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

Abstract

Stratospheric photochemistry leads to anomalous oxygen isotope enrichments in CO2 (for which Δ17O = δ17O - 0.516 × δ18O ≠ 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 Δ 17OCO2. The predicted magnitude of ∼0.15‰ is large enough that measurement of a globally averaged tropospheric Δ17OCO2 should provide a new constraint for gross carbon exchanges between the biosphere and atmosphere in terrestrial carbon cycle models. Importantly, Δ 17OCO2 should be complementary to the primary isotopic tracer of gross carbon exchanges, δ 18OCO2, but is not dependent on numerous hydrologic variables. Furthermore, with improved measurement precision, Δ17OCO2 could serve as a direct tracer of gross carbon exchanges and their variations. Copyright 2005 by the American Geophysical Union.