THE O-18/O-16 ratio in atmospheric CO2 is a signal dominated by CO2 ex
change with the terrestrial biosphere and it has considerable potentia
l to resolve the current importance of the oceans and individual terre
strial biomes as net sinks for anthropogenic CO2. Fractionation of the
oxygen isotopes of CO2 occurs in plants owing to differential diffusi
on of (COO)-O-18-O-16 and (CO2)-O-16 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 O-18 isotopic exchange fluxes, especially between th
e atmosphere and terrestrial biosphere, are large, with considerable s
patial variation. Near 70-degrees-N, where precipitation (and soil wat
er) is most depleted in O-18, photosynthesis and respiration both depl
ete the atmospheric CO2 of O-18. This provides an explanation for the
depletion of O-18 in atmospheric CO2 at high northern latitudes1.