Reconstructing the dissolved carbon dioxide of surface waters from the δ13C of organic carbon relies on accurate determination of the growth rate of phytoplankton contributing to sedimentary organic matter. We demonstrate that the Sr/Ca ratio in the coccoliths of Emiliania huxleyi is correlated with the rates of both organic carbon fixation and calcification. An investigation of biomineralization models suggests that these three factors may be linked by the differential pumping rates of calcium relative to strontium ions across the cellular membranes which supply ions for photosynthetic carbon fixation and calcification and derives energy from photosynthetic products. A quantitative relationship is derived between calcification, organic carbon fixation, and Sr/Ca such that a combined geochemical proxy approach using ϵp and Sr/Ca can be applied to the paleoceanographic record.