Impact of organic carbon on weathering and chemical denudation of granular basalt

Bench-scale experiments were conducted to determine rates and patterns of coupled organic matter infusion and weathering in a San Francisco volcanic field (Flagstaff, AZ) basalt sample under experimentally-modeled biotic and abiotic condition and to inform larger-scale collaborative studies at the landscape evolution observatory (LEO), Biosphere 2 (Tucson, AZ), where the same basaltic media is being used in a synthetic hillslope experiment. We postulated that mineral transformations depend significantly on the presence of organic carbon compounds including dissolved natural organic matter (DOM), with organic C simultaneously imprinting the chemical and mineralogical properties of primary and secondary solids undergoing incongruent dissolution. The present work reports on solute releases from Flagstaff basalt (FB) along laboratory-controlled gradients in DOM type and concentration. Loamy sand textured FB was subjected to flow-through, saturated column dissolution experiments using influent solutions with and without DOM compounds. Solutions included Ponderosa pine forest soil O-horizon extracts at three target concentrations: 7, 35, and 70mgL^-1C, malic acid (MA) solutions at 7, 35, 70, and 140mgL^-1C, and a control without DOM but having comparable inorganic solution composition. Chemical denudation rates for FB dissolution products were calculated from the concentration difference between outflow and inflow solutions. In addition, changes in the composition of the solid phase over the course of the experiment were determined using X-ray diffraction (XRD), X-ray fluorescence (XRF), and selective dissolution (SE). Column experiments supported dissolution rates derived from the literature and indicated a potentially strong effect of plant-derived organic ligands on mineral dissolution congruency and secondary phase precipitation. Both malic acid and DOM enhanced basalt dissolution, with malic acid having larger effect on per unit C basis. The largest relative effect of organic ligands was observed for Fe and Ti. Si/Al and Si/Fe chemical denudation rate ratios indicated non-stoichiometric dissolution, consistent with the observed precipitation of secondary phases, as confirmed by XRD and SE. DOM enhanced precipitation of secondary phases but this effect decreased with increase in amount of DOM added. Stoichiometry of the outflow solutions indicated that basaltic glass was preferentially dissolved in agreement with previous modeling predictions. Results suggest that biotic colonization of FB in the large-scale, long-term LEO experiment is likely to strongly influence the rate and congruency of FB weathering reactions and their distribution across the convergent hillslopes.