Comparison of Cretaceous OAE1a and OAE2 in two drill cores with unusually high sedimentation rates shows that long-lasting negative δ¹³C excursions precede the positive δ¹³C excursions and that the evolution of the marine δ¹³C positive excursions is similar during both OAEs, although the durations of individual phases differ substantially. Phosphorus speciation data across OAE2 and the Mid-Cenomanian Event suggest a positive feedback loop, enhancing marine productivity during OAEs.