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 CO_2 piston velocity should be revised downward (Sweeney et al. 2007; Muller et al. 2008). This article compares the transient ~(14)C distributions in the ocean obtained with different formulations of the isotopic ratio commonly used in modeling studies. It is found that both the CO_2 increase and the air-sea CO_2 flux significantly contribute to the 1990 ocean bomb ~(14)C 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 CO_2 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 ~(14)C cycle, a simplified form is more relevant for model calibration and piston velocity assessment based on currently available bomb ~(14)C inventories.
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