Ammonoid and nannoplankton abundance in Valanginian (early Cretaceous) limestone–marl successions from the southeast France Basin: carbonate dilution or productivity?

A quantification of calcareous nannofossil and ammonoid absolute abundance was carried out in the Valanginian limestone–marl alternations of the Vergol section (Drôme) located in the epicontinental Vocontian Basin (southeast France). This section is characterised by four centimetric organic carbon-rich layers (Barrande layers, B1–B4). The aim of this work is to reconstruct changes in the distribution of nutrients in the water column on the base of the analysis of fossil organisms at different levels of the food chain. Calcareous nannofossil abundance correlates negatively with carbonate content. Thus, calcareous nannoplankton probably did not produce the bulk of carbonates in the Valanginian of the studied section. Nannofossil abundance was more likely controlled by carbonate mud produced and exported from shallow platform environments to the basin. Variations of carbonate export, controlled by climate and hydrodynamics, are responsible for the limestone–marl alternations. The levels of highest abundance of nannoplankton are interpreted as mainly linked to a reduced accumulation rate and, to a minor extent, to variations in primary production. A dilution by allochthonous carbonate mud is further evidenced by the relationship between Total Organic Carbon (TOC) and calcium carbonate (CaCO 3). TOC content decreases with increasing CaCO 3 content, except for one of the organic carbon-rich layers (B4). Both taphonomy and dilution factors may have partly controlled the variations in macrofauna abundance. These can be more likely interpreted as a response to palaeoenvironmental changes of the various taxa with different palaeoecological affinities. The high abundance and dominance of bochianitids (orthocone ammonoids) in the layer B4 can be explained by their opportunistic behaviour and their deep-nektonic, possibly nektobenthic, mode of life. Their great capacity for vertical mobility may have permitted brief feeding incursions to the bottom waters when the trophic resources were higher in depth than in the surface waters. High trophic levels at the sea bottom may have also benefited the bivalves and gastropods which are very abundant in layer B4. The mode of life of different ammonoid families is also discussed.