CLUMPED ISOTOPIC EQUILIBRIUM AND THE RATE OF ISOTOPE EXCHANGE BETWEEN CO2 AND WATER

The reaction of CO2 hydration/dehydration controls the oxygen isotopic composition in both carbonate minerals and atmospheric CO2 through the exchange of oxygen isotopes with water. The use of delta O-18 as an environmental indicator typically assumes isotopic equilibrium, namely full oxygen isotope exchange between CO2 and water. Clumped isotopes is a new isotopic tracer that is used in both CaCO3 and atmospheric CO2 and reflects the thermodynamic preference of two heavy isotopes, C-13 and O-18 in this case (given as Delta(47)), to "clump" together into one chemical bond at low temperatures. As such, the use of Delta(47) as an indicator for temperature relies on the assumption of isotopic equilibrium. The experiments presented here examine the rate in which Delta(47) of CO2 that interact with water approaches the equilibrium values. This rate is indistinguishable between Delta(47) and delta O-18, suggesting that the isotope exchange with water also leads to reorganization of the isotopes among CO2 isotopologues thus controlling the Delta(47) values. The direct implication of the temporal link between Delta(47) and delta O-18 is that when one isotopic system shows disequilibrium, either in DIC or in gas phase CO2, so will the other. As CO2 clumped isotope values are independent of the oxygen isotopic composition of the water participating in the reaction, disequilibrium in Delta(47) is often identified more readily than in delta O-18. The combination of clumped isotopes and oxygen isotopes is therefore likely to elucidate cases of suspected disequilibrium also in delta O-18 (and vice versa).