Seasonal Extremes in Meltwater Chemistry at Bratina Island (Antarctica): Physical & Biogeochemical Drivers Of Compositional Change
Thesis DisciplineAntarctic Studies
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
In order to understand and predict the geochemical conditions in Antarctic meltwater ponds during winter, the geochemical extremes in Bratina Island meltwater ponds over a seasonal cycle were determined and compositional variation related to key physical, chemical and biological processes. A high resolution record of vertical temperature gradients in Skua Pond during freezing, winter and thaw, highlighted a significant seasonal temperature variation (10.3˚C to -41.8˚C) driven by air temperatures and the release of latent heat of fusion. A conceptual model of freeze-thaw involved heterogeneous melting, and explained how the presence of an ice plug near the base of the pond supports the strong chemical stratification observed, which can persist throughout summer. The geochemistry of Bratina Island meltwater ponds was shown to be catchment specific with correlation between geochemical parameters within ponds, but not between ponds. Basal brines that develop during freezing were nearer in composition to the brines preserved during summer, than to those present immediately post-melting. This is due to mineral precipitation during winter removing selected dissolved ions. Therefore winter brine predictions should be based on mid-late summer conditions, and allow for existing geochemical stratification. Nutrient concentrations were vertically stratified, by the same physical processes controlling major ion concentrations. However, the relatively low nutrient concentrations meant that biological processes exerted little influence over winter brine geochemistry. FREZCHEM62 modeled winter brine compositions were consistent with those of brines present during progressive freezing. Predicted mineral precipitation was also consistent with the presence of halite (NaCl), mirabilite (Na₂SO₄.10H₂O), thenardite (Na₂SO₄), magnesite (MgCO₃), gypsum (CaSO₄), sodium carbonate (NaCO₃) and calcite (CaCO₃) in pond sediments. FREZCHEM62 can therefore be used with confidence to predict winter conditions, as long as a reliable initial bulk pond water composition is calculated, and limitations, such as the over-prediction of carbonate mineral formation, are borne in mind.