VII. THE PHYSICS OF SEA-ICE 



E. R. Pounder 



1. Introduction 



Pure water freezes at 0°C. The presence of salts in sea-water depresses the 

 freezing point to — 1.9°C for a sahnity 8 = 35%^, if the various salts are in the 

 proportions normally found. The depression of the freezing point varies linearly 

 with the salinity. As the sea-water freezes, pure ice crystals are formed because 

 foreign atoms or ions cannot apparently fit into the solid H2O lattice. The 

 only known exception to this rule is the fluoride ion (Workman and Drost- 

 Hansen, 1954), which is rare in sea-water. Consequently, if sea-water were 

 frozen infinitely slowly, a cover of pure ice would form, with total rejection of 

 the salts into the underlying melt. The presence of salts in sea-ice thus results 

 from mechanical trapping by the growth of pure crystals, and the amount of 

 salt trapped will depend largely on the freezing rate. The salinity of new sea-ice, 

 very rapidly frozen, may be as high as 20 %o but most annual sea-ice has a 

 sahnity ranging from 10%o to about 2%o. The bulk of sea-ice has a salinity of 

 around 4%^. 



It is well known that pure water has its maximum density at 4°C. Hence a 

 lake or pond cooled by the air undergoes convective circulation until it reaches 

 a uniform temperature of 4°C. With further cooling the surface temperature 

 drops until ice forms, but the water below a certain depth, called the thermo- 

 cline, remains at 4°C throughout the winter. The depth of the thermocline 

 increases gradually as long as there is a net loss of heat through the ice. The 

 temperature of maximum density, or inversion temperature, of a saline solution 

 is lower than that of pure water. For normal sea-water the inversion tempera- 

 ture decreases linearly with salinity, and the freezing point and inversion 

 temperature are both equal to - 1 .3°C for aS' = 24.7 %o. For sea-water of a sahnity 

 greater than this critical value, the density increases steadily with decreasing 

 temperature, until the freezing point is reached. In dealing with the freezing 

 of saline water, it is convenient to call water with ;S<24.7%o brackish fresh 

 water, reserving the term sea-water for salinities higher than the critical value. 



The properties of sea-water just described make its behaviour on cooling 

 considerably different from that of fresh water. Since there is no inversion 

 temperature to limit convective circulation, in theory a still body of sea-water 

 would cool uniformly and no ice could form until the water right to the bottom 

 was at the freezing point. It is for this reason that sea-ice forms only in high 

 latitudes and after prolonged periods of cooling. In practice, sea-water at a 

 uniform temperature is not observed for great depths below an ice cover. One 

 limiting factor on vertical circulation is the compressibility of sea-water. In 



[MS received June, 1960] 826 



