IMMEDIATE SCIENCE REPORT
K022: ROV Grounding Line Sudy of the Mackay Glacier Tongue
Antarctica New Zealand November 1991 - December 1991
NZARP REPORT NO.1. IMMEDIATE SCIENTIFIC REPORT.
Event K042 - ROV grounding line study of Mackay glacier.
In November-December 1991 we explored the Mackay Glacier Tongue with a submersible remotely operated vehicle (ROV) to see whether sediment was being shielding by freezing or melting out onto the sea floor in the area of the grounding line. The ROV made ten dives at three sites, one on the southern side, and two on the northern side of the tongue. The tongue was grounded at both of the latter sites at a depth of 100 and 200 m, showing that the glacier was touching the sea floor for at least 1.9 km of the length of the tongue. This was unexpected, and shows that the debris transported beneath the ice and released can build up and out at a measurable rate, accommodating at least low rates of rising sea level. The sea floor on the south side has an abundant biota including bryozoans attached to the stones, indicating that this area has been ice free for a significant period of time.
The sea floor comprises a mixture of sand and mud with scattered cobbles and boulders (diamicton) draped with a thin layer of mud in many places. Transverse ridges and crag and tail features on the sea floor close to the grounding line suggest that shearing and lodgement are important processes when the ice is grounded.
Layers of debris rich ice in the basal zone of the tongue were observed and are similar to those in glaciers in more temperate settings and confirm that freezing on and erosion of debris is occuring in Mackay Glacier.
Two dives were made at the Blue Glacier and confirm that this glacier is a significantly different type of polar glacier. The Blue glacier terminus calves periodically into the marine environment but it is slow moving and free of basal debris where we observed it. No lodgement processes occur but a small (3 m high) push moraine is present immediately in front of the underwater terminal cliff.
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.
About a 1.5 days (6 - 7 December) were spent at the Blue Glacier tide water terminus and 2 ROV dives were made from one site approximately at the centre of the terminus cliff. At this site water depth was about 100 m and the sea floor covered with fine mud. A small (3 m high) push moraine is present infront of the underwater cliff which has recently calved and rebounded a few metres (less than 5 m) off the sea floor. Some current was observed but we are unsure if there was any associated salinity changes because the CTD was inoperative.
The ice cliff and sole of the glacier is free of basal debris and only clean "white" ice was observed.
Very little encrusting biota were present on the muddy sea floor but sea cucumbers were present on the sea floor under the sole of the ice cliffs.
Data from the tideguage - metmast was retrieved on 22 November and 4 December. We now have a continuous tidal record from this site in excess of 1 year. The transducer depth was measured with a small underwater camera and a 24 hour levelling done by the K191 surveyor. These tests should be carried out annually to check the transducer mounting and calibration.
A. Pyne meet NZARP visitors to Cape Roberts in the afternoon of 2 December to outline the CIROS ll drilling proposal and specifically the requirement for a camp at Cape Roberts to support drilling offshore.
The very successful ROV programme was made possible by inviting Dr Ross Powell (and his ROV) as a collaborative foreign scientist into our NZARP event. Dr Powell who is a Masters graduate of VUW now at Department of Geology, Northern Illinois University, USA has been working on similar glacial studies to us but in Alaska. This last season has enabled both our groups to work together in Antarctica and in proving how valuable a tool the ROV is to understanding polar glacial processes. We hope to continue collaboration in the future (1993-94) in a similar programme jointly supported by NZARP and USARP.
We are in the process of preparing a short paper to Nature on the Mackay Glacier Tongue. In the longer term we intend to sumitt a detailed article to Marine Geology.
No lasting impact has occured from this programme. All activity was carried out on the seasonal sea ice except K024 cargo unloading at Cape Roberts. Human and cooking wastes were consigned to sea ice holes and burnable and non-burnable materials ail returned to Scott Base. Small quantities of explosives (less than 1 kg per shot) were used to enlarge the ROV access holes in sea ice and these were over 2 km away from ice cracks used by seals.
Future Research and Management of Science in the Ross Dependency.
We hope to continue the vibrocorer programme in 1992-93 which was postponed this season. NZARP will also be in a better position to support this programme with a N.Z. nodwell-hiab crane vehicle at Scott Base in the 1992-93 season.
In the 1993-94 we hope to continue collaboration with Dr Powell and use the ROV to study the Mackay Glacier Tongue sedimentation processes and "retreat" in greater detail and then compare this to the much longer Nordenskjold Ice Tongue 80 km north of Granite Harbour.
We would like to thank the following people and organisations who contributed to the success and implementation of this programme.
At Victoria University. The mechanical workshop who built the prototype ROV dredge. Allison Thwaite (Teaching Aids) copied the video tapes during the Christmas 91 period.
At Northern Illinois University. Bob Bailey who prepared the ROV before shipment to New Zealand.
Mike Chapman (MECCO) who provided information on the ROV to help design the bucket dredge. Helen Phillips, Graham Shelly, Pete Baxter (DSIR) and Rodger Hargraves (Pengalleys) all helped to get our late cargo through customs and to Antarctica.
Dr Rob Dunbar (Rice University) and Dave Geddes (DSIR Antarctic) for help arranging for the use of the NSF nodwell.
All the staff of DSIR Antarctic and at Scott Base and in particular Phill Robins (Scott Base Operators Manager).
Lastly special thanks to our field personnel Sean Heaphy our "gourmet" mechanic/field assistant and Dave Statham N.Z. Army plant operator.page break