A two-box ocean model with a coupled carbon and phosphorus cycle is used to test which processes can realistically generate a rapid positive 2‰ δ13C excursion in the global carbon reservoir. Internal oceanic processes (enhanced preservation of organic matter under anoxic conditions, enhanced upwelling) cannot produce a substantial δ13C excursion under the assumption that burial rates of C and P are coupled. Loss of nutrients to the sediment acts as a negative feedback mechanism that prevents a substantial perturbation of the carbon cycle. A large and renewable source of nutrients is needed, either from increased continental weathering or from enhanced regeneration of sedimentary organic phosphorus under anoxic conditions. Measurements of total phosphorus and organic carbon in sediments from the Tarfaya Basin, Morocco, suggest that a large decrease in P burial occurred relative to organic carbon at the base of the Cenomanian–Turonian Oceanic Anoxic Event (OAE2), with the right magnitude to quantitatively explain a 2‰ shift in δ13C. It is concluded that the main underlying mechanism that allowed and sustained enhanced carbon burial during the mid-Cretaceous was a perturbation of the oceanic phosphorus cycle.