Australia: The Land Where Time Began

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Larsen C Ice Shelf, Antarctica – In situ Observations of Ocean Circulation Beneath it

Data were obtained from beneath 2 sites on the Larsen C Ice Shelf, 1 in the north and 1 in the south, by the use of holes drilled by hot water drills. The water was colder throughout the water column than the freezing point at the surface, and the temperature-salinity characteristics were found to be consistent with a High Salinity Shelf Water of maximum salinity 34.65 psu. The 0.08^oC thermal driving at the base of the ice and the 0.2 m/sec rms speed of the water resulted in a melt rate of 1.3 ± 0.2 m/a, as measured over a period of 8 days.  The evidence, when combined with the available ship-based data, suggests that the sub-ice cavity is flushed only by water at the surface freezing point. It is implied by this that the decrease in surface elevation that has been reported for Larsen C Ice Shelf is not likely to have resulted from an increasing basal melt rate.

Background

The atmosphere above the Antarctic Peninsula has been warming rapidly and this has led to a sequential breakup of several ice shelves along the east coast of the Peninsula, beginning in the northern part of the Peninsula and progressing to the south: Beginning with the Prince Gustav Ice Shelf in 1995, Larsen A Ice Shelf also in 1995 and the most recent, most of the Larsen B Ice Shelf in 2002). Surface elevation lowering of the Larsen C Ice Shelf between 1992 and 2001has been described (Shepard, 2003), which concluded that the lowering was evidence of ice shelf thinning that was being driven to a large extent by increased ocean melting of the ice shelf from below. A caveat was included in this Shepard’s report that part of the surface lowering that had been observed would have resulted from the densification of firn due increased summer air temperatures. A map has been presented (Holland et al., 2011) for the thickness of the air column for the ice shelf which showed a strong positive gradient from northwest to southeast, and from this Holland et al. deduced there had been stronger summer melting in the north. ICESat altimetry data was used (Pritchard et al., 2012) to show the surface lowering had continued between 2003 and 2008, and surface modelling was used to support the suggestion that surface processes were entirely responsible for the surface lowering.

According to Nicholls et al. if increased basal melting had a role in the surface lowering then either the currents or water temperatures beneath the ice shelf needed to be increasing. A changing salinity field over the continental shelf would be required as thermohaline and tidal currents are the only significant drivers of currents beneath the ice shelf.

The oceanographic conditions over the Larsen C continental shelf have been particularly poorly sampled because of adverse sea conditions in the northwest Weddell Sea. During cruise ANTX/7 of the research ship Polarstern the most comprehensive dataset was acquired when a conductivity-temperature-depth (CTD) section toward the ice front and XBT (eXpendable BathyThernograph) section along the southern 2/3 of the ice front, were acquired in January 2003 (Bathmann et al, 1994).

It was shown by those data that the continental shelf was flooded with relatively warm Modified Warm Deep Water (MWDW). A small number of CTD profiles obtained from the northern end of the ice front in March 2002 were described (Nicholls et al.,2004) and the properties of the water that was flowing out from beneath the ice shelf were used to show there was no evidence of the flow of waters above the surface freezing point into the cavity. During December 2004 and January 2005 a helicopter was used to deploy a CDT profiler as part of ISPOL, an ice drift experiment that was based on R.V. Polarstern (Hellmer et al., 2008) and the results made a further contribution to the limited dataset that was available. Some of the stations extended to the west on to the mid-shelf, though the experiment focused on the Larsen continental shelf break.

If the conclusion that the source waters for the circulation beneath the ice shelf are at the surface freezing point is both correct and applies to the entire ice shelf, not only in the northern part of the ice shelf (Nicholls et al., 2004), this would imply that the conditions beneath the ice shelf cannot be warming, therefore the flushing of the cavity is also increasing is the only way the basal melt rate of the ice shelf can be increasing significantly. An array of year-round moorings would be required along the ice front to sample the water masses crossing the ice front to test the conclusion. A different strategy is required as local sea ice conditions would not allow such an array to be deployed or serviced. An alternative approach has been offered by a project focused on the turbulent layer at the base of the ice shelves. In this paper Nicholls et al. report on temperature and salinity observations made from both sites, which allowed a new assessment of the characteristics of the source waters to the cavity beneath the ice shelf. They also report on currents they measured at the southern site.

Conclusions – summary

Nicholls et al. presented CTD profiles from beneath the Larsen C Ice Shelf that were obtained from 2 sites, 1 northern site and 1 southern site. It is shown by the profiles that at these 2 sites the water column was composed of ISW, the θ-S characteristics show that there is no evidence of water above the surface freezing point entering the cavity. Tides dominate currents in the south, and there is a weak mean flow from east to west. At the southern site temperature and current data from the boundary layer suggest a mean melt rate of about 1.9 m/a for the duration of the record, which was 8 days, compared with a value derived from radar data of 1.3 ± 0.2 m/a for the same period.

The salinity of the water entering the cavity is suggested by the evidence to be between 34.60 and 34.65. It is suggested by the upper limit that the water flowing in is not HSSSW that was formed originally over the Ronne continental shelf, but it results from cooling and salinification of MWDW in winter that has crossed the shelf break locally. It is suggested by the low temperature of the water flowing to the west, -1.9^oC, at the southern site, that the water flowing in has interacted with thin ice to the south of Kenyon Peninsula prior to entering the main cavity.

Nicholls et al. concluded that there is no evidence that there is water above the surface freezing point in the cavity beneath the Larsen C Ice Shelf, and that the basal melt rate of the ice shelf has not been increasing significantly during the period for which oceanographic data has been available.

Nicholls et al. based their analysis on snapshots from different years, and none of them are from the winter, though it could be argued that observations from the northern site are responding to winter conditions at the ice front. Nicholls et al. suggest it is possible that interannual variations may be confounding the analysis, with the cavity flickering between warm and cold conditions. To resolve this problem multiyear time series are required.

Sources & Further reading

Nicholls, K. W., K. Makinson and E. J. Venables (2012). "Ocean circulation beneath Larsen C Ice Shelf, Antarctica from in situ observations." Geophysical Research Letters 39(19): L19608.