OCO in Earth’s atmosphere

The chemistry and budgets of atmospheric gases are constrained by their bulk stable isotope compositions (e.g., δ¹³C values), which are based on mixing ratios of isotopologues containing one rare isotope (e.g., ¹⁶O¹³C¹⁶O). Atmospheric gases also have isotopologues containing two or more rare isotopes (e.g., ¹⁸O¹³C¹⁶O). These species have unique physical and chemical properties and could help constrain origins of atmospheric gases and expand the scope of stable isotope geochemistry generally. We present the first measurements of the abundance of ¹⁸O¹³C¹⁶O from natural and synthetic sources, discuss the factors influencing its natural distribution and, as an example of its applied use, demonstrate how its abundance constrains the sources of CO₂ in the Los Angeles basin. The concentration of ¹⁸O¹³C¹⁶O in air can be explained as a combination of ca. 1‰ enrichment (relative to the abundance expected if C and O isotopes are randomly distributed among all possible isotopologues) due to enhanced thermodynamic stability of this isotopologue during isotopic exchange with leaf and surface waters, ca. 0.1‰ depletion due to diffusion through leaf stomata, and subtle (ca. 0.05‰) dilution by ¹⁸O¹³C¹⁶O-poor anthropogenic CO₂. Some air samples are slightly (ca. 0.05‰) lower in ¹⁸O¹³C¹⁶O than can be explained by these factors alone. Our results suggest that ¹⁸O¹³C¹⁶O abundances should vary by up to ca. 0.2‰ with latitude and season, and might have measurable sensitivities to stomatal conductances of land plants. We suggest the greatest use of Δ₄₇ measurements will be to “leverage” interpretation of the δ¹⁸O of atmospheric CO₂.