Small-scale details and measurements of various crystal parameters were 

 obtained from thin sections of sea ice that were photographed under plain and crossed 

 polarized light to bring out different features. Most of the crystal measurements were 

 made from enlargements of the photographs. Ring-tensile strength data collected 

 after the ice sheet had become nearly isothermal were plotted and compared with 

 temperature, salinity, platelet width, and other crystal parameters in an attempt to 

 correlate these properties with the strength data. 



LARGE-SCALE FEATURES 



The most obvious features visible in a vertical section of an ice sheet are 

 horizontal banding (stratification), brine drainage channels and cavities, and bubble 

 zones. Under natural conditions these features are usually seen only in pressure 

 areas, where the ice may be uplifted, broken, or tilted in such a way as to expose 

 a vertical section. The easiest way to study these features is to extract complete 

 7.62-cm-diameter cores and photograph sections of the core against a dark background. 



Banding 



Horizontal bands of alternating blue and white ice are a prominent feature 

 in uplifted or tilted blocks of sea ice, especially when the ice has been exposed for 

 several days and the bands are emphasized by sublimation. Banding, or stratification, 

 in arctic sea ice has been briefly discussed by Schwarzacher (1959) and Smith (1964). 

 Bennington (1963) presents an excellent discussion of banding in arctic sea ice and 

 states (pp. 681-682) that bands are caused by one of the following processes: (1) an 

 accumulation of crystals with vertical c-axis orientation, (2) high-porosity zones 

 consisting of brine pockets along staggered intersections of platelets, and (3) the 

 expansion of trapped brine pockets. 



Banding in the sea ice of McMurdo Sound was seen at all levels, but occurred 

 most frequently and with closer spacing in the upper half of the ice sheet. The 

 frequency of the bands (number of bands per foot) and the spacing between them varied 

 from place to place and apparently reflected differences in the growth history of the 

 ice. The bands varied in width from 1 to several centimeters and all had diffuse 

 boundaries. The ice between bands was generally clear, although zones of a slightly 

 milky ice were common. Long, vertical, small-diameter brine drainage channels 

 were common throughout the ice sheet but increased in size and number towards 

 the bottom. The bands were easily seen in cores or blocks but were almost indistin- 

 guishable in thin section. 



Banding in the sea ice of McMurdo Sound consists of high-porosity zones 

 formed by horizontal rows of minute, closely spaced, elongated bubbles and brine 

 drainage cavities. They are apparently caused by the downward drainage of high- 

 density brine formed during platelet growth under supercooled conditions at the 



