Oxygen Isotopic Composition of Tropospheric CO2 as a Tracer of Terrestrial Biosphere Productivity: Investigating the Influence of the Stratosphere

Photochemistry in the stratosphere produces anomalously enriched CO₂ for which δ¹⁷O !=q 0.516 ṡ δ¹⁸O (or Δ¹⁷O = δ¹⁷O - 0.516ṡ δ¹⁸O !=q 0). Since there is no stratospheric sink for these enrichments, the stratospheric signatures in both δ¹⁸O and Δ¹⁷O of CO₂ are transported to the troposphere where their magnitudes are controlled by isotope exchange with liquid water, primarily in chloroplasts of leaves, soils and the ocean. Currently, δ¹⁸O of tropospheric CO₂ is used to constrain global models of gross primary production (GPP). Recent calculations and modeling will be presented that suggest that the stratospheric flux of ¹⁸O-enriched CO₂ to the troposphere has a significant impact on both the annual mean value of δ¹⁸O of CO₂ and its seasonality in the free troposphere. This stratospheric influence has yet to be included in global GPP models. Moreover, using δ¹⁸O of tropospheric CO₂ to constrain models of GPP requires knowledge of a number of parameters that are difficult to model on a global scale, such as the δ¹⁸O of leaf and soil water. In contrast, when modeling the global Δ¹⁷O of CO₂, the oxygen isotopic compositions of these surface water reservoirs need not be known explicitly because equilibration of the oxygen isotopes with liquid water resets the anomaly to Δ¹⁷O = 0. Model results exploring the use of Δ¹⁷O of tropospheric CO₂ as an additional constraint on GPP will also be presented.