Calcareous nannofossil paleoceanography of Cretaceous Oceanic Anoxic Event 3: from local to global perturbations

This study revises calcareous nannofossil biostratigraphy across the Coniacian-Santonian Oceanic Anoxic Event 3 (OAE3), sharpening supra-regional correlations, enhancing precision in dating and paleoenvironmental reconstructions. Quantitative analyses of nannofossil assemblages of sites in the eaquatorial and south Atlantic Ocean as well as Indian Ocean provided insights into paleoecological and paleoceanographic conditions during OAE3. The temperature index (TI) indicates overall warm conditions, though brief cooling events occurred in the Coniacian-early Santonian, followed by a more sustained cooling trend in the middle Santonian. The nutrient index (NI) revealed that trophic conditions during OAE3 were mostly local, with nutrient-rich conditions limited to upwelling zones and areas receiving enhanced runoff, rather than reflecting a global fertilization episode. All literature data available for Micula and Marthasterites abundance were integrated to obtain a large -scale picture across OAE3: the abundance peaks are synchronous and cannot be preservation artifact or local signals , thus implying imply large scale perturbations of oceanic surface waters during short time intervals. Two acmes of M. furcatus were identified, with the most significant peak at the onset of OAE3, indicating a deep nutricline in oligotrophic surface waters. Five acmes of M. staurophora were recognized, with a climax during the latest Coniacian-early Santonian, likely linked to extreme ocean stratification. These acmes also serve as biostratigraphic markers, improving the resolution of Late Cretaceous boundaries, implementing the resolution of the Turonian/Coniacian, Coniacian/Santonian, and Santonian/Campanian.