Fast-ice properties and structure in McMurdo Sound
Fast-ice properties and structure in McMurdo Sound 50
M.O. JEFFRIES and WE WEEKS Geophysical Institute University of Alaska Fairbanks, Alaska 99775-0800
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The fast ice in McMurdo Sound frequently is used as a platform for oceanographic and biological studies, and the annual sea ice runway is essential to the movement of personnel, equipment and supplies in and out of McMurdo Station. Considering the importance of the fast ice to the operation of McMurdo Station remarkably little is known about its annual growth history, properties, and structure. In early January 1991, we obtained 15 first-year cores from the fast ice in McMurdo Sound (figure 1). For comparative purposes, an additional core was obtained from Gerlache Bay, the site of the Italian antarctic station, Baie Terra Nova (see Jeffries and Weeks, Antarctic Journal, this issue, for location). Here we report some preliminary results of the ice core analysis program. The mean salinity of the individual fast ice cores ranged from 2.95 to 5.39 parts per thousand. The mean value of all the fast ice salinity measurements was 4.21 parts per thousand. This value, only slightly less than that of the western Ross Sea pack ice (Jeffries and Weeks, Antarctic Journal, this issue), is evidence of brine loss prior to sampling. Some idea of the magnitude of the brine loss in 1991 can be gained from the reported mean salinity of 6.0 parts per thousand for the McMurdo fast ice in October through November 1980 (Gow et al. 1982). The mean salinity profile in the fast ice has a roughly reverse S-shape (figure 2A), which almost certainly is the result of desalination. The relationship between mean ice salinity (S i , in parts per thousand) and mean ice thickness (h, in meters) of the fast ice, S = 0.007h + 2.846, is similar to that of the western Ross Sea pack ice (Jeffries and Weeks, Antarctic Journal, this
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Figure 2. Depth profiles of salinity and temperature in McMurdo Sound fast ice cores. The solid symbols (circles for salinity, squares for temperature) represent the mean values of the open symbols at a given depth. (% denotes parts per thousand. cm denotes centimeter.)
issue). Like the pack ice, the fast ice is also more saline than arctic sea ice of similar age and thickness. Also similar to the western Ross Sea pack ice (Jeffries and Weeks, Antarctic Journal, this issue), the fast ice, although close to the melting point and nearly isothermal (figure 2B), nevertheless retains a considerable amount of brine. The mean ice thickness at the 16 sites ranged from 1.25 to 2.32 meters. The mean value of all the ice thickness measurements was 1.94 meters. The data are consistent with recent fast ice thickness records for McMurdo Sound and adjacent waters (Leventer et al. 1987). The fast ice is almost twice as thick as the western Ross Sea pack ice (Jeffries and Weeks, Antarctic Journal, this issue). The greater thickness of the fast ice might be caused by a combination of lower oceanic heat fluxes and different ice accretion mechanisms in the fast ice zones. Congelation ice (figure 3) was observed at all fast ice sites, comprising from 30.4 percent to 93.6 percent of the individual cores. This ice type often was characterized by strongly aligned crystals, a feature also observed at many McMurdo Sound locations in October through November 1980 (Cow et al. 1982). The preferred orientation of congelation ice crystals in the McMurdo Sound area is consistent with water current control of crystal growth (Weeks and Cow 1978). Frazil ice, found only at the top of core RS-19 (figure 3) was a minor component of the fast ice. Unlike the western Ross Sea pack ice (Jeffries and Weeks, Antarctic Journal, this issue), the lower portions of most fast ice cores comprised layers of congealed, densely packed platelet ice (figure 3). The maximum percentage of platelet ice observed in a core was 50.7 percent. Only core RS-31, obtained from Erebus Bay, did not contain an appreciable amount of platelet ice (figure 3). ANTARCTIC JOURNAL
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Figure 3. Diagrammatic representations of the stratigraphy of some fast ice cores from McMurdo Sound (RS-19, RS-33, RS-31) and Gerlache Bay (RS-17). F denotes frazil ice; C denotes congelation ice; P denotes platelet ice; C/p denotes congelation ice with some platelets.
