Brine Movement in Antarctic Sea Ice

                             by 

                      Victoria Lytle

          (Antarctic CRC and Australian Antarctic Division
           University of Tasmania)

             JTB 120, 3:20pm  Monday, February 23, 1998


                          Abstract

Sea ice around Antarctica undergoes an annual change in area from 
about 4 million square kilometres in February to 19 million square
kilometres in  September - one of the largest seasonal changes of
surface characteristics on earth. This growth and melt cycle is
crucial in the regulation of climate, and may be a driving force
behind the biological productivity of the Southern Ocean.  The
dense, salty brine which is rejected as sea ice  grows can cause
deep ocean convection, increasing the upwelling of  nutrients, and
enhancing the global ocean circulation.  The snow covered sea ice
insulates the relatively warmer ocean from the cold winter
atmosphere, reducing heat loss. Compared with open water,
snow-covered sea ice reflects  much of the incoming solar radiation,
and reduces the amount of light  available for photosynthesis under
the ice.

Sea ice is not, however, a simple homogeneous layer; currents, 
winds and tides constantly deform the ice creating open water with
newly forming ice  and thicker ridged regions. The ice itself is a
multi-phase material, consisting of solid ice, liquid brine, and in
some cases air bubbles. In  the natural environment, it is found at
its melting point, and small changes in temperature will alter the
relative percentage of each of these phases. High salinity brine
is found in small, vertically elongated pockets and where these
pockets link up to form channels, they provide a conduit for the 
brine. Convection within these brine channels has been well
documented in the laboratory and as the brine moves, it transports
heat, salt and  nutrients.  In some cases, the weight of the snow
cover can depress the  surface of the ice below sea level, allowing
sea water to infiltrate the  snow, either from the side of the floe,
or through this network of brine channels. As this slushy snow
mixture freezes, snow ice is formed on the surface of the existing
ice cover.  Because of the reduced thermal insulation on the
surface, this ice can form much quicker than ice growth at the base
of the floe.

This talk will review recent measurements taken in the sea ice zone 
around Antarctica which provide evidence of brine movement and its
effect on algal blooms, heat fluxes and snow-ice formation. The
rate of snow ice formation can be an order of magnitude greater
than basal ice growth rates, and may be significantly
underestimated in global climate models. Significant ice algal 
blooms have been found which rely on the supply of nutrients by
brine  convection. These processes depend on the linkage of the
brine pockets,  allowing percolation of brine through the ice.


Requests for preprints and reprints to: v.lytle@utas.edu.au
This source can be found at http://www.math.utah.edu/applied-math/