Microstructure
Crystal structure
The microstructure of sea ice depends largely on ice formation processes. We differentiate coarsely between granular ice which is formed under dynamic and turbulent conditions and columnar ice which forms under quiet conditions. Small round crystals are formed in rough seas, while longer needle like crystals form when the sea surface is flat. Ice consisting of a mixture of both is also often encountered. Usually sea ice consists of a thin layer of granular ice (several centimeters) which is followed by a thicker layer of columnar ice. Such crystal structures are visible when very thin sections of ice are viewed under cross-polarised light.
Brine channels
When sea water freezes its crystal lattice consists of pure water molecules. The dissolved salts are not incorporated in the crystal. They are excluded and retained in the liquid phase. The salinity of the liquid phase thus increases and this lowers its freezing point. This saltier water is cold and dense. A portion sinks out of the ice as it forms but some remains between the crystals to form brine pockets or channels. These channels are tiny with average diameters of 100 micrometers. However, they may increase to millimeter sized tubes. The picture shows an electron micrograph of the brine channel system in columnar ice made visible by filling the system with epoxy resin under a vacuum.
Ice algae
In contrast to sea ice, a brine channel system does not develop in fresh water ice for obvious reasons. It is only because of this system that sea ice is inhabitable by microscopically small plants and animals. Particularly the diatoms have succeeded to adapt to the extreme conditions. Under optimal conditions these, mainly pennate forms, grow so successfully and in such high numbers that they will stain the sea ice a dark brownish green, a colour that is due to their red, yellow and green photosynthetic pigments. This brown coloration is most frequently observed on the underside of the sea ice, where the brine channels have connections to the underlying water and nutrients, important for growth, are replenished.