ice-rafted supraglacial debris, which is likely to be more abundant during interglacial rather than glacial periods.