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

Abstract

Of the six stable isotopic variants of O 2 , only three are measured routinely. Observations of natural variations in 16 O 18 O/ 16 O 16 O and 16 O 17 O/ 16 O 16 O ratios have led to insights in atmospheric, oceanographic, and paleoclimate research. Complementary measurements of the exceedingly rare 18 O 18 O and 17 O 18 O 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 18 O 18 O and 17 O 18 O proportions relative to the stochastic distribution of isotopes (i.e., Δ 36 and Δ 35 values, respectively) in tropospheric air. Measured enrichments in 18 O 18 O and 17 O 18 O yield Δ 36 = 2.05 ± 0.24‰ and Δ 35 = 1.4 ± 0.5‰ (2 σ ). Based on the results of our electrolysis experiment, we suggest that autocatalytic O( 3 P ) + O 2 isotope exchange reactions play an important role in regulating the distribution of 18 O 18 O and 17 O 18 O 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 Δ 36 and Δ 35 values. O( 3 P ) + O 2 isotope exchange in the stratosphere and troposphere is therefore expected to govern atmospheric Δ 36 and Δ 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.