Victoria University Antarctic Research Expedition Science and Logistics Reports 1991-92: VUWAE 36
This season we had proposed to run two programmes together; 1.,vibrocorer sampling of the sea floor in Granite Harbour and 2., the study of the Mackay Glacier Tongue grounding line with a remotely operated submersible vehicle (ROV). At the end of September we postponed the vibrocorer programme because the equipment was not ready and satisfactorily tested but received RDRC permission to continue with the ROV-grounding line study.
Mackay Glacier forms a 3 km wide, 300 m thick, "floating" glacier tongue that moves seaward at 250 ma"1 when it enters Granite Harbour, an embayment up to 900 metres deep on the Victoria Land coast. The project is to observe and describe, for the first time, the interaction between a polar glacier tongue, sediment and sea water at the detachment point (grounding line) using a remotely operated submersible vehicle (ROV).
We propose to study the subglacial delta presumbed to have formed as basal debris carried in the glacial tongue melts out. (Macpherson 1987, Alley et al. 1989). The existence of subglacial deltas and sedimentation processes requires testing to provide better models to compare with seismic data from the Antarctic continental shelf where a number of page break examples of delta like sedimentary bodies have been observed (Cooper et al. 1990).
|1.||Determine the character and thickness of the basa! debris layer in the glaciers.|
|2.||Evaluate grounding line processes including debris meltout, sediment deformation, meltwater production (if any) and grounding line stability.|
|3.||To check if a subglacial delta exists.|
Scientific Endeavours & Achievements
Mackay Glacier Tongue
Ten days were spent working around the Mackay Glacier Tongue (23 November - 2 December) at three dive sites, site 1 on the southside of the tongue, and Sites 2 & 3 on the north side (Figure 1).
Access holes (1.8 m × 1.2 m minimum) were made in the sea ice for the ROV by a combination of drilling with the 0.6 m auger on the NSF nodwell and explosives. In each case holes were made in existing ice cracks. Near the ice tongue several platelett cubic metres of platlett ice was cleared from holes at sites 2 & 3 especially before the holes were clear for the ROV.
The Phantom DHD2 ROV which belongs to Dr Ross Powell is equipped with colour and low light black and white video cameras. A sea bird CTD, an electromagnetic current meter, a optical back scatter sensor and still stereo photography comprise the instruments which were on the vehicle. In addition we designed and built a 5 bucket min-dredge at Victoria University to sample the sea floor for the ROV.
A total of 10 dives were made at the Mackay Glacier Tonge sites.
This site was on the south side of the tongue approximately 2.7 km east (seaward) of Cuff Cape. 705, 676.4 m N, 280, 347.0 m E (Location based on a local plane grid, origin Cape Roberts @ 700 000 m N 300 000 m E). Parts of the glacier tongue in this region appeared to be in the process of detaching from the main ice mass. The ice tongue did not contain debris and was floating with a minimum of 10 m of ice water above the sea floor.
The sea floor shallows to 150 m south away from the ice tongue side, and appeared to be in part a basement ridge parallel to the side of the ice tongue probably extending out from Cuff Cape. The sea floor comprises a rubbly diamicton with mud drapes in places and areas of well established encrusting benthos indicating the present sedimentation is primarily from suspension and that the ice tongue has not grounded in this area for a significant time period.
This site was the most westerly site occupied on the northern side of the glacier tongue (708 334.6 m N, 277 619.7 m E), as close as possible to the "change in slope" of the main ice flow. An embayment in the tongue has formed because the edge of the ice is slowed in velocity by a sub ice basement high creating a small icefall on this north side of the tongue.page break
Figure 1. Sketch map and cross section of the Mackay Glacier Tongue showing positions of ROV dive sites and interpretation of the sea floor grounding zone along the ice tongue axis.
The glacier tongue at this site is grounded approximately at 100 m depth although local edge rebound exposes areas of the sea floor under the ice cliffs. Ridge and trough relief on the sea floor trends across glacier showing active coupling of the sea floor and moving glacier tongue. Sedimentation is active with mud drapes on boulders and ice contacting the sea floor was observed in several places. No significant conductivity or temperature change in the water column or at the ice contact were measured confirming the absence of measureable meltwater discharge. Encrusting benthos was generally absent indicating much more active sedimentation than at site 1. The basal debris layer (alternating layers of debris rich and poor ice) in the tongue was 20m thick.
This site is also on the northern side of the tongue (708 495. m N, 279 479.9 m E) about 1.9 km east of site 1. The tongue is also grounded here but at a depth of 200m. The seafloor is similar to site 1 with mud drapes on many of the boulders, although in general there are less large boulders observed. A strong trough fabric was observed on the sea floor showing that coupling with the glacier and lodgement processes occur here also. The basal debris layer at this site had thinned to about 10 m thick compared to site 2.