Mineral Dynamics in Sea Ice Brines

The sea ice microstructure is permeated by millimetre to micrometre sized inclusions filled with concentrated seawater-derived brine. It is within these brines that the in-situ chemical and biological reactions occur. The brines are confined to a temperature-dependent composition, becoming more concentrated and reducing in volume with decreasing temperature. Upon sufficient cooling the coupled effects of lower temperature and higher salinity results in the brine exceeding the solubility of a mineral, which precipitates. Given the complex composition of seawater, there are several minerals that can exceed saturation within the polar temperature spectrum, each with their own dynamics and environmental significance. This thesis investigates mirabilite (Na2SO4 10H2O), gypsum (CaSO4 2H2O) and hydrohalite (NaCl 2H2O). The small crystal size ( m), temperature dependence, and solubility of these minerals acts to limit the scope for studying their existence and behaviour in sea ice with field experiments. For these reasons, their dynamics have been investigated in a laboratory setting using synchrotron X-ray powder distraction experiments, and measurements of mineral solubility in solutions representative of sea ice brines at thermal equilibrium. The experimental observations are supplemented with model predictions, and together are used to provide a comprehensive assessment of the existence, role and e ects of mineral precipitation in sea ice. Mirabilite and hydrohalite are found to cause substantial changes to brine composition and the sea ice microstructure, and are observed to interact in accordance with equilibrium crystallisation. The precipitation of mirabilite is also found to have implications for the measurement of sea ice brine salinity. In contrast, the solubility of gypsum displays complex dynamics between 0.2 and 25 C, and is shown to be highly dependent upon the SO2 4 concentration, resulting in the processes of mirabilite precipitation and dissolution controlling the fate of gypsum in sea ice