Calcareous nannofossil abundance and diversity during the Messinian Salinity Crisis: New evidence from Mezzavalle section (Central Italy)

The Messinian Salinity Crisis (MSC) is commonly portrayed as an extreme event of environmental stress, during which Mediterranean surface waters were widely assumed to have experienced severe ecological deterioration. Here, calcareous nannofossil abundance and diversity are investigated in the Mezzavalle section along the Conero Riviera (Central Italy), where a cyclic succession of resedimented gypsum and organic-rich laminated mudstones is exposed. The succession deposited during stage 2 of the MSC, which corresponds to the phase of maximum restriction. The organic-rich mudstones interbedded with gypsum represent background hemipelagic sedimentation within a persistently subaqueous basin. The occurrence of intact coccospheres within these mudstones suggests in situ nannoplankton production and argues against an extensive biological crisis of this group of organisms. The comparison of diversity patterns and reworked nannofossil proportions across different localities and MSC stages reveals a decoupling between strongly variable taxonomic richness and a largely stable dominance structure, with reworked specimens occurring within pre-MSC variability and likely reflecting enhanced freshwater runoff. This pattern likely points to eutrophic conditions, possibly combined with salinity variations, which favored a limited number of opportunistic taxa with broad environmental tolerance. Overall, the persistence of calcareous nannoplankton throughout the MSC argues against a complete collapse of calcareous nannoplankton production and instead suggests that Mediterranean surface waters were characterized by predominantly marine to mesohaline salinity conditions, rather than by persistent hypersalinity.