The enormous phosphorite deposits of the southeastern United States indicate intense upwelling but contain small amounts of organic carbon. We propose that deposition of organic-rich sediment on continental shelves in the southeastern United States and elsewhere during marine transgressions in the late Oligocene and early to middle Miocene resulted in global positive δ13C shifts and the formation of early diagenetic phosphorite. Multiple reworking and supergene weathering from subaerial exposure during Miocene marine regressions oxidized most of the organic carbon and resulted in the return of δ13C to its preexcursion value. The estimated phosphorite content of the southeastern United States requires sufficient organic carbon burial (>1015 kg carbon) to support a 1‰ δ13C excursion. We believe that our hypothesis may explain the relation among δ13C excursions, sea-level fluctuations, organic carbon burial, phosphogenic episodes, and possibly global cooling during the Cenozoic.