An Oceanographic Study of the Cavity Beneath the McMurdo Ice Shelf, Antarctica
This thesis reports the first observations of currents, temperature and salinity beneaththe McMurdo Ice Shelf, Antarctica. They are reviewed and discussed here in conjunctionwith results of a numerical modelling study used to simulate current flow and to investigatelocal sediment deposition. The McMurdo Ice Shelf lies behind Ross Island off theVictoria Land coast of Antarctica, and represents the northwest corner of the much largerRoss Ice Shelf. The site will be drilled by the ANDRILL consortium in 2006, passingthrough the ice shelf, the water column, and 1000 m into the sea floor, to obtain a recordof ice shelf and climate history in this area.
This study stems from a site survey carried out in early 2003, for which access holeswere melted at two locations on the McMurdo Ice Shelf. Current meters surveyed multipledepths simultaneously during spring tides, and profiles of temperature and salinitywere collected through a diurnal tidal cycle at each site. Maximum currents were recordedin 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 temporalvariability 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 oneof the sites. At the second site, where the draft of the ice shelf was deeper, temperaturescorresponding to basal melting were observed.
At a third site on the sea ice at the northwestern edge of the McMurdo Ice Shelf, acurrent meter surveyed the water column to 320 metres below sea level for 23 days. Thisallowed comparison of current behaviour through spring and neap tides, and between subseaice and sub-ice shelf environments in the same season. Net throughflow over springtides at each of the three sites was consistent with transport eastwards from McMurdoSound along the channel defined by local bathymetry. Profiles of temperature and salinityfrom beneath the ice shelf were likewise consistent with McMurdo Sound being the sourceof the observed water masses.
Flow along the sub-ice shelf channel was further investigated using an adaptation of atwo-dimensional thermohaline ocean model. Year-long profiles of temperature and salinityfrom 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 withsupercooling closer to the McMurdo Sound end of the domain. This change in regimemirrored the observations from the boundary layer beneath the McMurdo Ice Shelf.
Sediment transport and deposition were investigated, with settling velocities used to representsediment sizes ranging from biogenic pellets and fine sand through algal flocs to finemud, particle types known and described from the present day environment. This methodof incorporating sedimentation processes gave results similar to observations from surfacesediment cores collected beneath the ice shelf. The larger grains were preferentially depositedclose to the open water McMurdo Sound source, whereas fine-grained materialwas entrained into the general circulation and deposited by regions of down-welling. Asettling velocity of ∼1x10−4 m s−1, corresponding to a grain size of ∼5 μm, defined theboundary between these depositional behaviours.
Characteristics of the water beneath the ice shelf suggest that it had been transportedfrom 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 tidalsignal throughout the water column superimposed on the net transport eastward fromMcMurdo Sound and under the ice shelf. This net flow pattern was supported by theresults of the longer-term simulation experiments.