Clumped isotopic equilibrium and the rate of isotope exchange between CO2 and water

The reaction of CO~2~ hydration/dehydration controls the oxygen isotopic composition in both carbonate minerals and atmospheric CO~2~ through the exchange of oxygen isotopes with water. The use of δ^18^O as an environmental indicator typically assumes isotopic equilibrium, namely full oxygen isotope exchange between CO~2~ and water. Clumped isotopes is a new isotopic tracer that is used in both CaCO~3~ and atmospheric CO~2~ and reflects the thermodynamic preference of two heavy isotopes, ^13^C and ^18^O in this case (given as Δ~47~), to "clump" together into one chemical bond at low temperatures. As such, the use of Δ~47~ as an indicator for temperature relies on the assumption of isotopic equilibrium. The experiments presented here examine the rate in which Δ~47~ of CO~2~ that interact with water approaches the equilibrium values. This rate is indistinguishable between Δ~47~ and δ^18^O, suggesting that the isotope exchange with water also leads to reorganization of the isotopes among CO~2~ isotopologues thus controlling the Δ~47~ values. The direct implication of the temporal link between Δ~47~ and δ^18^O is that when one isotopic system shows disequilibrium, either in DIC or in gas phase CO~2~, so will the other. As CO~2~ clumped isotope values are independent of the oxygen isotopic composition of the water participating in the reaction, disequilibrium in Δ~47~ is often identified more readily than in δ^18^O. The combination of clumped isotopes and oxygen isotopes is therefore likely to elucidate cases of suspected disequilibrium also in δ^18^O (and *vice versa*).