Variations of marine plankton δ 13 C with latitude, temperature, and dissolved CO 2 in the world ocean

Variations of the 13 C content of marine participate organic carbon (δ 13 C POC ) in the modern ocean were studied using literature data to test the assumptions underlying the calculation of atmospheric pCO 2 through geological time from the δ 13 C of sedimentary organic matter. These assumptions are that (1) concentrations of CO 2 in the atmosphere and the surface ocean are at equilibrium at all times and latitudes and that (2) carbon isotopic fractionation of phytoplankton (ϵ p ) covaries primarily with concentrations of dissolved molecular CO 2 ([CO 2 ] aq ). Previous studies and compilations have shown that the first assumption does not strictly hold, although [CO 2 ] aq may be predicted with a reasonable degree of accuracy from sea surface temperature for specific regions of the world ocean. The second assumption is shown to be questionable due to the weak covariation of ϵ p and [CO 2 ] aq in the modern ocean. The large residual variance for regressions of ϵp against [CO 2 ] aq suggests that factors other than [CO 2 ] aq strongly affect carbon isotopic fractionation in phytoplankton. It is concluded that the relationship between ϵp and [CO 2 ] aq cannot be easily calibrated using δ 13 C POC data from the modern ocean.