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.

Formation

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. The faster that this freezing process occurs, the more brine cells are left in the ice. These leftover brine cells create a porous layer – the mushy layer – in which concentrated liquid surrounds nearly pure solid ice crystals.^[1] Once the mushy layer reaches a critical thickness, roughly 15 cm, the concentration of salt ions in the liquid around the ice begins to increase, as leftover liquid begins to leave the brine cells.^[2] This increase is associated with the appearance of strong convective plumes, which flow from channels within the ice and carry a significant salt flux. The brine that drains from the mushy layer 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 mushy layer, increasing the solidity of the ice.^[2]

While the sea ice ages, desalinization occurs to such a degree that some multiyear ice has a salinity of less than 1 PSU.^[3] This occurs in three different ways:

• solute diffusion - this depends on the fact that brine inclusions trapped in ice will begin to migrate toward the warmer end of the ice block. The ice block is warmest at the water-ice interface, thus pushing the brine out into the water surrounding the ice.^[4]
• gravity drainage - Gravity drainage involves the movement of brine due to differences in density between brine in the interior of the ice and brine in the seawater outside of the ice, which occurs due to the development of a buoyancy driven convection system.^[2]
• expulsion - the migration of brine due to cracking produced by thermal expansion of the ice, or pressure caused by the increased volume of the newly formed ice.^[4]

Impact of Brine Rejection

Brine drainage has a controlling influence on the mechanical, biological and transport properties, and hence upon the ecology of the polar oceans.^[5] The downwelling of denser, briny water from sea ice creation causes upwelling of deep bottom water, and helps initiate deep ocean currents, which helps to drive thermohaline circulation. Brine rejection is the dominant mechanism for formation of the deep and bottom waters in the Antarctic, and for formation of the densest part of the North Pacific intermediate water, and is a central process for changes in the water masses of the Arctic.^[3]

According to some hypotheses, the salty water masses caused by brine rejection fill the deep ocean basins and lead to stratification, which locks considerable amounts of CO2 in the deep oceans.^[6] Because these waters are able to contain a large amount of CO2, they have helped slow the process of climate change.

Climate change could have different effects on ice melt and brine rejection. Previous studies have suggested that as ice cover thins, it will become a weaker insulator, resulting in larger ice production during the autumn and winter. The consequent increase in winter brine rejection will drive ocean ventilation, and strengthen the inflow of warm Atlantic waters.^[7] Studies of the last glacial maximum (LGM) have indicated that a drastic reduction in the production of sea ice and thus reduction of brine rejection, would result in the weakening of the stratification in the global deep oceans and in CO2 release into the shallow oceans and the atmosphere, triggering global deglaciation.^[6]

Life in Brine Rejection Channels and Surrounding Waters

Life in sea ice is energetically demanding, and sets limits at any hierarchical organizational, and organismic level, ranging from molecules to everything that an organism does.^[6] Despite this fact, the brine-containing interstices and pockets found in sea ice host a variety of organisms, including bacteria, autotrophic and heterotrophic protists, microalgae, and metazoan.^[8]

References

1. Worster M.G. (1992) Instabilities of the liquid and mushy regions during solidification of alloys. J. Fluid Mech. 237 649 – 669.
2. Wettlaufer J.S., Worster M.G., Huppert H.E. (1997). Natural convection during solidification of an alloy from above with application to evolution of sea ice. J. Fluid. Mech. 344 291-316.
3. Talley L.D., Pickard G.L., Emery W.J., Swift J.H., 2011. Descriptive Physical Oceanography: An Introduction (Sixth Edition), Elsevier, Boston, 560 pp.
4. Lake R.A., Lewis E.L. (1970), Salt rejection by sea ice during growth. J. Geophys. Research. 75, 583-597.
5. Wells A.J., Wettlaufer J.S., Orszag S.A. (2011). Brine fluxes from growing sea ice. Geophys. Rev. Lett. 30 4501 – 4506.
6. Thatje S., Hillenbrand C.D., Mackensen A., Larter R. (2008) Life hung by a thread: endurance of Antarctic fauna in glacial periods. Ecology. Mar;89(3) 682-692. PMID 18459332
7. Holland M.M., Bitz C.,Tremblay B. (2006), Future abrupt reductions in the summer Arctic sea ice. Geophys. Res. Letters. 33, 1-5.
8. Giannelli V., Thomas D. N., Haas C., Kattner G., Kennedy H., Dieckmann G.S. (2001), Behaviour of dissolved organic matter and inorganic nutrients during experimental sea-ice formation, Ann. Glaciology. 33, 317-321.