Victoria University Antarctic Research Expedition Science and Logistics Reports 1984-85: VUWAE 29

GRANITE HARBOUR SEDIMENTATION STUDY (K042) - Tony Macpherson

GRANITE HARBOUR SEDIMENTATION STUDY (K042) - Tony Macpherson.

Abstract

Sedimentation in Granite Harbour-Mackay Glacier system (140 km north of Scott Base) is dominated by the basal debris of the Mackay Glacier, the flow rate of which averages 226.5 ma −1, both summer and winter. Basal debris approximate 7.5% by weight of basal ice. Aeolian debris is an important sediment Source contributing about 23,000 tonnes a −1 into the system. Summer biogenic Activity results in diatomaceous muds and faecal material collecting in the deepest sinks within the harbour. Redistribution of sediments as a result of current activity will be identifiable from vertical water column profiles of both salinity and suspended particulate.

Introduction

The 1984-85 season was the final in a Victoria University-Oceanographic Institute sponsored project into the main sediment sources, transport processes and sinks operating in the Granite Harbour-Mackay Glacier sedimentation system. Granite Harbour is essentially a closed system, 140 km north of Scott Base on the Victoria Land coast.

Glacial Debris

Englacial debris was again sampled from overturned bergs about the southern grounding line of the Mackay Glacier. Two valves of a scallop were found 10 cm within the ice, on its basal surface, implying some "freezing on" occurs, at least at the edge of the tongue where the southern ice stream meets the ocean. Previously, it had been considered that the Mackay was wet based over its entire bottom and actively melted dropping basal debris immediately it entered Granite Harbour.

Samples of clasts were taken for provenance determinations. However, ice samples taken in order to determine sediment/ice ratios melted in transit to New Zealand. Measurements of sediment charged ice collected in 1983-84 show the sediment to be approximately 7.5% by weight of the basal ice. This is considered a minimum sediment concentration of the basal ice of the Mackay Glacier.

Aeolian Sediment

This season we attempted to determine the abundance of aeolian debris on the sea ice at eight sites within Granite Harbour.

Fine and very find sand accumulates on the surface of the sea ice and, therefore, when collected in late November, represents the amount blown there since the sea ice had formed (early winter?).

At each site, a 5 × 2 m area was staked out perpendicular to the dominant wind direction. Surface snow (varying in depth from 0.01 to 0.12 m) and the top 0.02 m of sea ice (which was chipped off) was collected and melted in a copper drum using a petrol immersion heater. Some contamination from flaking paint proved unavoidable. Preliminary results are tabulated below.

In general, sediment abundance varies with proximity to source with some samples having a high mud component as a result of diatoms frozen into the top 0.02 m of the sea ice.

For a few months each year, the harbour is ice free, and windblown sediment passes directly into the system. During the rest of the year, the harbour is ice covered and, therefore, collected windblown sediment could be rafted out of the system as the sea ice breaks up.

Considering the average weight for 10 m 2, there is an input of approximately 23 × 10 3 tonnes/annum onto the sea ice or directly into the system.

Table 8 Aeolian Sediment
Site Wt. collected material/10 m 2
84-1 23.09 gms
84-2 26.86
84-8 18.44
84-9 215.19
84-11 288.81
84-14 15.09
84-15 54.0
Averages Sites 1; 2; 8; 14; 15 = 27.5/All sites = 91.64

Sediment Trapping

Four strings of three single traps were deployed for approximately 56 days at various sites in Granite Harbour. The trap design and deployment is outlined in Pyne (1984).

The string from the deepest sink proved impossible to recover. However, all other traps had measurable quantities of mainly biogenic material. Samples were preserved in alcohol and will be analysed for biogenic silica, organic carbon and sedimentation rate.

In addition, S.216 from Rice University, Texas, deployed a string of seven traps immediately east of Granite Harbour. Logistic support was provided by K.042 as part of a co-operative programme. The Rice University party have analysed some of the sediment trapped from Granite Harbour in 1983-84.

Equipment

No major scientific equipment problems were experienced by K.042 this year. However, mechanical failure of the 5 to 7 year old toboggans supplied by Antarctic Division reduced the planned field programme.