Brine Exclusion Process from Growing Sea Ice

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The exclusion process of brine from growing sea ice, which plays an important role in the polar ocean, has been quantitatively studied in the field and laboratory in which sea ice grew under various conditions. From oceanographic observations during the growth of sea ice in the Antarctic winter it was found that a haline convection induced by brine exclusion changed the salinity structure of shelf water from strarified to homogenized distribution, yielding the salinization of the water. The appearance of the excluded brine falling into the seawater was experimentally observed by means of a schlieren optical system, with the following findings : The excluded brine formed long vertical filaments starting at fixed positions on the water-ice interface and fell into the underlying seawater without appreciable diffusion. The characteristics of the brine filaments varied with the ice growth rate. At a higher ice growth rate, a larger number of thinner brine filaments fell at a lower velocity. Direct measurements were made of the salinity and volume of the excluded brine under various conditions of ice growth. The brine salinity increases and the volume flux decreases as the ice growth rate decreases. Consequently, the salt flux of the brine decreases with decreasing ice growth rate and hence the amount of salt excluded as brine depends highly upon its volume rather than its salinity. For sea ice growth rates between 1.7 ×10^<-5> and 1.4×10^<-4>cm・s^<-1>, and for a seawater salinity of 33.0‰, the brine salinity ranged from 42.3 to 92.7‰, the volume flux ranged from 6.3×10^<-6> to 3.4×10^<-5>cc・cm^<-2>・s^<-1>, and the salt flux ranged from 6.4×10^<-7> to 1.5×10^<-6>g・cm^<-2>・s^<-1>. The total volume of brine excluded during one sequence of ice formation depends upon the duration of formation as well as the ice growth rate. It increases with an increase in both growth rate and duration ; that is, with an increase in the mass of ice grown. The brine exclusion process thus disclosed sheds light on a change in salinity of natural sea ice under various conditions of ice growth. When it takes a longer time for a sea ice to grow to a fixed thickness, the sea ice has a lower salinity. This is due to the exclusion of a larger amount of brine with a higher salinity during the formation of sea ice. Meanwhile, a thicker sea ice grown during a fixed period of time has a higher salinity. This is due to the exclusion of a smaller amount of brine with a lower salinity per unit mass of ice grown during the period. The foregoing results suggest that the salinity and volume of brine excluded during the formation of sea ice can be estimated approximately in future by measuring the duration of formation, thickness and salinity of the sea ice.
北海道大学の論文
1984-03-28

Brine Exclusion Process from Growing Sea Ice

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