McMURDO SOUND SEDIMENT STUDIES (K042)
The CIROS 2 drill hole in Ferrar Fjord cored units of well sorted sand and laminated sand-mud "couplets", unusual lithologies within a glacial marine sequence. This season sea floor samples were taken near the outfall of two meltwater channels on the Ferrar Glacier snout to compare with the sediments cored in CIROS 2. The channels contain sorted and bedded sand "bars" and larger rock debris. Several debris covered bergy bits broken from the channel mouth were also present stuck in the sea ice at the glacier edge.
The seafloor samples collected were imbedded, poorly sorted sandy mud with dropped pebbles and sand lenses deposited by ice rafting processes. Apparently, sand from the channels becomes widely dispersed as it settles through the water column. Thus we have eliminated one possible source of well sorted sand, but have yet to explain the sorting in the CIROS 2 core.
Attempts to recover sediment traps left in New Harbour and Granite Harbour by Rice University were also unsuccessful as they had been carried away by sea ice break-out.
Introduction and Background
This programme is a continuation of our study of modern sediment in the McMurdo Sound region. We have collected over 80 seafloor samples and cores since 1980 which have been analysed and preliminary interpretations published (Barrett et al., 1983). Several questions remain unanswered about modern sedimentation processes on the floor of McMurdo Sound. We intended this season to attempt a programme of underwater photography, but realized the camera electronic componentry might not be completed in time this season's operation and informed Antarctic Division. A revised programme was proposed which would expand an accompanying sampling programme at the Ferrar Glacier snout (Fig. 1). The aim of this programme was to study the modern sedimentation regime at the snout for comparison with the upper sediments cored in the CIROS 2 hole (Barrett et al., 1985). CIROS 2 cored sands, and laminated muds for which we have yet to identify a modern example in the area.
Another aspect of the programme was to recover two sediment trap moorings on the Ferrar and Mackay Glaciers (Fig. 1) set by USARP party S-216 (Dr R. Dunbar) in cooperation with the VUW programme last season. S-216 was not funded for this season so K042 intended to recover and redeploy the moorings.
Preparation and inventory for the CIROS science programme was also carried out in anticipation of drilling in 1986.
Results from seafloor sampling
Sea floor samples were taken from the sea ice directly in front of two meltwater channels on the Ferrar Glacier (Fig. 2). The channels contain supraglacial sediment including sandy bedforms and coarser debris which could be deposited on the sea floor today. A grid pattern was established around the channel mouths in order to sample any areal changes of the sea floor sediments.
Samples were recoverd from depths between 257 m and 197 m (Table 1) and are all slightly sandy mud. No gross sediment differences between samples were observed in the field.page 3 page 4
It does not appear that the meltwater streams deposit thick sand bodies on the sea floor like those drilled at CIROS 2. The change in position of the glacier edge is likely to have the effect of spreading any sand from the channels over a larger area. The glacier edge near the two meltwater channels was accurately surveyed this season to begin recording the ice edge changes and will be resurveyed in future years.
Debris covered "bergy bits" enclosed by sea ice were present in front of the two meltwater channels. The debris on these small pieces of glacier ice ranged from silt to a boulder up to 4 m long. These debris bergs have calved from the meltwater channel mouths and show that ice rafting of sediment is a process which occurs in Ferrar Fjord.
Sampling was also attempted in water less than 100 m deep at Mt Barnes and Delta 4 (Fig. 2) to compare with samples from deeper water in New Harbour. Small quantities of sandy sediment were recovered confirming that hard bottom conditions are present in these areas like many other shallow areas of McMurdo Sound. Different sampling techniques such as a grab or box corer will be required to sample these sea floor sediments in the future.
Results from Sediment Trapping
Sediment traps in McMurdo Sound and Granite Harbour have been deployed by parties from Victoria University of Wellington and Rice University, Houston, Texas. Short term deployments (maximum time site occupied 61 days to date) have been successfully recovered. Long term deployments (1 year or more) have been attempted by suspending traps from ice tongues but have not been successfully recovered to this date.
Rice university assisted by K042 deployed two four trap moorings anchored from the Ferrar and Mackay Glaciers in December 1985. The traps were deployed through holes cut in the sea ice within a few metres of the glacier fronts with the intention of recovering the moorings the following season.
This season K042 were prepared to recover the two moorings, but in both instances although the anchor points were found the traps had broken out with the sea ice. The moorings would have remained intact if the sea ice breakout had not occurred at the glacier front like in 1982 at the Mackay Glacier. The probability of retrieving moorings anchored in this way is now considered unlikely after this season's experience.
These attempts show that for long term deployment to be successful, the moorings used must be unaffected by sea ice movement and breakout, and, to a lesser extent, ice tongue calving - ice berg movement. The need to measure sediment flux throughout the whole year still remains a major factor in the understanding of modern marine glacial sedimentation processes in this area and Antarctica in general.
Publication and future research
The Ferrar Glacier samples will form the basis of a BSc Honours thesis by David Kelly, who will compare the texture and mineralogy of glacier surface sediment and sea floor sediment to identify the sources of the latter. The results will be incorporated into a review paper by Barrett on modern sedimentation in McMurdo Sound.
We would like to continue work with Rice University to improve the recovery of sediment traps, the only satisfactory approach for finding out what is currently being deposited on the sea floor. A special design will be required to test the idea that most mud in the Ross Sea comes from density currents emanating from the major outlet glaciers.
Barrett, P.J., Pyne, A.R. and Ward, B.L. 1985. Modern sedimentation in McMurdo Sound. In Oliver, R.L., James, P.R. and Jago, J.B. (eds.), Antarctic Earth Science, Australian Academy of Science Press, Canberra: 550-553.
Barrett, P.J. and Scientific Staff, 1985. Plio-Pleistocene sequence cored at CIROS 2, Ferrar Fjord, Western McMurdo Sound. N.Z. Antarctic Record, 6(2): 8-19.
Dunbar, R.B., Macpherson, A.J., Wefer, G. and Biggs, C.D. 1984. Biogenic fluxes from sediment trap experiments on the Antarctic margin. Transactions American Geophysical Union 65: 916.
We are grateful to the University Mechanical Workshop, and technical assistance from C. Snell, J. Carter and D. Gilmore in Wellington, as well as help from Cas Roper and the technical staff at Scott Base. Help from Mr L. Pyne was also appreciated.