Ice in the Sea 247 



For sea-water of salinity 5" there will be a similar relationship 



(1 + a.')S.' = S'. 



Since Sr = St' it follows 



S S' 



-I = ~ = const., 



\ — a-r I — Or' 



that means, the amount of salt solution per gramme is proportional to the salinity 

 of the sea-water from which the ice has been formed. The first substance which begins 

 to separate at temperature below — 8-2°C is sodium sulphate (Na2S04). However, the 

 chlorine is retained since its separation begins only at — 23°C. This selective separation 

 process during freezing changes the composition of the salts in the sea-water (Ringer, 

 1906; see also O. Pettersson, 1883). Thus in the polar seas sulphate is expected to be 

 steadily withdrawn by the freezing process from the sea-water which thus becomes 

 enriched in chloride. On the other hand, in areas where the ice carried away by the 

 ocean currents melts sodium sulphate goes again into solution and the sea-water 

 should show a surplus in SO3. 



Malmgren and Sverdrup (1929) have found that deviations of this type from 

 normal behaviour are only very slight, and thus there occurs no selective process on a 

 large extent during ice formation in nature. On the other hand, the investigations of 

 Liakionoff, according to Wiese (1938), have shown that in the Barents Sea both in 

 sea ice, as well as in melt water, there is a deficit of chloride and a surplus of sulphate 

 (SO3). Further investigation is required to settle this point. 



(b) Density and Porosity of Sea Ice 



The density of pure ice at 0°C is 0-91676, while the density of water at the same 

 temperature is 0-999867. The density of sea ice which is free of air bubbles in- 

 creases with its salinity. If it increases at the same rate as the density of sea-water 

 increases with salinity, the density of sea ice is expected to increase by about 0-0008 

 for every l%o in sahnity. The density of sea ice free of air bubbles and with a sahnity 

 of 15%o would thus be about 0-9296. The first precise determinations of the density 

 of sea ice were made by Makaroff (1901) by extensive measurements of the mean 

 height It and the mean depth d (above and below the sea surface) of freely floating 

 ice floes. If a^ is the density of the sea-water then the density of the sea ice is given by 



This gives a mean value for the entire floe. Makaroff"'s measurements apply only to 

 summer floes of drift ice and give reliable values only for regular floes without any 

 snow cover. These observations gave results between 0-96 and 0-85. These large 

 variations are due to the considerable amounts of air and water which may be present 

 in sea ice. The greatest eff"ect is that due to air bubbles enclosed in the ice, which can 

 be of a twofold origin. One part originates already during the ice formation, due to a 

 separation of gases dissolved in the sea-water which cannot always escape from the 

 cells between the ice crystals. Thus the gas bubbles will be more numerous and larger 

 the faster the rate of freezing of the ice. The upper parts of freshly frozen ice thus 

 usually contain more air than the lower parts. 



