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.