Oxygen Isotopic Exchange Between CO₂ and Phosphoric Acid: Implications for the Measurement of Clumped Isotopes in Carbonates

Abstract

Experiments have been conducted in which CO2 gases with varying C and O isotopic compositions and with stochastic and nonstochastic Δ47 values have been allowed to equilibrate with phosphoric acid of two concentrations in reaction vessels of varying dimensions at temperatures of 25 and 90 °C. Rates of 13C18O and 18O exchange between the CO2 and the phosphoric acid varied as a function of the length of exposure, volume of reaction vessel, acid strength, and difference of the initial Δ47 and δ18O values of the CO2 from theoretical equilibrium values. The Δ47 values were also altered by heated stainless steel surfaces such as those found within the Kiel device and other preparation systems. These results have been used to explain variations in the differences in the fractionation between 25 and 90 °C reported for calcite by different workers as well as differences in the slopes between temperature and Δ47 values produced by reacting samples at different temperatures (25 and 90 °C).

Isotopic fractionation during acid digestion of calcite: A combined ab initio quantum chemical simulation and experimental study

“…In this study, we have calculated Δ 47 acid fractionation as a function of temperature for the phosphoric acid digestion reaction using a combination of ab initio density functional theory (DFT) 39,40 as implemented in the Gaussian package 41 and transition state theory 42,43 . We propose that the reaction commences with the protonation of CaCO 3 that results in the formation of CaHPO 4 , H 2 O, and CO 2 23,27,33 . Our calculations show that there is isotopic fractionation in the first step of the reaction that was not considered in the earlier studies and that the clumped isotopic abundance anomaly is a nonlinear function of 1/ T 2 .…”

“…In the SV method, the sample is allowed to completely react with phosphoric acid in an enclosed chamber (typically at 25°C). The product CO 2 remains in the reaction chamber for a prolonged period and can exchange its oxygen directly with water 27–30 or with phosphoric acid 31–33 . The isotopic exchange between the by‐products and the remaining reactants may alter the isotopic signature of CO 2 , compromising the accuracy of measurements.…”

“…The magnitude of the difference, however, decreases with increasing reaction temperature 27 . Therefore, the CAB method was preferred 33 for circumventing the post‐reaction isotope exchange processes.…”

Analytical effects on clumped isotope thermometry: Comparison of a common sample set analyzed using multiple instruments, types of standards, and standardization windows

“…Yet quantifying the impacts of standardization method on resultant Δ 47 values is critical for resolving why there are still discrepancies in Δ 47 ‐temperature calibrations, and for quantifying the magnitude of equilibrium and kinetic isotope effects. There are three end‐member hypotheses for the origin of discrepancies in reported calibrations: (a) there are measurable equilibrium and kinetic isotope effects that give rise to the range of values observed in the literature, (b) inter‐laboratory differences in sample digestion give rise to the variation observed in Δ 47 , and (c) inter‐laboratory differences in Δ 47 measurement and/or standardization give rise to the variation observed in Δ 47 ,…”

“…There are three end-member hypotheses for the origin of discrepancies in reported calibrations: (a) there are measurable equilibrium and kinetic isotope effects that give rise to the range of values observed in the literature, 11,[23][24][25][26][27] (b) inter-laboratory differences in sample digestion give rise to the variation observed in Δ 47 , [28][29][30][31] and (c) inter-laboratory differences in Δ 47 measurement and/or standardization give rise to the variation observed in Δ 47 , 32,33 This paper represents a contribution to partially testing hypotheses 2 and 3, with the caveat that all three hypotheses may in fact be true. Here we report the results of a multiyear study through which we quantify the differences that can be caused by the use of different standardization techniques.…”