The stable-carbon isotopic composition of marine organic material has varied significantly over geological time, and reflects significant excursions in the isotopic fractionation associated with the uptake of carbon by marine biota. For example, low 13C/12C in Cretaceous sediments has been attributed to elevated atmospheric (and hence oceanic) CO2 partial pressures. A similar depletion in 13C present-day Antarctic plankton has also been ascribed to high CO2 availability. We report, however, that this high-latitude isotope depletion develops at CO2 partial pressures (pCO2 levels) that are often below that of the present atmosphere (340 microatmospheres) , and usually below that of equatorial upwelling systems (> 340 microatmospheres). Nevertheless, because of the much lower water temperatures and, hence, greater CO2 solubility at high latitude, the preceding pCO2 measurements translate into Antarctic surface-water CO2 (aq) concentrations that are as much as 2.5 times higher than in equatorial waters. We calculate that an oceanic pCO2 level of > 800 microatmospheres (over twice the present atmospheric pCO2) in a warmer low-latitude Cretaceous ocean would have been required to produce the plankton 13C depletion preserved in Cretaceous sediments.