Measurements of 18O18O and 17O18O in the atmosphere and the role of isotope-exchange reactions

Of the six stable isotopic variants of O-2, only three are measured routinely. Observations of natural variations in (OO)-O-16-O-18/(OO)-O-16-O-16 and (OO)-O-16-O-17/(OO)-O-16-O-16 ratios have led to insights in atmospheric, oceanographic, and paleoclimate research. Complementary measurements of the exceedingly rare (OO)-O-18-O-18 and (OO)-O-17-O-18 isotopic variants might therefore broaden our understanding of oxygen cycling. Here we describe a method to measure natural variations in these multiply substituted isotopologues of O-2. Its accuracy is demonstrated by measuring isotopic effects for Knudsen diffusion and O-2 electrolysis in the laboratory that are consistent with theoretical predictions. We then report the first measurements of (OO)-O-18-O-18 and (OO)-O-17-O-18 proportions relative to the stochastic distribution of isotopes (i.e., Delta(36) and Delta(35) values, respectively) in tropospheric air. Measured enrichments in (OO)-O-18-O-18 and (OO)-O-17-O-18 yield Delta(36) = 2.05 +/- 0.24 parts per thousand and Delta(35) = 1.4 +/- 0.5 parts per thousand(2 sigma). Based on the results of our electrolysis experiment, we suggest that autocatalytic O(P-3) + O-2 isotope exchange reactions play an important role in regulating the distribution of (OO)-O-18-O-18 and (OO)-O-17-O-18 in air. We constructed a box model of the atmosphere and biosphere that includes the effects of these isotope exchange reactions, and we find that the biosphere exerts only a minor influence on atmospheric Delta(36) and Delta(35) values. O(P-3) + O-2 isotope exchange in the stratosphere and troposphere is therefore expected to govern atmospheric Delta(36) and Delta(35) values on decadal timescales. These results suggest that the 'clumped' isotopic composition of atmospheric O-2 in ice core records is sensitive to past variations in atmospheric dynamics and free-radical chemistry.