By late December high ambient temperatures and strong solar radiation in the 

 McMurdo area caused the ice sheet to become nearly isothermal and to approach the 

 melting point of sea ice. Brine drainage was accelerated as the cells enlarged and 

 coalesced (Figure 8) and formed long columnar brine drainage channels normal to 

 the surface of the ice. Many of the brine drainage channels were open to the sea 

 below and filled with seawater to a level depending upon hydrostatic conditions of 

 the ice sheet. Solar radiation transmits considerable energy to the ice sheet, especially 

 if the ice has no protective layer of snow. Because of this, a zone of unusually 

 weak ice occurred between the 30- and 91-cm depth, where brine cavities became 

 numerous and large. 



Bottom Growth 



The bottom of a growing sea ice sheet is unlike that of freshwater ice and is 

 characterized by an irregular surface with numerous disconnected ice platelets 

 protruding downward into the seawater. This layer has been termed the "skeleton 

 layer" by Assur (in Butkovich, 1956, p. 1) and results from the separation of pure ice 

 platelets freezing from seawater, as explained by Weeks (1958, p. 97). 



The small amount of available information regarding the thickness of the 

 skeleton layer comes from various sea ice studies in the arctic and subarctic. The 

 skeleton layer has been observed in arctic sea ice to be 2.4 to 2.8 cm (Weeks and 

 Anderson, 1958, p. 644), 1 to 2 cm (Schwarzacher, 1959), and up to 2 cm (Bennington, 

 1963, p. 685). 



Direct observation revealed that the bottom surface of the growing ice sheet 

 In McMurdo Sound had a skeleton layer 10 to 15 cm thick, with a maximum of 30 cm 

 in isolated locations. Individual sheet- like platelets that protruded downward 10 to 

 15 cm beyond the bottom of the ice sheet were common. The bottom surface was 

 undulating and extremely irregular, with a possible relative relief of at least 60 cm 

 (2 feet). It must be emphasized that the above observations were made when the 

 ice was more than 1.8 meters thick and about 10 months old. 



The idealized concept of platelet-crystal growth at the bottom of a sea ice 

 sheet shows the platelets growing vertically downward with their c-axes truly 

 horizontal (Assur and Weeks, 1964, p. 4), (Stehle, 1965, p. 3), and (Peyton, 1963, 

 p. 109). Weeks and Anderson (1958, p. 643) describe the skeleton layer as consisting 

 of unconnected vertical ice plates. Two photographs by Bennington (1963, pp. 679, 

 682) of vertical thin sections from the bottom of thick ice clearly show that c-axis 

 horizontal crystals predominate. 



The c-axis orientation of platelet-crystal growth at the bottom of thick sea 

 in McMurdo Sound was random, with only a slight majority of platelets growing 

 vertically downward. Figure 9 shows a slab cut vertically from the bottom of 

 2. 44-meter- thick sea ice. The slab was about 25 cm wide, 25 cm long, and 1 cm 

 thick. The dendritic platelet growth with the c-axes inclined at a wide variety of 



13 



