Interactions of calcareous suspended sediment with glacial meltwater: a field test of dissolution behaviour

Abstract

Abstract Dissolution of calcite and associated interactions of suspended sediment with aqueous solution were investigated in a tributary-free 600 m reach of the main meltstream draining the Tsanfleuron glacier, Switzerland, over a 24-h cycle during which solute concentrations varied inversely with discharge. Downflow, solute calcium, strontium, and alkalinity increased because of calcite dissolution. Using flow-through times from salt-dilution gauging, a consistent small sulphate excess at the downstream site was observed. Given the slowness of sulphate supply by pyrite oxidation, this excess sulphate can be attributed to mixing of around 1% of ion-rich water (seeping from till banks) with the main meltstream. Calcite dissolution is normally directly proportional to exposed surface area of the mineral, yet only a small increase in calcite dissolution was observed when suspended sediment increased by a factor of 25 to 1.3 g/l at peak flow. The suspended sediment displays little variation in size distribution with total suspended load, and contains 30–40% calcite with a minimum specific surface area (S) of 0.25 m2/g sediment. Application of the Plummer–Wigley–Parkhurst (PWP) model predicts dissolution rates broadly similar to those found at lower suspended sediment concentrations given this value of S. At higher suspended sediment loads predicted dissolution rates are too high. This discrepancy is reduced by use of the Buhmann–Dreybrodt (B–D) model which takes explicit account of the slowness of hydration of aqueous carbon dioxide, and the problem of mass transfer of H2CO3 given the surface area of calcite to volume of solutions considered. The remaining discrepancy implies less interaction than expected of suspended sediment particles with turbulent meltwater at high suspended sediment concentrations. The effects of proglacial modification of meltstream geochemistry in this case is a strong decrease in PCO2 accompanied by an increase in total ion load, but decreases in Mg/Ca and Sr/Ca, from the high values characteristic of low water–rock ratio interactions in subglacial environments and till. Nevertheless, the distinctive chemical imprint in meltstream chemistry of non-congruent mineral dissolution in low water–rock ratio glacial weathering environments remain. In contrast, in terrains where calcite is scarce, it will tend to dissolve congruently, contributing significantly to total solutes, and its dissolution will be less limited by CO2 reaction kinetics.