On the relations between the oceanic uptake of CO2 and its carbon isotopes

The recent proposals to estimate the oceanic uptake of CO2 by monitoring the oceanic change in C-13/C-12 isotope ratio [Quay et al., 1992] or the air-sea C-13/C-12 isotopic disequilibrium [Tans et al., 1993] is reviewed. Because the history of atmospheric CO2 and (CO2)-C-13 since preindustrial times is almost the same and increasing in an almost exponential fashion, the oceanic penetration depth of both tracers must be the same. This dynamic constraint permits the establishment of yet a third method to estimate the global ocean uptake of CO2 from C-13 measurements. Using available observations in conjunction with canonical values for global carbon cycle parameters, the three methods yield inconsistent oceanic CO2 uptake rates for the time period 1970-1990, ranging from 0.6 to 3.1 GtC yr(-1). However, uncertainties in the available carbon cycle data must be taken into account. Using a nonlinear estimation procedure, a consistent scenario with an oceanic CO2 uptake rate of 2.1 +/- 0.9 GtC yr(-1) can be established. The method also permits an investigation of the sensitivities of the different approaches. An analysis of the results of two three-dimensional simulations with the Hamburg model of the oceanic carbon cycle shows that the C-13 isotope indeed tracks the oceanic penetration of anthropogenic CO2. Because of its different time history, bomb produced radiocarbon, as measured at the time of the Geochemical Ocean Sections Study (GEOSECS), correlates not as well to excess carbon.