Figure: The global ``pre-anthropogenic'' steady-state budget for the oxygen isotopes of atmospheric CO based on Farquhar et al. [1993] showing annual fluxes of CO in units of 10 moles of carbon and showing the isotopic composition of CO in equilibrium with dominant exchangeable water reservoirs [see also Keeling, 1993]. CO exchange with soil water involves uptake of CO by leaves, respiration within the soil, and diffusion of the respiratory CO out through the soil. The budget shown here assumes that the kinetic isotope fractionation that results from diffusion through stomata and through the soil cancel each other out (see also Table 2, Eq. (F)). According to this budget, the bulk composition of atmospheric CO can be explained by assuming that 45% of the oxygen atoms come from chloroplast water at an average isotopic composition of +5 , 34% come from soil water at an average of -7 , and 21% come from sea water at an average of 1 . This combination yields atmospheric CO at approximately 0 . All numbers here are relative to the PDB standard.

 
Figure: Latitudinal averages of the effective discrimination factor on uptake of CO by leaves , the O of CO emitted by soils , the O of CO in equilibrium with surface seawater , and the observed atmospheric O of atmospheric CO (circles), and the sum , from Farquhar et al. [1993]. Here was computed from the isotopic composition of precipitation minus 7.6 . Latitudinal averages for and are weighted by GPP. and are expressed relative to the PDB standard.

 
Figure: The global steady-state budget for the oxygen isotopes of atmospheric O per Bender et al. [1994]. Fluxes are in units of 10 moles O yr. The O values represent estimates of global averages of spatially and temporally variable quantities. Photorespiration and photooxidation reactions are grouped here as part of total terrestrial respiration. The O flux from leaves thus exceeds the net O production by leaves, i.e., the O production associated with gross primary production, by the amount required by balance photorespiration and photooxidation reactions.

 
Figure: Measurements of (O/N) and CO mole fraction at (a) Alert, (b) La Jolla, and (c) Cape Grim as reported previously by Keeling and Shertz [1992]. The axes are scaled (5 per meg 1 ppm) so that changes in (O/N) and CO are directly comparable on a mole O to mole CO basis. Supplemental CO data from the Climate Monitoring and Diagnostics Laboratory of the National Oceanic and Atmospheric Administration and from the Scripps Institution of Oceanography are also shown.