GRANITE HARBOUR SEDIMENTATION STUDY (K042) - Tony Macpherson.
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.
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.
Mackay Glacier Movement
A pole network established in late November 1982 was resurveyed this season by Lands and Survey Department surveyors, both in November 1984 and January 1985. In addition, surface ablation was measured in November 1984. Results are tabulated below (Table 6).
In addition, a traverse along the length of the Mackay Glacier Tongue was attempted, to survey surface elevation. However, the tongue is badly crevassed and the survey was abandoned after several hours.page 15
Annual movement of the Mackay Glacier, above its grounding line, is 226.5 m, and on the floating tongue is 277 m. The two surveys this season show that there is no measurable change in the flow rate of the Mackay Glacier from winter to summer.
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.
Previous attempts to measure currents in the area have not revealed any currents greater than 5 cm/sec (measurement threshold of the instruments). This season an indirect method of determining currents by measuring salinity changes and suspended particulate concentrations was attempted.
|(a)||Salinity: 143, 250 ml aliquots have been taken from 22 sites within Granite Harbour. These were equally spaced within the water column; however, the density of the spacing was increased in strategic areas, e.g. around the Mackay Glacier Tongue and bergs within the harbour.|
|(b)||Suspended Particulate: 79 filtrations of 2000 ml aliquots were executed in the field through 0.8 mm "Nuclepore filters". Sampling density was similar to that carried out for salinity.|
The samples from these programmes have yet to be analysed; however, it is hoped that vertical water column profiles will give an indication of gross water mass movement. It is unlikely that tidal currents play a major role in redistributing sediment within the Granite Harbour system as average tidal flows are calculated to be approximately 0.1 cm sec−1. It is not known yet whether tidal flow is constant over the whole cycle or whether surges occur. Tide gauge measurements from previous years are interpreted to show that steady flow conditions are most likely.page 16
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.page 17
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 m2, there is an input of approximately 23 × 103 tonnes/annum onto the sea ice or directly into the system.
|Site||Wt. collected material/10 m2|
|Averages||Sites 1; 2; 8; 14; 15 = 27.5/All sites = 91.64|
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.
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.