246 



Ice in the Sea 



shows that the faster the ice is formed (at lower temperatures) the less salt solution 

 can escape and the higher therefore the salinity of sea ice (Tabic: 91). Since ice is 

 formed more slowly in the deeper layers than at the surface some dependence on depth 

 can also be expected. For a young ice layer that began to freeze in November 1924, 

 Malmgren found in April 1925 the values shown in Table 92. Referring finally to the 

 age of the ice, the older the ice the smaller its salinity. The salt solution leaks through 

 continuously and this process is accelerated by changes in temperature. Blocks of 

 ice lifted by the pressure of the ice become almost completely salt-free in the summer by 

 this process of deconcentration, and can be used after melting for drinking water. 



The changes in salinity in winter ice occurring during the course ola year have been 

 summarized by Malmgren in a diagram given in Fig. 111. The ice formed in October 

 gradually increases in thickness, and initially the salinity decreases from the surface 

 downwards. Corresponding to their age the middle layers have the lowest salinity. 



12345678 9 



Fig. 111. Salinity changes in winter ice during the course of the year (schematic, according 



to Malmgren). 



but at the lower surface of the ice layer the salinity again increase:* since the water 

 freezes here from below. This is due to melt water sinking below the ice layer and 

 freezing again immediately due to temperatures below freezing point (about — 1-6°C). 

 In a dilute aqueous solution freezing proceeds with the formation of pure ice only 

 until the eutectic point is reached, the concentration of the solution increasing at the 

 same time. This critical point depends on the salts dissolved in the watoi . When sea- 

 water freezes the separation of the salts dissolved in the water begins only at — 8-2°C. 

 For sea-water the situation is simplified only in so far as: (1) all types of water have the 

 same salt composition; and (2) ice is always the first substance to freeze out. In conse- 

 quence, no matter how great the salinity, the freezing process always proceeds in the 

 same way. For a given temperature the concentration and the composition of the salt 

 solution is to a close approximation the same for all types of sea-water, regardless of 

 their original salinity (Malmgren). If Tj is the freezing temperature of 1 g of sea- 

 water of salinity S, then for a temperature t between t^ and --8-2°C only pure ice 

 will separate out according to the above discussion, and at this temperature there 

 will be a-r g of pure ice and (1 + a^) g of salt solution. If the sali ity of the salt solu- 

 tion is S-T, then necessarily 



(1 -f a;)S. = S. 



