248 



Ice in the Sea 



A second source of air-bubble formation is the penetration of air during the meUing 

 process (Hamberg, 1895). In the upper part of a mass of ice which begins to melt 

 from the inside the rise in temperature first widens the small intermediate spaces con- 

 taining the salt solution. As the ice particles melt their volume decreases and empty 

 spaces are formed into which air is pressed in due to the atmospheric pressure. These 

 spaces finally become so enlarged that the melt water, together with the salt solution, 

 can flow out and finally they are replaced entirely by air. The originally pure and clear 

 ice thus becomes a porous mass penetrated by a number of air channels. On top the 

 drift ice floes in the summer thus always appear white simulating a snow cover. 

 The lower parts below the water surface are still cold and hard (solid). They do not 

 melt from the inside and show, at first, only very little porosity. However, when the 

 ice disintegrates more and more and still drifts in sea-water, the temperature of which 

 is above freezing point, the already existing empty spaces become filled with water 

 and the ice density increases rapidly. 



The air enclosed in sea ice, according to Hamberg, has an oxygen content greater 

 than that of atmospheric air but less than that of the air mixture absorbed by sea-water 

 (24-26% as compared with 20-95 for atmospheric air and 34-6 for sea-water at 0°C 

 and 35%o salinity). 



The most accurate determinations of the density of sea ice in situ have been made by 

 Malmgren (1927) on the "Maud" Expedition. These were made by determination of 

 the loss of weight of a piece of ice on immersion in petroleum of specific weight Pf 

 If the weight of the ice sample in air is G and in petroleum g grammes then the density is 

 given by 



G 



Pi = 



Pf 



Table 93. Density of sea ice 



(According to Malmgren ("Maud" Expedition)) 



^ Young ice partly broken open. 



* Young ice from a freshwater pool on a thick old ice floe. 

 ^ Young ice frozen in autumn from low salinity water. 



* Thick broken young ice some time exposed to the sun. 

 ^ Top peak of ice exposed to sun ("gesommert"). 



* Sample of old ice at the place of temperature measurement. 



