The operation of the carbon cycle forms an important part of the processes relevant to future changes in atmospheric carbon dioxide. The balance of carbon between terrestrial and oceanic reservoirs is an important factor and here we focus in particular on the oceans. Future changes in the carbon cycle that may affect air–sea partitioning of CO2 are difficult to quantify but the palaeoceanographic record and modern observational studies provide important evidence of what variations might occur. These include changes in surface nutrient use, the oceanic inventory of nutrients, and the elemental composition and rain–rate ratio of marine particles. Recent work has identified two inter–linked processes of potential importance that we consider in some detail: the response of marine calcification to changes in surface water CO2 and the association of particulate organic carbon with ballast minerals, in particular biogenic calcite. We review evidence from corals, coccolithophores and foraminifera, which suggests that the response of reduced calcification provides a negative feedback on rising atmospheric CO2. We then use a box model to demonstrate how the calcification response may affect the organic carbon rain rate through the ballast effect. The ballast effect on export fluxes of organic and inorganic carbon acts to counteract the negative calcification response to increased CO2. Thus, two oceanic buffers exert a significant control on ocean–atmosphere carbonate chemistry: the thermodynamic CO2 buffer; and the ballast/calcification buffer. Just how tightly coupled the rain–rate ratio of CaCO3/Corg is to fluxes of ballast minerals is an important question for future research.