Carbon 13 exchanges between the atmosphere and biosphere

We present a detailed investigation of the gross C-12 and C-13 exchanges between the atmosphere and biosphere and their influence on the delta(13)C variations in the atmosphere. The photosynthetic discrimination Delta against C-13 is derived from a biophysical model coupled to a general circulation model [Sellers et al., 1996a], where stomatal conductance and carbon assimilation are determined simultaneously with the ambient climate, The delta(13)C of the respired carbon is calculated by a biogeochemical model [Potter et al., 1993; Randerson et al., 1996] as the sum of the contributions from compartments with varying ages, The global flux-weighted mean photosynthetic discrimination is 12-16 parts per thousand, which is lower than previous estimates. Factors that lower the discrimination are reduced stomatal conductance and C-4 photosynthesis. The decreasing atmospheric delta(13)C causes an isotopic disequilibrium between the outgoing and incoming fluxes; the disequilibrium is similar to 0.33 parts per thousand for 1988. The disequilibrium is higher than previous estimates because it accounts for the lifetime of trees and for the ages rather than turnover times of the biospheric pools. The atmospheric delta(13)C signature resulting from the biospheric fluxes is investigated using a three-dimensional atmospheric tracer model. The isotopic disequilibrium alone produces a hemispheric difference of similar to 0.02 parts per thousand, in atmospheric delta(13)C, comparable to the signal from a hypothetical carbon sink of 0.5 Gt C yr(-1) into the midlatitude northern hemisphere biosphere, However, the rectifier effect, due to the seasonal covariation of CO2 fluxes and height of the atmospheric boundary layer, yields a background delta(13)C gradient of the opposite sign. These effects nearly cancel thus favoring a stronger net biospheric uptake than without the background CO2 gradient. Our analysis of the globally averaged carbon budget for the decade of the 1980s indicates that the biospheric uptake of fossil fuel CO2 is likely to be greater than the oceanic uptake; the relative proportions of the sinks cannot be uniquely determined using C-12 and C-13 alone. The land-ocean sink partitioning requires in addition, information about the land use source, isotopic disequilibrium associated with gross oceanic exchanges, as well as the fractions of C-3 and C-4 vegetation involved in the biospheric uptake.