The mid-Valanginian Weissert Event as recorded by Calcareous nannoplankton in the Vocontian Basin

International audienceThe mid-Valanginian Weissert Event represents one of the most significant paleoceanographic events of the Early Cretaceous and is characterized by a major perturbation of the carbon cycle testified by a positive carbon isotope shift, a crisis of both neritic and pelagic carbonate producers, and climatic fluctuations. Here we propose a paleoreconstruction of paleoenvironmental changes that occurred in the reference Vergol and La Charce sections (Vocontian Basin, SE France) during the Weissert Event based on the analysis of calcareous nannofossil absolute abundance and assemblages. These latter are compared to newly acquired and already published sedimentological and organic geochemical analyses. Our approach is novel for the time interval considered. Indeed, Principal Component Analysis was applied for the first time to the entire nannofossil assemblage to reconstruct environmental conditions, instead of using paleoecological preferences of single species. The comparison of calcareous nannofossils and biomarker analyses indicates that a phase of severe sea–water stratification occurred prior to the carbon positive excursion of the Weissert Event in the Vocontian Basin. This was followed by a raise in fertility of surface waters, as attested by increased nannofossil abundances, in particular, of meso–eutrophic taxa, and biomarkers likely produced by dinoflagellates. This increase in fertility was likely triggered by a humid climate and enhanced continental input of clays and nutrients to surface oceanic waters. Calcareous nannofossils also proved to react to sea-level changes that occurred in the Valanginian, as inferred by previous works. Species likely inhabiting proximal areas were recorded in higher proportions during a major sea-level drop in the Peregrinus Ammonite Zone, Late Valanginian. The results of this study also permit to revise previously proposed paleoecological affinities of some nannofossil species. We suggest that Watznaueria barnesiae, one of the most-widely used species in the literature, should be used cautiously because of its high plasticity with respect to environmental conditions. Also, nannoconids that are usually regrouped in papers with paleoceanographic purposes should be analyzed separately because they show distinct species-specific ecological preferences