Revised Calcareous Nannofossil Biostratigraphy and Paleoceanography of Cretaceous Oceanic Anoxic Event 3: From Local to Global Perturbations

The Coniacian‐Santonian (89.8–83.6 Ma) Oceanic Anoxic Event 3 (OAE3) is the youngest episode of sea‐floor deoxygenation mainly restricted to areas of the Atlantic Ocean and adjacent epeiric and shelf seas. The associated organic carbon‐rich black shale deposition varied in time/intensity at different sites. Climatic and paleoceanographic perturbations possibly triggering OAE3 remain elusive. Quantitative analyses of nannofossil assemblages of sites in the Atlantic and Indian Ocean provided new insights into paleoecological and paleoceanographic conditions across OAE3. The nutrient index revealed that during OAE3 fertility varied locally, with nutrient‐rich conditions limited to equatorial upwelling zones and areas receiving enhanced runoff rather than reflecting a global fertilization episode. The nannofossil‐based temperature indices based on different cold‐water taxa show disparate results at the same region. Literature data available for Micula and Marthasterites abundance were integrated in this study to obtain a comprehensive summary of nannofossil changes across OAE3. Abundance peaks of Micula and Marthasterites are synchronous and argue against preservation artifacts or local signals, implying large scale perturbations of surface waters over 100–500 kyr-long time intervals. A large Marthasterites furcatus acme was identified at the onset of OAE3, indicating a deep nutricline and oligotrophic surface waters. Five acmes of Micula staurophora were recognized, with a climax during the latest Coniacian–early Santonian, likely linked to extreme ocean stratification. These acmes suggest synchronous perturbations of surface waters at supra‐regional scale independently from the development of seafloor anoxia. The Micula and Marthasterites acmes represent additional biostratigraphic markers to approximate the Turonian/Coniacian, Coniacian/Santonian, and Santonian/Campanian boundaries.