Large discrepancies exist among data-based estimates and model reconstructions of the ocean bomb radiocarbon inventory. In order to resolve this gap, it has been proposed that the CO2 piston velocity should be revised downward (Sweeney et al. 2007; Muller et al. 2008). This article compares the transient 14C distributions in the ocean obtained with different formulations of the isotopic ratio commonly used in modeling studies. It is found that both the CO2 increase and the air-sea CO2 flux significantly contribute to the 1990 ocean bomb 14C inventory, by around 10% each. Moreover, these 2 processes explain more than 25% of the inventory difference between 1974 and 1990. These results imply that, as already argued by Naegler (2009), inventories based on observations that lack information about CO2 invasion are underestimated. Further, this work provides insight into the reasons for discrepancies among model results. It suggests that while a comprehensive isotopic formulation is needed when addressing the global 14C cycle, a simplified form is more relevant for model calibration and piston velocity assessment based on currently available bomb 14C inventories.
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