Biosphere-atmosphere gross carbon exchange flux and the δ13CO2 and Δ14CO2 disequilibria constrained by the biospheric excess radiocarbon inventory

Estimates of the global biospheric excess C-14 inventory I-B(14,E) from Naegler and Levin (2009) were used to constrain the age distribution a(tau) in heterotrophically respired CO2 with a simple (radio) carbon model of the global biosphere. Subsequently, a(tau) could be used to estimate the global gross carbon exchange F-eq(C) (net primary productivity, NPP, and heterotrophic respiration) between atmosphere and biosphere as well as both the delta C-13 and Delta C-14 signatures in heterotrophically respired CO2(delta C-13(RH) and Delta C-14(RH), respectively). Our estimates of F-eq(C) range from 41 to 64 petagrams carbon per year (Pg C a(-1)), with a best estimate of 55 Pg C a(-1). The uncertainty of this value is dominated by the uncertainties of I-B(14,E) and of the net biospheric uptake of anthropogenic CO2. Limitations intrinsic to our approach as well as uncertainties in effective global average atmospheric Delta(CO2)-C-14 add an uncertainty of +/- 3 Pg C a(-1). The delta C-13(RH) of heterotrophically respired CO2 lags the delta C-13 of assimilated CO2 by similar to 10-17 years. This leads to a somewhat smaller estimate of the biospheric (CO2)-C-13 disequilibrium flux than previously assumed. Delta C-14(RH) increased from similar to-20 parts per thousand in the early 1950s to maximum values of 300-325 parts per thousand in the late 1960s/early 1970s. In the 1980s, when the maximum I-B(14,E) occurred, Delta C-14(RH) was in a transient equilibrium with the atmosphere. The Delta C-14 disequilibrium between atmosphere and biosphere increased to Delta C-14(DIS) = 20-50 parts per thousand in the mid-1990s, before it dropped to 15-40 parts per thousand in 2005.