Large degrees of isotope disequilibrium during precipitation-controlled degassing of CO2 from a CaCO3-H2O-CO2 solution

Understanding the process of CO₂ outgassing during precipitation of calcite from a CaCO₃-H₂O-CO₂ solution is crucial for interpreting isotope compositions in the calcite precipitates. Unlike diffusion-controlled outgassing i.e., dissolved CO₂ escapes from the solution, outgassing controlled by precipitation of calcite is accompanied by a large degree of carbon isotope fractionation due to breaking of C-O bond via HCO₃^- dehydration or dehydroxylation, with an equilibrium fractionation of ca. -9‰ between CO₂ gas and bicarbonate ion (Δ¹³C_g-b) at ambient temperature. Such fractionation has a great influence on carbon isotope compositions (δ¹³C) of DIC (dissolved inorganic carbon) reservoir in the solution and thus on that of calcite precipitated from it. However, knowledge on isotope fractionation during CO₂ outgassing controlled by precipitation in a supersaturated solution is limited where rapid calcite precipitation drives CO₂ outgassing out of isotopic equilibrium. Here we show the data of water chemistry and carbon isotope compositions of DIC and carbonate precipitates in a mountain stream at Baishuitai, China. Model results show there exist large degrees of carbon isotope disequilibrium during CO₂ outgassing driven by precipitation. Average Δ¹³C_g-b value calculated from the our dataset is about -21‰ which is far away from the equilibrium value. Moreover, an inverse correlation between the Δ¹³C_g-b and precipitation rate was observed. This dependence can be used to understand abnormal δ¹³C values of the carbonate minerals in a non-equilibrium carbonate system influenced by rapid precipitation. By compiling the data from this study and literature, we infer that disequilibrium fractionation of carbon isotope is common during the formation of speleothem and travertine from a supersaturated CaCO₃-H₂O-CO₂ solution. When P_CO2 of cave air varies due to ventilation, variable precipitation rates of calcite will cause inconstant degrees of disequilibrium isotope fractionation between CO₂ and DIC and thus perturb the time-series of speleothem's δ¹³C records.