angles Is quite evident. Figure 10 is a vertical thin section at a depth of 246 to 

 254 cm, where the ice was 2.7 meters thick. Crystals with the c-axis inclined far 

 from the horizontal predominate in this section. Figure 1 1 is a horizontal thin 

 section from within 5 cm of the bottom of ice 2.87 meters thick. Again, the variety of 

 c-axis angles is obvious, as shown by the large, irregular clear areas. 



When the sea ice stopped growing and bottom melting began, the skeleton 

 layer was quickly destroyed and, as melting progressed upward, crystals with horizontal 

 c-axis orientations predominated. This suggests that during the early stages of ice 

 growth in McMurdo Sound, the skeleton layer is thin with mostly horizontal c-axis 

 crystal growth, as reported for arctic sea ice. The unusually thick skeleton layer 

 with its variety of c-axis orientations may not begin to form until late in the winter, 

 when the growth rate becomes extremely slow (about 10 cm for November) and ? 

 ice is usually more than 2.4 meters thick. The bottom surface of the melting ic 

 sheet became smooth and attained a reversed sun-cupped appearance similar to rne 

 surface of an ablating snowfield. 



STRUCTURAL PARAMETERS 



Structure, as used herein, defines the relationship of sea ice crystals to each 

 other as well as their size, shape, subcrystal features, and c-axis orientation. In a 

 horizontal section, the structure consists of an intensely interlocking mosaic of 

 elongate, sharply angular crystals. Brine layers and cavities occur both at crystal 

 and subcrystal boundaries and vary in size and shape depending upon the age and 

 temperature of the ice. In a vertical section, individual crystals appear as long, 

 lenticular, spindle-shaped grains with their long axis normal to the surface of the 

 ice sheet. Brine features are also commonly elongate in a vertical section, as 

 compared to round or elliptical in a horizontal section. C-axis orientation is 

 predominantly random in the horizontal plane except in the upper few centimeters and 

 at the bottom of a growing ice sheet. 



Gross crystal parameters, such as length, length-width ratio, relative size in 

 terms of crystals per unit area, and subcrystal platelet width, were plotted against 

 depth in the McMurdo ice sheet. All measurements were made in the horizontal 

 plane from photographs of thin sections and are somewhat subjective. Each point 

 plotted is an average of 8 to 12 measurements. Most of the measurements are restricted 

 to a 7.62-cm-diameter core specimen and do not truly represent crystal dimensions 

 at the bottom of the thick, growing ice sheet. For example, actual crystal lengths 

 were as much as 15 cm (horizontally) and some crystals were so large that only a 

 small fraction of the crystal occupied 1 cm . 



14 



