Relative weathering intensity of calcite versus dolomite in carbonate‐bearing temperate zone watersheds: Carbonate geochemistry and fluxes from catchments within the St. Lawrence and Danube river basins

Abstract

Calcite and dolomite solubilities in open weathering environments are proportional to p CO 2 and inversely proportional to temperature, and dolomite solubility is progressively greater than calcite below 25°C. The continent‐scale weathering budget reveals the significance of the Northern Hemisphere (NH) to globally integrated riverine fluxes of Ca 2+ , Mg 2+ , and HCO 3 − . The NH contributes 70% of the global HCO 3 − flux while only 54% of the riverine discharge. We present results of a comparative hydrogeochemical study of carbonate mineral equilibria and weathering fluxes in two NH carbonate‐rich river basins. Surface water geochemistry and discharge were determined for headwater streams in Michigan and Slovenia within the St. Lawrence and Danube river basins. Michigan watersheds are established atop carbonate‐bearing glacial drift deposits derived from erosion of Paleozoic strata with thick soil horizons (100–300 cm). Slovenia watersheds drain Mesozoic bedrock carbonates in alpine and dinaric karst environments with thin soil horizons (0–70 cm). Carbonate weathering intensity is a parameter that normalizes river runoff and HCO 3 − concentration to catchment area (meq HCO 3 − km −2 s −1 ), summing calcite and dolomite contributions, and is used to gauge the effects of climate, land use, and soil thickness on organic‐inorganic carbon processing rates. Importantly, Michigan riverine discharge is one‐tenth of Slovenian rivers, providing the opportunity to evaluate the kinetics of carbonate mineral equilibration. The study rivers are HCO 3 − − Ca 2+ − Mg 2+ waters, supersaturated for calcite at p CO 2 values in excess of the atmosphere. As discharge varies, HCO 3 − concentrations differ by less than 20% for any location, and Mg 2+ /Ca 2+ remains relatively fixed for Michigan (0.5) and Slovenia streams (0.4), requiring that dolomite dissolution exceed calcite on a mole basis. The ability of calcite and dolomite dissolution to keep pace with increased discharge indicates carbonate weathering is limited only by water flux and temperature‐dependent solubility in these watersheds. Carbonate weathering intensity in Michigan and Slovenia exceeds the world average by factors between 2 and 20, and dolomite weathering intensity, estimated from riverine Mg 2+ fluxes, exceeds the world average by factors between 2 and 15. Thus global fluxes of carbonate‐related weathering products appear heavily skewed toward carbonate‐bearing environments at higher latitudes with relatively low mean annual temperatures and high discharge.