Variations of marine plankton δ13C with latitude, temperature, and dissolved CO2 in the world ocean

Abstract

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.