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

[ 1] Stratospheric photochemistry leads to anomalous oxygen isotope enrichments in CO2 ( for which Delta(17)O = delta(17)O - 0.516 x delta(18)O not equal 0). This isotope anomaly is not lost until air returns to the troposphere and CO2 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 Delta(17)O(CO2). The predicted magnitude of similar to 0.15parts per thousand is large enough that measurement of a globally averaged tropospheric Delta(17)O(CO2) should provide a new constraint for gross carbon exchanges between the biosphere and atmosphere in terrestrial carbon cycle models. Importantly, Delta(17)O(CO2) should be complementary to the primary isotopic tracer of gross carbon exchanges, delta(18)O(CO2), but is not dependent on numerous hydrologic variables. Furthermore, with improved measurement precision, Delta(17)O(CO2) could serve as a direct tracer of gross carbon exchanges and their variations.