Modeling of water-rock interaction in the Mackenzie basin: Competition between sulfuric and carbonic acids

The chemical weathering of continental surfaces depends on many factors including lithology, climate, tectonics, erosion, soil or vegetation, and also on the availability of acidity in the weathering zone. In addition to atmospheric/soil CO2, sulfide oxidation may represent a significant source of protons for weathering reactions as shown previously for the rivers draining the Mackenzie Basin, Canada. Here, a numerical model describing continental weathering reactions based on laboratory kinetic laws is coupled to a dynamic global vegetation model (the B-WITCH model) in order to estimate the impact of sulfide oxidation on chemical weathering and associated atmospheric CO2 consumption fluxes throughout the Mackenzie River basin. The model is first calibrated with available field data for the various sub-basin of the Mackenzie watershed and then, a simulation is performed removing the contribution of sulfuric acid to the weathering system. The main outcomes of the present study are that 1) the dissolved silica export is controlled by secondary minerals reactivity in soils and the calcium and magnesium fluxes are driven by the dissolution of calcite and dolomite, 2) the atmospheric CO2 consumption by weathering processes in the Mackenzie river is limited, accounting for only 24% of the total bicarbonate flux to the ocean, confirming a previous study and suggesting that weathering processes in the Mackenzie watershed act as a source of carbon to the atmosphere at the geological timescale, and 3) the presence of sulfide minerals within a river basin decreases the apparent net consumption of atmospheric CO2 through chemical weathering. We show that the actual production of sulfuric acid in the weathering zone of the Mackenzie basin reduces the net atmospheric CO2 consumption by about two, compared to what the atmospheric CO2 consumption should be in the absence of sulfide minerals. Sulfuric-acid-driven chemical weathering has thus to be considered at both short and long timescales when looking at the link between chemical weathering and the global carbon cycle. (C) 2011 Elsevier B.V. All rights reserved.