Abstract

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.