ice -water interface. According to Bennington (1963) the growing platelets separate 

 brine with a progressively increasing concentration and density. This unstable brine 

 layer then cascades to a lower level and leaves a greater proportion of brine pockets 

 (p. 683). In discussing a possible driving mechanism for brine drainage, Bennington 

 (1966) also states that temperature fluctuations create internal pressure changes that 

 result in brine expulsion from the zone of growing platelets. Each temperature-related 

 episode of brine expulsion would, therefore, form a band and, in turn, record the 

 temperature fluctuation. Dykins (1966, p. 19) describes banding that occurred while 

 freezing seawater in a laboratory tank and states that it may have been caused by 

 varying the temperature in the freezing chamber. 



That the development of banding is related to temperature effects during the 

 growth of the ice is strikingly evident in the different band frequency seen in the 

 cores from stations 1, 2, and 4. At station 1 (Figure 4), where the ice grew over the 

 deep, open water of McMurdo Sound, there were 19 bands in the upper 60 cm (2 feet). 

 At station 2 (Figure 5), where the ice grew in a protected zone of deep, quiet water 

 between the ice shelf and a stranded iceberg, there were eight bands in the upper 

 60 cm. At station 4 (Figure 6), where an early snow cover 91 cm thick dampened 

 the effect of temperature fluctuations, there were only five bands in the upper 60 cm. 



A temperature decrease promotes the growth of platelets and the attendant 

 expulsion of brine. As the temperature of the ice becomes colder during the early 

 stages of ice growth, pure ice platelets are able to grow from seawater of increasing 

 brine concentration. Zones of clear ice between bands probably represent periods 

 of steady growth when convection, tidal currents, or other circulation allowed 

 platelets to grow from seawater of normal salinity. 



In McMurdo Sound the decrease in frequency and the increased spacing 

 between bands from the top of the ice sheet downward results from the dampening of 

 temperature fluctuations by the thickening ice sheet. From the 1.2-meter depth 

 (4 feet) to the bottom of the ice sheet, banding is either rare or absent. Thus, it 

 appears that banding accurately records the temperature fluctuations of an ice sheet 

 during its growth. 



Brine Drainage 



During the early growth stages of sea ice, brine is expelled by the growing 

 platelets of pure ice and becomes trapped as layers and as vertical elongated cells 

 at interplatelet boundaries. It is this localization of brine cells and cavities that 

 so clearly defines the subcrystal structure of sea ice. Figure 7 is a horizontal thin 

 section of sea ice that shows brine cells and layers outlining the platelets of pure 

 ice. While the ice sheet grows during the coldest part of the winter, most brine 

 features are small and closely spaced except for occasional large vertical drainage 

 channels. As the ice sheet warms during the summer, the size and shape of brine 

 drainage features change considerably. 



