Living at extremes

The Biochemist — February 2005. © 2005 Biochemical Society 12 of metres thick. The majority of the ice in the Southern Ocean lasts only less than 1 year, and the average Antarctic sea ice thickness is 1 m. In contrast, in the Arctic Ocean sea ice can last several years and the average thickness is generally 2 m. When ice forms from freshwater, the result is a hard brittle solid with the primary inclusions being gas bubbles. In contrast. when seawater freezes the resultant ice is a semi-solid matrix, permeated by a labyrinth of brine-filled channels and pores. The volume of ice occupied by the brine channels is directly proportional to the temperature of the ice, as is the brine concentration within the channels: at 6 C the brine salinity is 100 g/l, at 10 C it is 145 g/l and at 21 C it is 216 g/l. The brines are not static and gravity drainage results in a gradual desalination of sea ice as it ages and brines are expelled into the underlying waters1. The temperature at the upper surface of an ice floe is determined by the air temperature (down to Frozen seawater