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 δ18O 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, 13C and 18O in this case (given as Δ47), to "clump" together into one chemical bond at low temperatures. As such, the use of Δ47 as an indicator for temperature relies on the assumption of isotopic equilibrium. The experiments presented here examine the rate in which Δ47 of CO2 that interact with water approaches the equilibrium values. This rate is indistinguishable between Δ47 and Δ18O, suggesting that the isotope exchange with water also leads to reorganization of the isotopes among CO2 isotopologues thus controlling the Δ47 values. The direct implication of the temporal link between Δ47 and Δ18O 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 Δ47 is often identified more readily than in 18O. The combination of clumped isotopes and oxygen isotopes is therefore likely to elucidate cases of suspected disequilibrium also in Δ18O (and vice versa).