STRONG ACIDS AND THE CARBONATE MINERAL WEATHERING ATMOSPHERIC CARBON SINK

Weathering of carbonate rocks within karst aquifers on the continents has been recognized as a sink for atmospheric CO₂. Global carbon budget models, however, have assumed that this is offset by CO₂ resulting from reprecipitation of that carbon in the oceans as solid mineral phases. Measurements of this sink have typically assumed that half of the dissolved inorganic carbon leaving a given catchment comes from the mineral and half from the atmosphere, based on the reaction:

CaCO₃ + H₂CO₃^* « Ca²⁺ + 2HCO₃^-

Recently, however, we have shown a non-unity mineral/atmospheric inorganic C source ratio, defined here as y , of 1.13 (or 57/43) for a 25 km² limestone-dominated Kentucky karst drainage system over one year, which suggests that varying geochemical environments can influence inorganic carbon source partitioning. Here we provide a simple theoretical model based on elementary reactions for carbonate mineral dissolution in acid solutions that allows prediction of y, and show that strong- and/or organic acid-influenced recharge can influence the ratio's value. To the extent that y depends on geochemical environments of mineral-fluid contact, the assumed continental sink/oceanic source CO₂ balance from carbonate mineral interactions may be impacted.

Our model, which has no adjustable parameters, predicts the ratio y as a function of mean rainfall pH, total dissolved inorganic carbon, and temperature, respectively, of waters flowing through and leaving a limestone, dolomite, or mixed karst aquifer over the period of measurement, along with atmospheric background carbon dioxide concentration. Using the measured y value above (1.13) obtained from a year long, high resolution monitoring program, along with the other relevant measured parameters from the study, the model's predicted rainfall pH of 4.31 agrees to within 0.1 pH of the measured mean rainfall pH over that year (n=44) of 4.41 (mean 1983-2000, n=734 is 4.39).

We are now making additional, similar measurements within karst flow systems in California (alpine, higher rain pH) and southern China (warm, monsoon-influenced subtropical, low rain pH) to test the expression, and additional monitoring sites are being planned for a variety of temperature, total precipitation, and precipitation pH conditions.