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

Abstract

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 δ¹⁸O. The combination of clumped isotopes and oxygen isotopes is therefore likely to elucidate cases of suspected disequilibrium also in δ¹⁸O (and vice versa).