An Oceanographic Study of the Cavity Beneath the McMurdo Ice Shelf, Antarctica

Abstract

This thesis reports the first observations of currents, temperature and salinity beneath the McMurdo Ice Shelf, Antarctica. They are reviewed and discussed here in conjunction with results of a numerical modelling study used to simulate current flow and to investigate local sediment deposition. The McMurdo Ice Shelf lies behind Ross Island off the Victoria Land coast of Antarctica, and represents the northwest corner of the much larger Ross Ice Shelf. The site will be drilled by the ANDRILL consortium in 2006, passing through the ice shelf, the water column, and 1000 m into the sea floor, to obtain a record of ice shelf and climate history in this area. This study stems from a site survey carried out in early 2003, for which access holes were melted at two locations on the McMurdo Ice Shelf. Current meters surveyed multiple depths simultaneously during spring tides, and profiles of temperature and salinity were collected through a diurnal tidal cycle at each site. Maximum currents were recorded in the boundary layer at the base of the ice shelf, reaching 0.22 m s−1 during the flood tide. The salinity and temperature profiles were similar at the two sites, with greater temporal variability observed at the site closer to the open water of McMurdo Sound. Supercooling, due to the pressure-dependence of the in-situ freezing temperature, was observed at one of the sites. At the second site, where the draft of the ice shelf was deeper, temperatures corresponding to basal melting were observed. At a third site on the sea ice at the northwestern edge of the McMurdo Ice Shelf, a current meter surveyed the water column to 320 metres below sea level for 23 days. This allowed comparison of current behaviour through spring and neap tides, and between subsea ice and sub-ice shelf environments in the same season. Net throughflow over spring tides at each of the three sites was consistent with transport eastwards from McMurdo Sound along the channel defined by local bathymetry. Profiles of temperature and salinity from beneath the ice shelf were likewise consistent with McMurdo Sound being the source of the observed water masses. Flow along the sub-ice shelf channel was further investigated using an adaptation of a two-dimensional thermohaline ocean model. Year-long profiles of temperature and salinity from southern McMurdo Sound were used to seasonally force the model, resulting in annual variation in all parameters. The rate of melting decreased monotonically from ∼0.6 m yr−1 at the deep end of the ice shelf, into a region of freezing associated with supercooling closer to the McMurdo Sound end of the domain. This change in regime mirrored the observations from the boundary layer beneath the McMurdo Ice Shelf. Sediment transport and deposition were investigated, with settling velocities used to represent sediment sizes ranging from biogenic pellets and fine sand through algal flocs to fine mud, particle types known and described from the present day environment. This method of incorporating sedimentation processes gave results similar to observations from surface sediment cores collected beneath the ice shelf. The larger grains were preferentially deposited close to the open water McMurdo Sound source, whereas fine-grained material was entrained into the general circulation and deposited by regions of down-welling. A settling velocity of ∼1x10−4 m s−1, corresponding to a grain size of ∼5 μm, defined the boundary between these depositional behaviours. Characteristics of the water beneath the ice shelf suggest that it had been transported from McMurdo Sound, being modified through interaction with the base of the ice shelf. This pattern of throughflow was also seen in the current meter data, with a strong tidal signal throughout the water column superimposed on the net transport eastward from McMurdo Sound and under the ice shelf. This net flow pattern was supported by the results of the longer-term simulation experiments.