The fate of CO2, Ca, and Mg after terrestrial rock weathering

The silicate−carbonate cycle controls atmospheric CO2 concentrations and moderates Earth’s climate over geologic time scales. Chemical weathering of silicate minerals by CO2 results in the release of cations and the neutralization of CO2 to HCO3− or CO32−. The precipitation of Ca- and Mg-carbonate minerals is expected once waters are supersaturated. However, quantifying the magnitude of supersaturation required, particularly for Mg-carbonates, has remained challenging. Here we present a database of 854 water samples from the Central Plateau, British Columbia, Canada, representing a wide range of salinities, including both Na−(SO4)−HCO3−CO3 and Mg−Na−SO4 hypersaline lakes, to determine the geochemical thresholds for Ca- and Mg-carbonate formation. For HCO3-dominated waters, the data indicate maximum alkalinity thresholds of ∼5 mEq/kg for Ca-carbonates and ∼40 mEq/kg for Mg-carbonates. Activity plots of Ca−CO32− and Mg−CO32− suggest that maximum saturation thresholds for both Ca-carbonates and Mg-carbonates are applicable to HCO3-dominated, SO4-dominated, and CO2-rich waters. These geochemical thresholds may be used to optimize geochemical carbon dioxide removal (geoCDR) technologies, such as enhanced rock weathering, as HCO3− has up to double the CDR efficiency of carbonate minerals.