Large and unexpected enrichment in stratospheric¹⁶O ¹³C¹⁸O and its meridional variation

The stratospheric CO₂ oxygen isotope budget is thought to be governed primarily by the O(¹D)+CO₂ 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 ¹⁶O ¹³C¹⁸O proportions observed in the midlatitudes are determined primarily by the O(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 CO ₂ or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric ¹⁶O ¹³C¹⁸O enrichments may impose additional isotopic constraints on biosphere-atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.