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Physical characteristics of MSSTS Sites 1 & 2 (Peter Barrett and Phil Bentley)

The purpose of this part of VUWAE 23 was to locate and determine the physical conditions at the two sites proposed for drilling in late 1979 in western McMurdo Sound (Fig. 1). For each site local bathymetry was determined and an access hole then opened for current measurements and sea floor sampling. Nothing was found at either site to suggest that drilling should not proceed.

The 1978 winter was unusually cold and the sea ice thickness greater than expected. Around site 1 the thickness ranged from 2.5 to 2.8 m, though the ice must have formed that year. At Site 2 the thickness was 4.0 m and the two debris layers in snow patches on the ice surface suggest the ice was entering its 3rd season. We had hoped to determine water depth by using a depth sounder and placing the transceiver (the energy source) on the sea ice surface in a puddle to minimise energy loss. Neither of our tow instruments (SIMRAD, frequency kHz, power watts; FURONO, frequency 50 kHz, power 10 watts) displayed a return signal, and we found it necessary to drill a hole through the ice. On some occasions a signal was received when the transceiver was only 1 m down the hole, but on others no signal was received even when the transceiver was lowered to the base of the ice. When a signal was received the depth indicated was within 2% of that obtained from a line. Because of the uncertainty of the depth sounders, and the need to drill a hole through the ice anyway to obtain a depth, we opted for the time-honoured (and time-consuming) method of the weighted line. However, there is no doubt that a depth sounder that could operate through the sea ice would be a major advance for marine scientists using sea ice as an operating platform; Physics and Engineering laboratory, DSIR, are currently investigating ways of improving commercial depth sounders for such an operation.

The bathymetry in the region of MSSTS Site 1 (Fig. 2) shows a shelf at 180 to 200 metres over a large area with a dome close to the proposed drill site. Both major northwest-trending lines were expected to encounter a 300 m deep valley revealed by previous surveys (Northey et al., 1975, Figs. 2 & 3), though it could lie between two stations, which are about 1 km apart for much of the survey. The dome, which rises to less than 120 m below sea level is an odd feature of unknown significance. It could be a volcanic core or reflect a basement high or be one of several other possibilities. In any event it is suggested that the drill site be moved about 2 km to the southwest so that a flat (typical) part of the sea floor is drilled. The bathymetry at MSSTS Site 2 is simple (Fig. 2) and indicates a basin with a maximum depth of a little over 110 m.

We planned to cut the access holes with chain saws and to break out the last few 30 cm or so with crowbars and heavy chisel points attached to SIPRE rods. At Site 1 it took 8 hours of very hard work to make a hole 1.0 × 1.2 m × 2 m deep, by which time we were not at all reluctant to use eight pounds of explosive (2 in each comer) to finish the job. However, one of the holes for the explosive penetrated the bottom filling the excavation with sea water. The nearby tent proved to be too close (Plate V). No damage was sustained, but the sleeping bags were uncomfortably wet, and were almost impossible to dry properly. At Site 2 we cut a hole 1 m deep and sliced the ice below this for a further 0.8 m. Four 4 lb charges were used, and another small charge was necessary to break a hole 1.5 m across through the 4.0 m thickness of ice. A third access hole was attempted at the snout of Ferrar Glacier using a single 30 lb charge three-quarters of the way through the 4.8 m thick ice. This shattered the ice reasonably well but the hole was not large enough to use the grab for sea floor sampling.

Current measurements were carried out every three hours over a 24 hour period at both drill sites and at Ferrar Glacier snout using rotors (Plate V) borrowed from the N.Z. Oceanographic Institute. The meter was read half way down and 1 m above the sea floor. Before the rotor was lowered, it was moved in the water to check that a signal was reading in the meter. No currents were recorded, indicating velocities less than 2.7 cm s-¹.

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Sea floor sampling with a McIntyre grab (Plate VII), also borrowed from the N.Z. Oceanographic Institute, produced much more positive results.

Sampling was successful about two times in three, with the sea floor sediments filling about two-thirds of the grab. The sea floor itself was little disturbed, and at Site 1 showed a varied fauna (Plate VIII), in addition to a dense muddy mat from 1 to 5 cm thick of sponge spicules. Site 2 had very few creatures by comparison. The sea floor was well preserved by subsampling with a metal box and storing in plastic containers (Plates IX and X).

The texture of the sea floor sediments at both MSSTS Sites 1 and 2 is moderately sorted medium to coarse grained sand with admixtures of gravel (<4 mm) typically from 0.3 to 0.8% and of mud from 3 to 13%. The gravel fraction from both sites includes both granitic and basaltic stones, though the latter which include both floor rocks and tuffs, are dominant. Of particular interest are pieces of light olive grey claystone, possibly from early Cenozoic Strata. The sand fraction appears far more distinctive at each site, being basaltic at MSSTS Site 1 and quartzo feldspathic at Site 2 (New Harbour).

An important feature of the sediment is the burrowing, seen in Plates IX and X. As the grab samples were washed for pebbles, mucus-cemented sand tubes collected in the sieve and were disaggregated only with some effort. The tubes are best seen as the sediment surface in Plate X, but occurred in abundance at both sites. The faint mottling in the section of Plate X is also an indication of burrowing. We were not able to determine how far beneath the sea floor present day burrows extend, but they are commonly 8 cm long and this may therefore be taken as a minimum. We noted above that the sponge spicules at Site 1 were abundant only in the upper few centimetres. Perhaps their paucity beneath is due to comminution by burrowing organisms.