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Kinetics of CO2(g)-H2O(1) isotopic exchange, including mass 47 isotopologues


The analysis of mass 47 isotopologues of CO2 (mainly 13C18O16O) is established as a constraint on sources and sinks of environmental CO2, complementary to δ13C and δ18O constraints, and forms the basis of the carbonate clumped isotope thermometer. This measurement is commonly reported using the δ47 value - a measure of the enrichment of doubly substituted CO2 relative to a stochastic isotopic distribution. Values of δ47 for thermodynamically equilibrated CO2 approach 0 (a random distribution) at high temperatures (≥ several hundred degrees C), and increase with decreasing temperature, to ≈0.9% at 25°C. While the thermodynamic properties of doubly substituted isotopologues of CO2 (and, similarly, carbonate species) are relatively well understood, there are few published constraints on their kinetics of isotopic exchange. This issue is relevant to understanding both natural processes (e.g., photosynthesis, respiration, air-sea or air-groundwater exchange, CO2 degassing from aqueous solutions, and possibly gas-sorbate exchange on cold planetary surfaces like Mars), and laboratory handling of CO2 samples for δ47 analysis (e.g., re-equilibration in the presence of liquid water, water ice or water adsorbed on glass or metal surfaces). We present the results of an experimental study of the kinetics of isotopic exchange, including changes in δ47 value, of CO2 exposed to liquid water between 5 and 37°C. Aliquots of CO2 gas were first heated to reach a nearly random distribution of its isotopologues and then exposed at low pressure for controlled periods of time to large excesses of liquid water in sealed glass containers. Containers were held at 5, 25 and 37°C and durations of exchange ranging from 5min to 7days. To avoid the formation of a boundary layer that might slow exchange, the tubes were vigorously shaken during the period of exchange. At the end of each experiment, reaction vessels were flash frozen in liquid nitrogen. CO2 gas was then recovered from the head space of the reaction vessel, purified and analyzed for its δ47, δ13C and δ18O by gas source isotope ratio mass spectrometry. Equilibrium was reached for both δ18O and δ47 after durations of a few hours to tens of hours. δ18O values at equilibrium were consistent with known fractionation factors for the CO2-H2O system. The evolution of δ18O and δ47 with experiment duration were consistent with first-order reactions, with rate constants equal to each other (within error), averaging 0.19h-1 at 5°C, 0.38h-1 at 25°C and 0.65h-1 at 37°C. We calculate an activation energy for this isotopic exchange reaction of 26.2kJ/mol. By comparison, Mills and Urey (1940) measured the rate of 18O exchange between CO2(aq) and water to have a rate of 11h-1 at 25°C and an activation energy of 71.7kJ/mol. Our finding of a slower rate and lower activation energy is consistent with the rate limiting step of our experiment being the CO2(g)-CO2(aq) exchange, even when samples are shaken during the partial equilibration. Our results broadly resemble those from the study of (Affek, 2013), though this prior study found a lower rate constant for δ47. We propose that the difference is due to analytical uncertainties and explore the theoretical consequences of unequal reaction rates between 12C18O16O and 13C18O16O with a forward model.

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