Vegetation effects on the isotope composition of oxygen in atmospheric CO2

The (18)O/(16)O ratio in atmospheric CO2 is a signal dominated by CO2 exchange with the terrestrial biosphere and it has considerable potential to resolve the current importance of the oceans and individual terrestrial biomes as net sinks for anthropogenic CO2. Fractionation of the oxygen isotopes of CO2 occurs in plants owing to differential diffusion of C(18)O(16)O and C(16)O2 and to isotope effects in oxygen exchange with chloroplast water. Here we investigate the consequences of these effects for the global distribution of oxygen isotopes in CO2. We predict that (18)O isotopic exchange fluxes, especially between the atmosphere and terrestrial biosphere, are large, with considerable spatial variation. Near 70 degrees N, where precipitation (and soil water) is most depleted in (18)O, photosynthesis and respiration both deplete the atmospheric CO2 of (18)O. This provides an explanation for the depletion of (18)O in atmospheric CO2 at high northern latitudes.