Congelation ice

Abstract: The distribution of temperature throughout an ice sheet has been considered, taking into account the influence of ice movement as well as other items previously considered, such as conduction, the geothermal outflow of heat and heat generated by ice movement. By making certain simplifying assumptions, a quantitative method of estimating the temperature distribution near the centre of an ice sheet has been put forward. It is shown that even a small mean annual accumulation will have considerable effect on the temperature distribution in a large ice sheet. For a moderate rate of accumulation a substantial fraction of the total thickness of ice at the centre of a large ice sheet may be isothermal at the prevailing surface ice temperature. Under these conditions at some distance from the centre, the change in the surface ice temperature with elevation may produce a temperature gradient opposite to normal, that is the temperature falls with increasing depth below the surface, due to the outward movement of the ice. Observed temperature gradients on ice sheets fit the proposed hypotheses roughly, but it appears that climatic change should also be taken into account. It is suggested that a rise from temperatures below melting point at the base of ice sheets may provide an explanation of the occasional catastrophic advances of certain glaciers.

Abstract: During the first leg of the Winter Weddell Sea Project (Antarktis V/2) cruise of F.S. Polarstern the entire width of the Antarctic sea ice zone was traversed in the vicinity of 0° longitude in the period July 18 to September 10, 1986. Ice thicknesses were measured by direct drilling and by helicopter profiling using an Exstar 100-MHz impulse radar system. In addition, aerial photography of the ice cover was done from 100- to 2000-m altitude using a 70-mm aerial camera mounted in the helicopter. The results of the point measurements (drilling) are reported in this paper together with an indication of how the radar and photography data will be used to extend them so as to yield area-averaged ice thickness distributions. It was found that the main ice type across the entire width of the ice cover was consolidated pancake ice occurring in vast floes; this formed out of a 250-km-wide band at the advancing ice edge which comprised a concentrated field of individual pancakes in a matrix of frazil ice. Preferred thicknesses of undeformed floes were 40–60 cm of ice covered with 5–15 cm of snow. The individual pancakes attained almost all of this thickness before consolidation; subsequent congelation growth was slow, estimated at 0.4 cm d−1. The floes contained much small-scale roughness on the upper and lower surfaces due to rafting of pancakes at the time of consolidation, but pressure ridging was modest except in the far south. A few very thick (8–11 m) multiyear floes were observed embedded in the pack at latitudes beyond 66° S.

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Abstract: We report on the development and physical properties of sea ice in the central and eastern Weddell Sea. The investigations were part of the Winter Weddell Sea Project 1986, which extended over the months of July through December. Major elements of the glaciological part of this study included continuous shipborne observations of sea-ice conditions and occasional helicopter reconnaissance flights, extensive measurements of snow and ice thicknesses at daily ice stations, and detailed analyses of sampled ice cores from each ice station. Textural investigations of the sampled ice revealed the dominance of frazil ice in the central Weddell Sea and the occurrence of an additional ice class, called platelet ice, together with the commonly known frazil and congelation ice in the coastal region of the eastern Weddell Sea. These results, in combination with the visual ice observations, reveal two major mechanisms for sea-ice generation in the Antarctic, which were not sufficiently well accounted for in previous investigations. In the central Weddell Sea, a cycle of pancake-ice formation and its growth into consolidated floes seems to be the dominant process of the advancing sea-ice edge. In the coastal waters, the growing sea-ice cover consists, to a considerable degree, of ice platelets which are formed in the underlying water column in front of the ice-shelf edges. Thus, congelation-ice growth, which is mainly controlled by atmospheric, thermodynamic forcing, seems to be of less importance in the central and south-eastern Weddell Sea than, for example, in the Arctic Basin.