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Victoria University Antarctic Research Expedition Science and Logistics Reports 1991-92: VUWAE 36

NZARP REPORT #3 POPULAR SUMMARY 1991-92. GROUNDING-LINE SEDIMENTATION OF THE MACKAY GLACIER TONGUE

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NZARP REPORT #3 POPULAR SUMMARY.

GROUNDING-LINE SEDIMENTATION OF THE MACKAY GLACIER TONGUE

Antarctica New Zealand

NZARP REPORT #3 POPULAR SUMMARY.
GROUNDING-LINE SEDIMENTATION OF THE MACKAY GLACIER TONGUE

The Mackay Glacier flows from the Antarctic ice sheet through the Transantarctic Mountains and into Granite Harbour, where it terminates as a floating glacier tongue up to 500m thick, 3km wide and 3.5 km long. In November/December 1991 we explored the tongue with a submersible remotely operated vehicle (ROV) to see whether sediment was being shielded by freezing or melting out onto the sea floor in the area of the grounding line. The question is important because it tells us how far sediment is likely to be carried offshore by polar marine glaciers, and also how the glaciers are likely to behave as sea level rises and fails.

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 200m, 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, accomodating 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.

Sea floor photographs from the ROV show the sea floor to comprise 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. The mud layer is presumed to result from post-grounding rain-out. Samples of sea floor sediment were taken for analysis with a purpose built five bucket mini-dredge.

The north side of the ice tongue at the landward end contained numerous layers of rock debris in the lower 20 m. This zone thinned seaward to 10 m over 1.8 km, though it still remaineed coupled to the sea floor, indicating progressive basal melting and presumably deposition of debris by lodgement. However salinity measurements showed no significant deviation from marine values (range 34.66-34.68%.), indicating the very slow rate of basal melting.

The study has shown for the first time the physical conditions at the grounding line of polar glacier, that basal debris is emplaced by lodgement and that meltwater plays little if any role in sediment transport and deposition. It also shows the potential, given the moderate transport rate for basal debris, for marginal marine glaciers building up their beds of subglacial debris to maintain their grounding-line position as sea level rises.