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

Of the six stable isotopic variants of O₂, only three are measured routinely. Observations of natural variations in ¹⁶O¹⁸O/¹⁶O¹⁶O and ¹⁶O¹⁷O/¹⁶O¹⁶O ratios have led to insights in atmospheric, oceanographic, and paleoclimate research. Complementary measurements of the exceedingly rare ¹⁸O¹⁸O and ¹⁷O¹⁸O 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 O₂. Its accuracy is demonstrated by measuring isotopic effects for Knudsen diffusion and O₂ electrolysis in the laboratory that are consistent with theoretical predictions. We then report the first measurements of ¹⁸O¹⁸O and ¹⁷O¹⁸O proportions relative to the stochastic distribution of isotopes (i.e., Δ₃₆ and Δ₃₅ values, respectively) in tropospheric air. Measured enrichments in ¹⁸O¹⁸O and ¹⁷O¹⁸O yield Δ₃₆ = 2.05 ± 0.24‰ and Δ₃₅ = 1.4 ± 0.5‰ (2σ). Based on the results of our electrolysis experiment, we suggest that autocatalytic O(³P) + O₂ isotope exchange reactions play an important role in regulating the distribution of ¹⁸O¹⁸O and ¹⁷O¹⁸O 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 Δ₃₆ and Δ₃₅ values. O(³P) + O₂ isotope exchange in the stratosphere and troposphere is therefore expected to govern atmospheric Δ₃₆ and Δ₃₅ values on decadal timescales. These results suggest that the `clumped' isotopic composition of atmospheric O₂in ice core records is sensitive to past variations in atmospheric dynamics and free-radical chemistry.