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

Dissolution of calcite and associated interactions of suspended sediment wi th aqueous solution were investigated in a tributary-free 600 m reach of th e 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 c alcite 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 ti ll banks) with the main meltstream. Calcite dissolution is normally directl y proportional to exposed surface area of the mineral, yet only a small inc rease 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 conta ins 30-40% calcite with a minimum specific surface area (S) of 0.25 m(2)/g sediment. Application of the Plummer-Wigley-Parkhurst (PWP) model predicts dissolution rates broadly similar to those found at lower suspended sedimen t concentrations given this value of S. At higher suspended sediment loads predicted dissolution rates are too high. This discrepancy is reduced by us e 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 tr ansfer of H2CO3 given the surface area of calcite to volume of solutions co nsidered. 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 meltstre am 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 hig h values characteristic of low water-rock ratio interactions in subglacial environments and till. Nevertheless, the distinctive chemical imprint in me ltstream chemistry of non-congruent mineral dissolution in low water-rock r atio glacial weathering environments remain. In contrast, in terrains where calcite is scarce, it will tend to dissolve congruently, contributing sign ificantly to total solutes, and its dissolution will be less limited by CO2 reaction kinetics. (C) 1999 Elsevier Science B.V. All rights reserved.