Calcareous nannofossil changes during the late Callovian–early Oxfordian cooling phase

Calcareous nannofossil quantitative and biostratigraphic analyses integrated with carbon and oxygen stable isotopes were carried out on the core ANDRA (Agence Nationale pour la gestion des Déchets Radio-Actifs—FRANCE) HTM 102 across the Callovian/Oxfordian boundary drilled at Cirfontaines-en-Ornois, Départment de Haute-Marne, eastern France. Calcareous nannofossil assemblages at the Callovian–Oxfordian transition are dominated by the genus Watznaueria. An increase in abundance of Biscutum spp. and A-group, which consists of Axopodorhabdus spp. (A. atavus, A. rahla, and A. cylindratus), Podorhabdus grassei, Octopodorhabdus decussatus, Hexapodorhabdus cuvillieri (family Axopodorhabdaceae), and Triscutum spp., correlates with a significant positive excursion in δ18O suggesting that these groups were probably adapted to cooler surface waters. A positive increase in δ13C values is coupled with high abundances of eutrophic taxa such as Zeughrabdotus erectus, Biscutum spp., and small-sized Watznaueria britannica, and a decrease in abundance of the big and oligotrophic taxa Schizosphaerella punctulata and Watznaueria manivitae. Climate cooling across the Callovian/Oxfordian boundary probably triggered a breakdown in stratification of surface waters leading to more intense nutrient recycling and higher primary productivity that favoured the shift in abundance of small-sized eutrophic taxa in the East Paris Basin.