IMMEDIATE SCIENCE REPORT
K042 : Last Retreat of the Antarctic Ice Sheet in the Ross Region
Antarctica New Zealand November 1993 - December 1993
1. Popular Summary
The main aim of this programme to recover sea floor cores from Granite Harbour to study the Holocene retreat of the MacKay Glacier has been unfortunately postponed because of the loss of our developmental vibracorer this season. Recovery options for the corer are currently under consideration but if recovery is attempted then this will probably be done from the sea ice in November 1994.
2. Proposed Programme
The principle objective of this seasons programme was to recover sea floor cores from Granite Harbour to date and track the retreat of Holocene ice in this region, specifically the Mackay Glacier since the Glacial Maximum 20,000 years ago. Minor objectives were to recover data from the Cape Roberts tide/Meterological instrumentation and measure sea ice thickness offshore to help planning the Cape Roberts Project.
3. Scientific Endeavours and Achievements
The main part of the programme to recover sea floor cores was not achieved due to the loss of the vibracorer in Granite Harbour. Annex 1 to this report describes in detail the loss of the device, including testing in New Zealand and at Scott base and the loss in Granite harbour on its first deployment.
The operation of sophisticated oceanographic programmes from the sea ice away from Scott Base such as the vibracorer operation is difficult because large equipment operated through the sea ice is also exposed to surface weather conditions. Significantly greater logistic resources would be required to operate this equipment under cover to avoid the weather at on the ice and this is not considered practical at present when all equipment must be transported from and returned to Scott Base each season. We then have a very limited period to operate from the sea ice starting from about 20-25 November when air temperatures have usually warmed up sufficiently and ending 5-7 December when sea ice travel with heavy plant normally must be completed because of the sea ice deterioration at some places along the coastal return route to Scott Base. The ice conditions in the Granite Harbour area however usually remain workable for a least another 7-10 days.
For future successful sea ice operations such as the vibracorer programme we need to maximise the working period. We could expect up to 25 days (20 Nov. to 15 Dec.) working in the area if NZAP logistic resources allowed prepositioning of some equipment in Granite Harbour and winter storage of plant and some equipment at Cape Roberts at the end of the work period.
Cape Roberts Tide/Meterological Programme
Data from the Tide/Met. installation at Cape Roberts was downloaded on 21-11-93 and again on 30-11-93. The wind speed sensor, which had seized at low speed, and the wind direction sensors, were replaced with refurbished sensors on 21-11-93.
Review of the downloaded data showed that the tide gauge transducer failed on 26-09-93. The transducer resistance was measured and still appears to be connected but may have failed internally or have somehow become frozen. A visual inspection of the transducer head with a underwater video camera showed that the ice foot-pressure ridge extended deeper than usual near the transducer and that a film of submarine ice covered the rock where the transducer head exited. We attempted to unfreeze the transducer with isopropyl alcohol for inspection on 30-11-94 but this had not worked within 24 hours and the attempt was abandoned this season.
If the transducer has failed because of external icing then it should resume measurements by February when the ice melts around Cape Roberts. New data will then continue to be recorded. If the transducer has failed electronically then it should be replaced. This will require active thawing of the transducer which would take 3-5 days in November 1994. We would also plan to refurbish the CR10 data logger at this time to overcome a minor problem with an excitation channel detected this season.
The failure of the tide gauge transducer in 1993 should not detract from the overall success of this programme which has continuously collected 1041 days of hourly tidal data since 20 November 1990 from continental Antarctica. The design of the installation will permit accurate replacement of the transducer without extensive recalibration and only a 26 hour survey calibration.
Sea Ice Measurements
This season the sea ice in McMurdo was expected to be significantly thinner than in most previous seasons. A major winter storm in early June caused the seaice to breakout nearly to the Mcmurdo Ice Shelf and Hut Point consequently it was expected by some people that the ice would be thin or absent later in the spring/summer. By the time we travelled to Granite Harbour on 18-20 November the seaice along our coastal route was between 1.68 and 2.0 m thick. Offshore of Cape Roberts in the area interest for drillsites ice thickness varied from a normal thickness of 2.3 m within Granite Harbour to 1.15 m thick about 18.3 km ENE offshore of Cape Roberts. At 21 km offshore pack ice had refrozen against a developed ice edge with about 300 mm of ice between floes.page break
The sea ice surface was generally smooth within Granite Harbour and offshore north of the thickness measurement positions in Table 1. Immediately to the south however the ice was rough from winter breakout and refreezing and was likely to be thicker. The ice thickness was certainly a bit thinner than usual in areas affected by the early June breakout but of normal thickness in Granite Harbour. If this season had been a drilling season then it would have unlikely that heavy transport could have traversed to Cape Roberts until early October. One or 2 of the planned inshore drillsites could probably have been occupied this season especially along the Rice Seismic line PD90-10. The planned study of winter sea ice formation from satellite imagery is expected to confirm this.
We were grateful to all the Scott Base personnel who helped us with a difficult season and especially mention Rick Morgan and Jeremy Ridgen who worked with us in the field and Neville Jones (operations manager) whose rapid response with unscheduled help operations kept us operational.
NZAP staff once again provided efficient pre season field/technical and cargo/personnel assistance including acting on our suggestions of improvements from previous seasons.
Several people have helped with the development of the vibracorer and they are acknowledged in the vibracorer report attached.