18O 13C 16O IN EARTH’S ATMOSPHERE

Abstract

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.