Geochemistry of Subglacial Calcites: Implications for the Hydrology of the Basal Water Film

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Abstract
Three types of calcite precipitate are found in the proglacial area of the Glacier de Tsanfleuron, Switzerland. Sparite and micrite form part of an evolutionary series precipitated in response to solute concentration as a result of progressive freezing of the subglacial water film. Sparite forms under open-system conditions or during the early stages of freezing under closed-system conditions, while micrite forms during the final stages of closed-system freezing. Sparite has a .delta.18O composition which is heavier than that of micrite, higher Ca/Sr and Ca/Mg rations, and a lower impurity content. Micrite is precipitated from water which is more supersaturated with respect to calcite than that from which sparite is precipitated. At times when precipitates are forming, film flow is dominated by basally produced waters which are richer in 18O than glacier ice as a result of selective drainage of 16O-rich residual waters into the main linked-cavity drainage system beneath the glacier. Compositional characteristics of the subglacial calcites indicate that neither type of precipitate could form if the film was transporting large volumes of water derived from bulk glacier ice. The origin of white deposits is less well understood. They have a pustular morphology reminiscent of some bacterial calcites and some samples display unusually heavy .delta.13C values. These could be explained by precipitation of calcite inequilibrium with atmospheric CO2 or as a result of fractionation due to photosynthetic removal of 12C from the water by cyanobacteria. In either case, formation in submarginal cavities where there is free access to light and to the atmosphere may be favored.

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