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The stratospheric C[O.sub.2] oxygen isotope budget is thought to be governed primarily by the O([sup.1]D)+C[O.sub.2] isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare C[O.sub.2] isotopologue [sup.16]O[sup.13]C[sup.18]O, in concert with [sup.18]O and [sup.17]O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric [sup.16]O[sup.13]C[sup.18]O, observed as proportions in the polar vortex that are higher than in any naturally derived C[O.sub.2] sample to date. We show, through photochemical experiments, that lower [sup.16]O[sup.13]C[sup.18]O proportions observed in the midlatitudes are determined primarily by the O([sup.1]D)+C[O.sub.2] isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher [sup.16]O[sup.13]C[sup.18]O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O([sup.1]D)+C[O.sub.2]. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of C[O.sub.2] or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric [sup.16]O[sup.13]C[sup.18]O enrichments may impose additional isotopic constraints on biosphere--atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change. clumped isotopes | C[O.sub.2] | mesosphere | polar vortex | stratosphere

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 atmosoheric 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 water18 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). [ABSTRACT FROM AUTHOR]

The stratospheric CO2 oxygen isotope budget is thought to be governed primarily by the O(1D)+C02 isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO2 isotopologue 16O13C18O, in concert with 18O and 17O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric 16O13C18O, observed as proportions in the polar vortex that are higher than in any naturally derived CO2 sample to date. We show, through photochemical experiments, that lower 16O13C18O proportions observed in the midlatitudes are determined primarily by the 0(1D)+CO2 isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher 16O13C18O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O(1D)+CO2. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO2 or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric 16O13C18O enrichments may impose additional isotopic constraints on biosphere-atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change. [ABSTRACT FROM AUTHOR]