Calcareous nannofossils of section A5_VB_MP, mid-Cretaceous

A high-resolution calcareous nannofossil record is presented from the Lower Albian Niveau Paquier black shale from the Vocontian Basin (SE France). The Niveau Paquier black shale represents the regional equivalent of the supraregionally distributed Oceanic Anoxic Event 1b (OAE 1b). To reconstruct surface water fertility, a nutrient index based on Zeugrhabdotus erectus, Discorhabdus rotatorius (high fertility indicators), and Watznaueria barnesae (low fertility indicator) was established using principal component analysis. In addition, the distribution of Nannoconus spp. and absolute abundances of coccoliths (coccoliths per gram) were used for reconstructing nutricline dynamics of the surface waters. High surface water fertility coincides with low percentages of nannoconids and vice versa. Moreover, high percentages of nannoconids correlate with low absolute abundances of all other coccoliths. Based on the observed nannoplankton distribution pattern and a suggested similarity in ecological requirements between nannoconids and the modern taxon Florisphaera profunda, a model is proposed that couples nannoconid abundances with dynamics of the nutricline. Time series analyses of the nutrient index show fluctuations within the precessional band. The precession-controlled fluctuations of the surface water fertility may represent a monsoonal signal, with the nutrient supply in the surface waters depending on the strength of monsoonal activity. During periods of enhanced monsoonal activity, which were characterized by humid conditions and stronger winds, mixing of the upper water column was enhanced. During that time, the abundance of nannoconids decreased as a consequence of enhanced wind stress that improved vertical mixing and led to an entrainment of nutrients into the surface waters. This resulted in an increase of primary production. During periods of reduced monsoonal activity, marked by drier conditions and reduced wind stress, the surface waters were depleted in nutrients. As a result of a deep nutricline and a reduced nutrient supply to the upper photic zone in a stratified water column, percentages of nannoconids increased. According to the mechanisms outlined above, fluctuations of the nutrient index and nannoconid percentages have been used as a proxy for reconstructing the physical structure of mid-Cretaceous oceans steered by precession-forced monsoonal activity. The results of the study show that the formation of the Lower Albian OAE 1b from the Vocontian Basin occurred under strongly fluctuating surface waters fertility