Search

Understanding the process of CO 2 degassing during precipitation of calcite from a Ca2+–HCO 3 − solution is crucial for interpreting isotope compositions in the calcite precipitates. Unlike diffusion-controlled outgassing, i.e., dissolved CO 2 escaping from the solution via diffusion, degassing caused by precipitation of calcite is accompanied by a large carbon isotope fractionation between CO 2 (g) and HCO 3 − due to breaking of C O bond, with an equilibrium fractionation of ca. −9‰ at ambient temperature. Such a magnitude of fractionation has a great influence on carbon isotope compositions (δ13C) of DIC (dissolved inorganic carbon) reservoir in the solution and thus on δ13C of calcite precipitated from it. However, knowledge on isotope fractionation is limited during precipitation-associated degassing of CO 2 in a supersaturated solution where rapid calcite precipitation drives CO 2 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 CO 2 via HCO 3 − dehydration and dehydroxylation. Average carbon isotope fractionation between CO 2 (g) and HCO 3 − ( ε C O 2 (g) - HC O 3 - ) calculated from our dataset is about −20‰ which is much lower than the equilibrium value. Moreover, an inverse correlation between ε C O 2 (g) - HC O 3 - and precipitation rate was observed, indicating higher precipitation rates cause ε C O 2 (g) - HC O 3 - farther from equilibrium value. By compiling the data from this study and literatures, we infer that disequilibrium isotope fractionation of carbon between CO 2 (g) and HCO 3 − may be common during the growth of speleothem and travertine from a solution supersaturated with respect to calcite. The rate dependence of ε C O 2 (g) - HC O 3 - has special implications for speleothem archives from ventilated caves. As partial pressure of CO 2 in cave atmosphere evolves with ventilation, variable precipitation rates of calcite will cause inconstant degrees of disequilibrium isotope fractionation between CO 2 and DIC and thus perturb the time-series of speleothem's δ13C records for paleo-environmental reconstruction. [ABSTRACT FROM AUTHOR]

• Calcites precipitated from the solutions with identical chemical and isotopic compositions have different δ 13 C values. • The DBL thickness affects the pH and the δ 13 C value of HCO − 3 at the mineral-solution interface. • Our finding offers a mechanistic interpretation on the variability of the ε calcite / DIC in riverine or cave environments. It is often assumed that the carbon isotope composition (δ 13 C) of carbonate minerals records that of the dissolved inorganic carbon (DIC) species with insignificant disequilibrium effect. However, results from field observations and laboratory experiments have shown that the δ 13 C difference between calcite and solution can vary up to 3‰ even under a similar set of solution composition and temperature, raising uncertainties on the δ 13 C's paleoclimate and paleoecology implications. One likely cause is the variable calcite precipitation rates and pH values induced by different thicknesses of the stagnant liquid layer between solid and well mixed bulk solution (i.e., diffusion boundary layer, DBL). To test this hypothesis, we selected a well-studied natural travertine deposit (Baishuitai, SW China) which consists of meter-scale travertine terraces. Calcite crystals at both the pool bottom and the rim of the terraces are inorganically precipitated from solutions identical in chemical, isotopic composition and temperature, but different only in hydrodynamics. This difference results in thicker DBL for water at the pool-bottom than for water flowing across the rim. We found that the δ 13 C and Mg/Ca ratios are higher while Sr/Ca ratios are lower for the pool-bottom calcites than for the rim calcites. By applying a mass transfer model, we quantitatively link the differences in carbon isotope and elemental compositions of the abiogenic calcites to the different hydrodynamic conditions. The inverse variation in Mg/Ca and Sr/Ca ratios in calcites arises from the different precipitation rates between at the pool-bottom and at the rim, while the consistently lower δ 13 C for calcites at the rim is due to their higher pH at the solid-solution interface than for calcites precipitated at the pool-bottom. In contrast, calcite precipitation rate has little influence on carbon isotope fractionation between calcite and HCO − 3 . Our results demonstrate the role of DBL thickness in governing the δ 13 C of HCO − 3 at mineral surface, which can assist to interpret the variable δ 13 C values of calcites in riverine or cave environments. [ABSTRACT FROM AUTHOR]