Revised microbial and photochemical triple-oxygen isotope effects improve marine gross oxygen production estimates.

The biogeochemical fluxes that cycle oxygen (O) play a critical role in regulating Earth's climate and habitability. Triple-oxygen isotope (TOI) compositions of marine dissolved O are considered a robust tool for tracing oxygen cycling and quantifying gross photosynthetic O production. This method assumes that photosynthesis, microbial respiration, and gas exchange with the atmosphere are the primary influences on dissolved O content, and that they have predictable, fixed isotope effects. Despite its widespread use, there are major elements of this approach that remain uncharacterized, including the TOI dynamics of respiration by marine heterotrophic bacteria and abiotic O sinks such as the photochemical oxidation of dissolved organic carbon (DOC). Here, we report the TOI fractionation for O utilization by two model marine heterotrophs and by abiotic photo-oxidation of representative terrestrial and coastal marine DOC. We demonstrate that TOI slopes associated with these processes span a significant range of the mass-dependent domain (λ = 0.499 to 0.521). A sensitivity analysis reveals that even under moderate productivity and photo-oxidation scenarios, true gross oxygen production may deviate from previous estimates by more than 20% in either direction. By considering a broader suite of oxygen cycle reactions, our findings challenge current gross oxygen production estimates and highlight several paths forward to better understanding the marine oxygen and carbon cycles.