It is believed that platelet ice growth in McMurdo Sound results from supercooling brought about by adiabatic decompression of low density seawater flowing northward into the sound from below the McMurdo Ice Shelf (Lewis and Perkin 1985). The accretion of the platelet ice against the base of the overlying congelation ice sheet and its subsequent consolidation allows increased growth of the fast ice, resulting in a final thickness greater than would otherwise be possible by heat conduction from platelet-free water alone. The basic two-layer structure of the McMurdo fast ice, i.e., congelation ice overlying platelet ice, undoubtedly is a recurrent feature. The significant contribution of platelet ice to the fast
Summer pack-ice properties and structure in the western Ross Sea M.O. JEFFRIES and WE WEEKS Geophysical Institute University of Alaska Fairbanks, Alaska 99775-0800
Most of the available information on the properties and structure of antarctic sea ice has been collected in studies in the Weddell Sea. This report describes some of the preliminary findings from the first field and laboratory investigation of the properties and structure of western Ross Sea pack ice (figure 1) undertaken during the period from December 1990 to March 1991 REVIEW
ice sheet has been noted since the 1960's (Paige 1966; Lewis and Perkin 1985; Cow personal communication). The occurrence of platelet ice in the Gerlache Bay fast ice (Core RS-17, figure 3) was somewhat unexpected, but it suggests a nearby source of low-density water for platelet ice growth. At Cerlache Bay, the inclusion of platelets in the congelation ice clearly began somewhat earlier than in McMurdo Sound. This was interrupted before resuming and then dominating the ice-accretion process (figure 3). This work was supported by National Science Foundation grant DPP 89-15863. Thanks go to the helicopter pilots and crews of the U.S. Navy VXE-6 Squadron and the U.S. Coast Guard's Polar Sea Aviation Detachment for flying us around McMurdo Sound and cheerfully assisting us with our work. The Cerlache Bay core was obtained with the assistance of the Polar Sea.
References
Cow, A.J. 1991. Personal communication. Cow, A.J., S.F Ackley, WE Weeks, and J.W. Covoni. 1982. Physical and structural characteristics of Antarctic sea ice. Annals of Glaciology, 3, 113-117 Jeffries, MO., and WE Weeks. 1991. Summer pack-ice properties and structure in the western Ross Sea. Antarctic Journal of the U.S., 26(5). Leventer, A., R.B. Dunbar, M.R. Allen, and R.V. Wayper. 1987 Ice thickness in McMurdo Sound. Antarctic Journal of the U.S., 22(5), 9496. Lewis, E.L., and R. Perkin. 1985. The winter oceanography of McMurdo Sound, Antarctica. In S.S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf. (Antarctic Research Series, Vol. 43.) Washington, D.C.: American Geophysical Union. Paige, R.A. 1966. Crystallographic studies of sea ice in McMurdo Sound, Antarctica. (Technical Report R494.) Port Hueneme, Calif.: Naval Civil Engineering Laboratory. Weeks, WE, and A.J. Cow. 1978. Preferred crystal orientations in the fast ice along the margins of the Arctic Ocean. Journal of Geophysical Research, 83(C10), 5105-5121.
1991. Ice thicknesses, salinities, temperatures, and structuralstratigraphic data from 17 cores are discussed. The mean thickness of the sampled floes ranged from 0.59 to 1.53 meters. The mean value of all the ice thickness measurements is 1.09 meters. The ice thickness statistics are similar to the more extensive ice thickness observations obtained from the Weddell Sea (Cow et al. 1987; Wadhams et al. 1987; Lange and Eicken 1991). The Ross Sea ice thickness data complement previous observations that antarctic sea ice is thinner than arctic sea ice of similar age. The mean salinity of the individual cores ranged from 3.44 to 6.01 parts per thousand. The mean value of all the pack ice salinity measurements was 4.5 parts per thousand. This is identical to the value determined for first-year ice in February and March in the Weddell Sea (Cow et al. 1982, 1987). Both of these values are lower than the mean salinity value of 7.3 parts per thousand for cold sea ice sampled in the period from October through December in the Weddell Sea (Eicken and Lange 1989). 95
