A CONCEPTUAL-MODEL OF SOLUTE ACQUISITION BY ALPINE GLACIAL MELTWATERS

Solute acquisition by Alpine glacial meltwaters is the result of the c oupling of different pairs of reactions, one of which usually involves dissolved gases. Hence, the availability of atmospheric gases to solu tion is an important control on the composition of glacial meltwaters. The chemical compositions of the two main components of the bulk melt water, quick flow and delayed flow, are dominated by different geochem ical processes. Delayed flow waters are solute-rich and exhibit high p (CO2) characteristics. The slow transit of these waters through a dist ributed drainage system and the predominance of relatively rapid react ions, such as sulphide oxidation and carbonate dissolution, in this en vironment maximize solute acquisition. Quick-flow waters are dilute, b oth because of their rapid transit through ice-walled conduits and ope n channels, and because the weathering reactions are fuelled by relati vely slow gaseous diffusion of CO2 into solution, despite solute acqui sition being dominated by rapid surface exchange reactions. As a conse quence, quick flow usually bears a low or open-system p(CO2) signature . Bulk meltwaters are more likely to exhibit low p(CO2) values when su spended-sediment concentrations are high, which promotes post-mixing r eactions. This conceptual model suggests that the composition of both quick flow and delayed flow is likely to be temporally variable, since kinetic, rather than equilibrium, factors determine the composition.