Triple oxygen isotope composition of tropospheric carbon dioxide as a tracer of terrestrial gross carbon fluxes

Stratospheric photochemistry leads to anomalous oxygen isotope enrichments in CO 2 (for which Δ 17 O = δ 17 O − 0.516 × δ 18 O ≠ 0). This isotope anomaly is not lost until air returns to the troposphere and CO 2 undergoes isotope exchange with water primarily in the terrestrial biosphere and oceans. A two‐box model is used to investigate the contribution of stratospheric production and contemporary surface carbon fluxes to tropospheric Δ 17 O CO2 . The predicted magnitude of ∼0.15‰ is large enough that measurement of a globally averaged tropospheric Δ 17 O CO2 should provide a new constraint for gross carbon exchanges between the biosphere and atmosphere in terrestrial carbon cycle models. Importantly, Δ 17 O CO2 should be complementary to the primary isotopic tracer of gross carbon exchanges, δ 18 O CO2 , but is not dependent on numerous hydrologic variables. Furthermore, with improved measurement precision, Δ 17 O CO2 could serve as a direct tracer of gross carbon exchanges and their variations.