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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]

Abstract: In order to understand the behavior of oxygen isotopic fractionation and obtain more reliable paleoclimatic data from surficial travertine deposits, the hydrochemistry, carbonate precipitation rates, and the temporal and spatial variations in δ18O values of the modern travertine in two low-temperature travertine-depositing systems (canal and pool) with differing hydrodynamic conditions at Baishuitai, Yunnan, SW China, were investigated during the warm rainy season (May 24 to November 2) in 2010. It was found that the oxygen isotopic composition of travertine in the two systems showed distinct controlling mechanisms mainly due to different carbonate precipitation rates and water residence times. For the canal system with fast flow, large deviations between the calculated and measured temperatures indicated that travertine precipitated out of isotopic equilibrium. This was due to rapid carbonate precipitation with intense CO2 degassing. δ18Otravertine values and water temperature increased downstream (∼1‰ and 4°C, respectively) while δ18Owater values remained relatively stable. It was shown that the δ18O values of travertine in the canal system recorded the δ18O values of dissolved carbonates (HCO3 −) which also increased along the canal, controlled by Rayleigh-distillation effects. In contrast, in the pools system with slow flow, oxygen isotopic equilibrium between dissolved carbonates (HCO3 −) and H2O was achieved and the δ18Otravertine values did not display spatial variations. Calculated water temperatures using the equilibrium fractionation factors of Coplen (2007) were identical to the measured ones, which indicates that travertine in such pools system is suitable for the reconstruction of paleo-precipitating temperatures. Finally, by comparing these two systems we have obtained a simple criterion to determine the suitability of travertine to estimate temperatures – the δ18O value of BaCO3 precipitated from water samples is compared with the theoretical δ18O value of dissolved carbonate calculated using water temperature and δ18Owater, to check if the oxygen isotopes of the dissolved carbonate are in equilibrium with the water. [Copyright &y& Elsevier]

Abstract: Travertine δ18O values can be used to reconstruct paleo-temperatures if the oxygen isotope fractionation factors between travertine and water are accurately understood. For this purpose, the δ18O values of pool travertine and its parent water, and the deposition rates of the calcite were investigated at Baishuitai (Yunnan, SW China) over the course of the full hydrological year, April 23 2006–April 25 2007. The results show that the travertine–water isotope fractionation factors are close to the commonly accepted equilibrium line of Kim and O’Neil (1997). This differs from the results obtained by Yan et al. (2012) who found that the oxygen isotope fractionation factors in the travertine-depositing pools were close to the line suggested as equilibrium relationship by Coplen (2007). The average calcite deposition rate (2.30mgcm−2 d−1) in the present study is six times larger than that (0.38mgcm−2 d−1) in Yan et al. (2012). If slower calcite precipitation leads to equilibrium oxygen isotopic fractionation, then the results of this study support the results of Coplen (2007) that indicate that the equilibrium fractionation factor may be greater than the commonly accepted one derived by Kim and O’Neil (1997). The relationship between oxygen isotope fractionation factor and calcite deposition rate in our study also agrees with the results of Dietzel et al. (2009) who found that the kinetic-isotope effect favors preferential incorporation of 16O in solid calcite as the calcite deposition rate increases. There was a threshold for calcite precipitation rate control on oxygen isotopic equilibrium. In our case of travertine-depositing pools, when the calcite deposition rate was lower than 0.38mgcm−2 d−1, oxygen isotopic equilibrium between calcite and water was attained. Therefore, calcite deposition rate is a potentially important consideration when using δ18O in natural carbonates as a proxy for terrestrial and ocean temperature. [Copyright &y& Elsevier]

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]