Geochemical weathering at the bed of Haut Glacier d'Arolla, Switzerland - a new model

Waters were sampled from 17 boreholes at Haut Glacier d'Arolla during the 1993 and 1994 ablation seasons. Three types of concentrated subglacial water were identified, based on the relative proportions of Ca2+, HCO3- and SO42- to Si. Type A waters are the most solute rich and have the lowest relative proportion of Si. They are believed to form in hydrologically inefficient areas of a distributed drainage system. Most solute is obtained from coupled sulphide oxidation and carbonate dissolution (SO-CD). It is possible that there is a subglacial source of O-2, perhaps from gas levels found (up to 1200 mueq/L) are greater than could be bubbles released during regelation, because the high SO42- achieved if sulphides are oxidized by oxygen in saturated water at 0degreesC (c, 414 mueg/L). A more likely alternative is that sulphide is oxidized by in anoxic environments. If this is the case, exchange reactions involving Fe-III and Fe-II from silicates are possible. These have the potential to generate relatively high concentrations of HCO3- with respect to SO42-. Formation of secondary weathering products, such as clays, may explain the low Si concentrations of Type A waters. Type B waters were the most frequently sampled subglacial water. They are believed to be representative of waters flowing in more efficient parts of a distributed drainage system. Residence time and reaction kinetics help determine the solute composition of these waters. The initial water-rock reactions are carbonate and silicate hydrolysis, and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites. Hydrolysis is followed by SO-CD. The SO42- concentrations usually are