Brine rejection

Brine rejection is a process that occurs during sea ice formation where salt is pushed from forming ice into the surrounding seawater, creating saltier, denser brine.

As water reaches the temperature where it begins to crystallize and form ice, salt ions are rejected from the lattices within the ice and either forced out into the surrounding water, or trapped among the ice crystals in pockets called brine cells. Generally, sea ice has a salinity ranging from 0 psu at the surface to 4 psu at the base. The faster that this freezing process occurs, the more brine cells are left in the ice. Once the ice reaches a critical thickness, roughly 15 cm, the concentration of salt ions in the liquid around the ice begins to increase, as leftover brine is rejected from the cells. This increase is associated with the appearance of strong convective plumes, which flow from channels and within the ice and carry a significant salt flux. The brine that drains from the newly formed ice is replaced by a weak flow of relatively fresh water, from the liquid region below it. The new water partially freezes within the pores of the ice, increasing the solidity of the ice.

As sea ice ages and thickens, the initial salinity of the ice decreases due to the rejection of brine over time [Fig. 2]. While the sea ice ages, desalinization occurs to such a degree that some multiyear ice has a salinity of less than 1 PSU. This occurs in three different ways:

Brine rejection occurs around at the north and south poles of the earth [Fig. 3]. The Arctic Ocean has historically ranged from roughly 14-16 million square kilometers in late winter to roughly 7 million square kilometers each September. The annual increase of ice plays a major role in the movement of ocean circulation and deep water formation.  The density of the water below the newly formed ice has increased density due to the brine rejection.  These water parcels are pushed down in the salinity gradient as salt content increases.  The dense waters that form in the Arctic are called North Atlantic Deep Waters (NADW), while the Antarctic Bottom Water (AABW) forms in the southern hemisphere.  These two areas of brine rejection drive the thermohaline circulation in all of earth’s oceans.

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