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

Abstract

Estimates of the global biospheric excess 14C inventory I B14,E from Naegler and Levin (2009) were used to constrain the age distribution a(τ) in heterotrophically respired CO2 with a simple (radio)carbon model of the global biosphere. Subsequently, a(τ) could be used to estimate the global gross carbon exchange FeqC (net primary productivity, NPP, and heterotrophic respiration) between atmosphere and biosphere as well as both the δ13C and Δ14C signatures in heterotrophically respired CO2 (δ13CRH and Δ14CRH, respectively). Our estimates of FeqC 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 IB14,E and of the net biospheric uptake of anthropogenic CO2. Limitations intrinsic to our approach as well as uncertainties in effective global average atmospheric Δ 14CO2 add an uncertainty of ±3 Pg C a-1. The δ13CRH of heterotrophically respired CO 2 lags the δ13C of assimilated CO2 by ∼10-17 years. This leads to a somewhat smaller estimate of the biospheric 13CO2 disequilibrium flux than previously assumed. Δ14CRH 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 IB14,E occurred, Δ 14CRH was in a transient equilibrium with the atmosphere. The Δ14C disequilibrium between atmosphere and biosphere increased to Δ14CDIS = 20-50‰ in the mid-1990s, before it dropped to 15-40‰ in 2005. Copyright 2009 by the American Geophysical Union.