Sea Ice 



The embayment south of Hut Point Peninsula is covered by ice for at least 

 10 months each year. The maximum breakout commonly occurs in late January or in 

 February, and the sea is usually frozen over again by late March (Heine, 1963, 

 p. 399). During the winter months from March to November the ice sheet grows at 

 a fairly steady rate. In late November the growth rate decreases, and usually by 

 mid-December the ice reaches its maximum thickness of 8 to 11 feet. Surface 

 melting is negligible, but in late December the ice begins to deteriorate internally 

 and starts to thin because of bottom melting. 



The thickness and growth rate of the sea ice vary with time and location and 

 are also affected by circulation, depth of water, exposure to wind, and snow cover. 

 Figure 2 shows the maximum ice thickness at four different localities during the 

 austral summer of 1965-66. Figure 3 shows the growth rate from the end of May 1965 

 to breakout early In February 1966. Data on the initial stages of ice growth in the 

 fall of 1965 are not available; however, data from previous yeors (Tate, 1963) show 

 that the growth rate is rapid: about 3.2 centimeters per day from the end of March 

 to about the middle of April. A decrease in growth rate during mid-August and the 

 final decrease starting at the end of September are reflected by systematic changes 

 in crystal parameters, as discussed later in this report. 



Sample Sites and Study Methods 



The four sample sites for the sea ice crystal studies were located, as shown on 

 Figure 1, within the embayment south of Hut Point Peninsula. Samples were collected 

 from three sites where the Ice sheet was thought to have had different growth histories 

 significant enough to be reflected by different crystollographlc features. The fourth 

 site was used for thickness measurements only. 



Core samples were collected with a standard coring auger and stored In a cold 

 room at -12 C to -9°C. Thin sections were made from the cores and from blocks cut 

 from the upper part of the ice sheet at station 1. Several blocks illustrating growth 

 features at the bottom of the ice sheet were collected from broken and overturned 

 ice behind the icebreaker USS Glacier in the area opposite McMurdo Station, 



Large-scale features easily seen in a sea ice sheet, such as banding and large 

 brine drainage channels, were studied to determine their origin and relation to the 

 history of the ice. Data concerning the nature of ice growth, crystal size and 

 orientation, and the morphology of the bottom surface of a thick, growing ice sheet 

 are rare because direct measurements are difficult to obtain. During Deep Freeze 66, 

 direct observations and estimates of crystal size and relative relief at the bottom 

 were made from an under-ice observation chamber installed by the U. S, Antarctic 

 Research Program. 



