Résumé
This chapter uses the mass-conservation equations for CO2 and its isotopomers 13CO2 and CO18O that can be used to infer globally biospheric and oceanic net fluxes in the case of 18C, and gross terrestrial biospheric fluxes in the case of 18O. The quantitative use of atmospheric measurements of 13C and 18O in CO2 to better constrain those fluxes requires knowledge of various processes specific to each isotopomer. The chapter is divided into two parts, one on each isotope. For 13C, it reviews existing work that calculated isofluxes either using global estimates or derived isofluxes from spatially and temporally explicit models. In addition, it estimates the magnitude of new isofluxes that were not addressed in former studies. These cover the effects of biomass burning, rock weathering and volcanism, and the oxidation of reduced carbon gases into CO2 within the atmosphere.