Figure 17 shows ring-tensile strength,* temperature, and salinity, versus depth. 

 A comparison of the ring-tensile strength curves with the data shown in Figures 12, 

 13, 14, and 15 indicates that there is no apparent correlation between the vertical 

 distribution of strength and the various crystal parameters in a mature, isothermal ice 

 sheet. The progressive strength decrease with time occurring in the 0.3- to 0.9-meter 

 depth interval (Figure 17a) results predominantly from solar radiation energy causing 

 accelerated brine drainage and the enlargement of brine layers and cavities. This 

 phenomenon also has no relation to crystal structure. The strength increase at the 

 surface of the 12 January 1966 plot resulted from freshening of the sea ice by almost 

 complete brine drainage. 



FINDINGS 



1. There is apparently no distinct correlation between crystal structure and ring-tensile 

 strength in the thick, mature isothermal ice sheet in McMurdo Sound. 



2. Subcrystal platelet width increased from about 5 mm at the surface to more than 

 10 mm at a depth of 3 meters, but had no apparent effect on the variation of ring- 

 tensile strength. 



3. Crystal length (in a horizontal plane) increased systematically with depth from 

 less than 1 cm near the surface to as much as 15 or 20 cm (as observed) at the bottom 

 of the ice sheet. 



4. The crystal length-to-width ratio changed from about 2 to 1 in the upper half of 

 the ice sheet to more than 5 to 1 near the bottom. 



5. The spacing and frequency of horizontal bands of cloudy, white ice alternating 

 with zones of clear ice was attributed to temperature fluctuations during early 

 growth of the Ice sheet. 



6. Wavy extinction in single crystals and a strong preferred c-axis orientation in 



ice from a pressure ridge indicated that the stress conditions in sea ice can be deduced 

 by detailed crystal studies. 



7. The bottom surface of the thick, growing ice sheet was extremely irregular, with 

 a possible relative relief of at least 60 cm (2 feet). 



8. Brine drainage features varied in size and shape in relation to temperature and 

 solar radiation intensity. This was especially true in the 0.3- to 0,9-meter depth 

 interval, where a zone of weakness was caused by enlarged and closely spaced brine 

 drainage cavities. 



''The ring-tensile strength test (Butkovich, 1956) was performed by drilling a l/2-inch- 

 diameter hole coaxial ly through a 3-inch-diameter core specimen and applying a load 

 normal to the axis at a loading ram rate of 8 in./min. 



27 



