Biosphere-atmosphere gross carbon exchange flux and the d13CO2 and D14CO2 disequilibria constrained by the biospheric excess radiocarbon inventory

摘要

Estimates of the global biospheric excess 14C inventory IB 14,E from Naegler and Levin (2009) were used to constrain the age distribution a(t) in heterotrophically respired CO2 with a simple (radio)carbon model of the global biosphere. Subsequently, a(t) could be used to estimate the global gross carbon exchange Feq C (net primary productivity, NPP, and heterotrophic respiration) between atmosphere and biosphere as well as both the d13C and D14C signatures in heterotrophically respired CO2 (d13CRH and D14CRH, respectively). Our estimates of Feq C range from 41 to 64 petagrams carbon per year (Pg C a1), with a best estimate of 55 Pg C a1. The uncertainty of this value is dominated by the uncertainties of IB 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 D14CO2 add an uncertainty of ±3 Pg C a1. The d13CRH of heterotrophically respired CO2 lags the d13C of assimilated CO2 by 10–17 years. This leads to a somewhat smaller estimate of the biospheric 13CO2 disequilibrium flux than previously assumed. D14CRH increased from 20% in the early 1950s to maximum values of 300–325%in the late 1960s/early 1970s. In the 1980s, when the maximum IB 14,E occurred, D14CRH was in a transient equilibrium with the atmosphere. The D14C disequilibrium between atmosphere and biosphere increased to D14CDIS = 20–50% in the mid-1990s, before it dropped to 15–40% in 2005.