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Triple oxygen isotope (Δ′17O) analysis of CO 2 or carbonate is increasingly applied in diverse fields in earth and environmental sciences. One of the widely-used analytical methods for Δ′17O of CO 2 is through Pt-catalyzed oxygen isotope exchange between CO 2 and O 2. However, large inter-laboratory inconsistencies in oxygen isotope fractionation between the two gases have been reported, potentially hindering data comparison across different laboratories. By examining the oxygen isotope fractionations in thermal diffusion experiments for pure O 2 and CO 2 gases as well as CO 2 -O 2 mixtures, we demonstrate that thermal-gradient-induced (TG-induced) isotope fractionation is the main reason for the inconsistencies. The degree of TG-induced oxygen isotope fractionation is affected by temperature contrasts and volume ratios between the hot and cold parts of the extraction line as well as gas composition. Larger thermal contrasts generated larger oxygen isotope fractionations. CO 2 fractionated less than O 2 in the same thermal gradient conditions. For CO 2 -O 2 mixtures, TG-induced oxygen isotope fractionation of O 2 was reduced while that of CO 2 was enhanced, compared with the pure gases. Based on the results of thermal diffusion experiments, we found that the anomalous mass fractionation exponents (0.2 ∼ 0.9) observed in the Pt-catalyzed CO 2 -O 2 isotope exchange experiments can be rationalized quantitatively by a combination of isotopic exchange equilibrium and TG-induced oxygen isotope fractionation. Our model suggests that by weakening TG-induced oxygen isotope fractionation and by increasing the volume fraction of hot zone of a reactor, we can achieve a high precision (<10 per meg) for Δ′17O CO2 through CO 2 -O 2 exchange. Considering the widely existing thermal-gradient conditions in nature, the findings in this study may shed light on the origin of non-mass-dependent isotope fractionation. [ABSTRACT FROM AUTHOR]

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]

Conventional carbonate–water oxygen isotope thermometry and the more recently developed clumped isotope thermometer have been widely used for the reconstruction of paleotemperatures from a variety of carbonate materials. In spite of a large number of studies, however, there are still large uncertainties in both δ 18 O- and Δ 47 -based temperature calibrations. For this reason there is a need to better understand the controls on isotope fractionation especially on natural carbonates. In this study we analyzed oxygen, carbon and clumped isotopes of a unique set of modern calcitic and aragonitic travertines, tufa and cave deposits from natural springs and wells. Together these samples cover a temperature range from 6 to 95 °C. Travertine samples were collected close to the vents of the springs and from pools, and tufa samples were collected from karstic creeks and a cave. The majority of our vent and pool travertines and tufa samples show a carbonate–water oxygen isotope fractionation comparable to the one of Tremaine et al. (2011) with some samples showing higher fractionations. No significant difference between the calcite–water and aragonite–water oxygen isotope fractionation could be observed. The Δ 47 data from the travertines show a strong relationship with temperature and define the regression Δ 47 = (0.044 ± 0.005 × 10 6 )/ T 2 + (0.205 ± 0.047). The pH of the parent solution, mineralogy and precipitation rate do not appear to significantly affect the Δ 47 -signature of carbonates, compared to the temperature effect and the analytical error. The tufa samples and three biogenic calcites show an excellent fit with the travertine calibration, indicating that this regression can be used for other carbonates as well. This work extends the calibration range of the clumped isotope thermometer to travertine and tufa deposits in the temperature range from 6 °C to 95 °C. [ABSTRACT FROM AUTHOR]