Understanding the process of CO2 degassing during precipitation of calcite from a Ca2+-HCO3(-) solution is crucial for interpreting isotope compositions in the calcite precipitates. Unlike diffusion-controlled outgassing, i.e., dissolved CO2 escaping from the solution via diffusion, degassing caused by precipitation of calcite is accompanied by a large carbon isotope fractionation between CO2(g) and HCO3- due to breaking of CAO bond, with an equilibrium fractionation of ca. similar to 9% at ambient temperature. Such a magnitude of fractionation has a great influence on carbon isotope compositions (delta C-13) of DIC (dissolved inorganic carbon) reservoir in the solution and thus on delta C-13 of calcite precipitated from it. However, knowledge on isotope fractionation is limited during precipitation-associated degassing of CO2 in a supersaturated solution where rapid calcite precipitation drives CO2 degassing 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. Results from numerical models show there exist large degrees of carbon isotope disequilibrium during precipitation-associated degassing of CO2 via HCO3- dehydration and dehydroxylation. Average carbon isotope fractionation between CO2(g) and HCO3- (epsilon(CO2 (g)) - (HCO3-)) calculated from our dataset is about similar to 20% which is much lower than the equilibrium value. Moreover, an inverse correlation between eCO(2) ogTHORN - HCO3- and precipitation rate was observed, indicating higher precipitation rates cause (epsilon(CO2 (g)) - (HCO3-)) farther from equilibrium value. By compiling the data from this study and literatures, we infer that disequilibrium isotope fractionation of carbon between CO2(g) and HCO3- may be common during the growth of speleothem and travertine from a solution supersaturated with respect to calcite. The rate dependence of epsilon(CO2 (g)) - (HCO3-) has special implications for speleothem archives from ventilated caves. As partial pressure of CO2 in cave atmosphere evolves with ventilation, variable precipitation rates of calcite will cause inconstant degrees of disequilibrium isotope fractionation between CO2 and DIC and thus perturb the timeseries of speleothem's delta C-13 records for paleo-environmental reconstruction. (C) 2020 Elsevier Ltd. All rights reserved.
| GEOCHIMICA ET COSMOCHIMICA ACTA Volume: 273 Pages: 244-256 DOI: 10.1016/j.gca.2020.01.012 Published: MAR 15 2020 |
| Yan, Hao; Liu, Zaihua; Sun, Hailong |
YAN Hao; LIU Zaihua
Chinese Acad Sci, Inst Geochem, State Key Lab Environm Geochem, Guiyang 550081, Peoples R China.
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