CO 2 and H 2SO 4 consumption in weathering and material transport to the ocean, and their role in the global carbon balance

Abstract

Consumption of CO 2 in mineral weathering reactions is one of the major fluxes in the global carbon cycle that drives the weathering and transport of its products by surface water from land to the ocean. In the weathering cycle, carbon dioxide, as an acid derived directly from the atmosphere and(or) remineralization of organic matter in soil, is supplemented by small, but perhaps regionally important, amounts of sulfuric acid forming in the oxidation of pyrite (FeS 2). Reactions of dissolved CO 2 and H 2SO 4 with carbonate and silicate minerals in continental sediments and crystalline crust produce the bicarbonate ion HCO 3 − and release metal cations, such as the four major cationic components of river water, Ca 2+, Mg 2+, Na +, and K +, and dissolved silica to solution. Depending on the reactions that may either only consume CO 2 or uncommonly also produce it, a general relationship describing the CO 2 consumption by weathering reactions with carbonate and silicate minerals is a weathering potential ψ = (net CO 2 consumed) / (HCO 3 − produced). The lower values of this ratio, about 0.54, are for carbonate rocks and evaporites, about 0.75 for shales and sandstones, and 1 for the crystalline igneous continental crust. In an average world river (of which there is more than one estimate of chemical composition), the mass proportions of the main cations and anions differ from those in the weathering source that consists of the sediments and part of the continental crust because of the differences in mineral solubilities and dissolution rates. A dissolution model of a weathering source that consists of 63 wt% average sediment and 37 wt% upper continental crust gives the concentrations of the major dissolved constituents in an average river that agree very well with the range of composition given by other investigators. This dissolution model also provides an average CO 2 consumption potential of ψ = 0.72 and a sequence of relative stability or order of persistence in the weathering of the mineral constituents of the sedimentary carbonate, silicate, and evaporitic rocks, and the crustal silicates. The CO 2 consumption rate translates into a weathering flux of about 22 × 10 12 mol C/yr, derived mainly from soil–atmosphere CO 2 that forms by decomposition of organic matter in soils. Anthropogenic emissions of SO 2 to the atmosphere, as projected for the future and at the upper bound of the projection, may provide H 2SO 4 to the continental surface that is 3 to 5 times greater than the natural H 2SO 4 production by the oxidation of pyrite in sediments. The higher input rates of H 2SO 4 may increase the dissolved ionic solid concentrations in river waters by about 13%, without significantly affecting the CO 2 consumption in weathering. In the global carbon cycle, the CO 2 uptake in weathering is comparable to other interreservoir fluxes in the atmosphere–land–ocean system.