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Variations of the 13C content of marine participate organic carbon (δ13CPOC) in the modern ocean were studied using literature data to test the assumptions underlying the calculation of atmospheric pCO2 through geological time from the δ13C of sedimentary organic matter. These assumptions are that (1) concentrations of CO2 in the atmosphere and the surface ocean are at equilibrium at all times and latitudes and that (2) carbon isotopic fractionation of phytoplankton (ϵp) covaries primarily with concentrations of dissolved molecular CO2 ([CO2]aq). Previous studies and compilations have shown that the first assumption does not strictly hold, although [CO2]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 ϵp and [CO2]aq in the modern ocean. The large residual variance for regressions of ϵp against [CO2]aq suggests that factors other than [CO2]aq strongly affect carbon isotopic fractionation in phytoplankton. It is concluded that the relationship between ϵp and [CO2]aq cannot be easily calibrated using δ13CPOC data from the modern ocean.

We have measured the carbon isotopic composition of particulate organic matter suspended in surface waters (POM) between 59°S and 30°S in the SW Indian Ocean during the austral summer. In an attempt to further document the pattern and causes of covariance between POC‐δ13C and [CO2aq], we concurrently measured surface water pCO2, temperature, salinity, nitrate concentration, POM concentration, chlorophyll a and the δ13C of total dissolved inorganic carbon. While we found the previously reported general negative correlation between POC‐δ13C and [CO2aq], we also observed a prominent maximum in POC‐δ13C in the region immediately north of the Subtropical Convergence, coinciding with a maximum in [POM] and chlorophyll a, and with a minimum in pCO2. The increase in POC‐δ13C between 59°S and the Subtropical Convergence is consistent with the trend expected if [CO2aq] were the main factor controlling the isotopic composition of POM. In contrast, data from the region north of the Subtropical Convergence clearly illustrate that POC‐δ13C can also vary independently of [CO2aq] as a 5 per mil decrease in POC‐δ13C was found in a region characterized by nearly constant [CO2aq]. We review several physiological factors which may account for these observations and discuss their implications for paleoceanographic reconstruction of [CO2aq] from the carbon isotopic composition of sedimentary organic matter.

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