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A SPOT satellite image of the McMurdo region acquired in December 1994 reveals a dark line of the McMurdo Ice Shelf parallel to the calving front ( figure).
A first thought was that this line is the trace of a crevasse and a precursor of a major calving event. The dark line does not appear on earlier images or photographs. The operational importance of the line arises because it crosses the road that connects McMurdo Station and Scott Base with Williams Field Skiway. A calving event along the line would make access to and from the skiway considerably more difficult.
An alternative explanation is that the line may be a step in surface elevation due to a collapse of firn seaward of the line caused by brine infiltration (Kovacs et al. 1982). These types of fronts have been observed up to 15 kilometers (km) from the outer edge of the ice shelf (Morse and Waddington 1994). Perhaps the snow surface seaward of the dark line has collapsed because of the weight of brine or because of weakening of the firn structure by percolating sea water.
Neither hypothesis can be tested from the imagery alone. A field program was undertaken in October and November 1995 to determine if a crevasse is opening at the site.
Strain grids were installed across the dark line and surveyed twice using global positioning system (GPS) methods. If the dark line is due to a sagging crevasse bridge, large strain rates are expected across the line. The dark line should be in the same position with respect to the ice edge as it appears in the image. Moreover, surface elevation profiles should show a depression. If the line is caused by infiltrating brine, surface elevations will show a step rather than a depression; horizontal strain rates will not be especially large at the line; and the position of the line will likely have migrated since the time of the image.
The methods and interpretation are described in Whillans, Merry, and Hamilton (in press). The results do not support the interpretation of the dark line as the trace of an opening crevasse. Strain rates across it are too small for an opening crevasse, and no sagging bridge is evident. Moreover, the feature is moving with respect to the ice. The concern raised by initial interpretations of the image can now be allayed.
Most of the observations fit the model of the line marking the limit of firn collapse due to brine infiltration. These observations include the shape of the surface, the migration of the feature and the simple pattern of horizontal strain rates.
Unresolved by this study is the cause of the bright line just seaward of the dark line observed on the image. This bright line was interpreted to be the far side of a sagging crevasse bridge, but elevation surveys show only a very minor reverse slope, which cannot account for the bright line on the image. We suggest that the reverse slope present in December 1994 is a transient feature that was no longer present in October 1995.
A new discovery is that major mechanical control on the ice shelf is compression originating at Dailey Islands. These islands lie some 40 km west of the study area. The ice shelf is losing contact with the islands; only two of the original six Dailey Islands now contribute to drag (Gow and Govoni 1994). The islands interact most directly with ice from Koettlitz Glacier, which in turn blocks the McMurdo Ice Shelf. It is important to continue to monitor the interaction of the ice shelf with Dailey Islands. Loss of contact with the islands could have major consequences to the skiway.
Fieldwork was conducted by Gordon Hamilton, Erik Venteris, and Ian Whillans, who received support from National Science Foundation grant OPP 95-28608.
References
Gow, A.J., and J.W. Govoni. 1994. An 80-year record of retreat of Koettlitz Ice Tongue, McMurdo Sound, Antarctica. Annals of Glaciology, 20, 249-253.
Kovacs, A., A.J. Gow, J.H. Cragin, and R.M. Morey. 1982. The brine zone in the McMurdo Ice Shelf, Antarctica (CRREL Report 82-39). Hanover, N.H.: Cold Regions Research and Engineering Laboratory.
Morse, D.L., and E.D. Waddington. 1994. Recent survey of brine infiltration in McMurdo Ice Shelf, Antarctica. Annals of Glaciology, 20, 215-218.
Whillans, I.M., C.J. Merry, and G.S. Hamilton. In press. Investigation of a possible crevasse near the main airstrip on McMurdo Ice Shelf. Annals of Glaciology.