Early Jurassic coccolith diversification and response to pre-Toarcian environmental changes: A perspective from the Paris Basin

The early Jurassic saw large fluctuations in global temperature, sea-level and bottom-water oxic conditions prior to the Toarcian-Oceanic Anoxic Event (T-OAE), in several oceanic basins of NW Europe. Links between such changes, ocean nutrient concentrations and progressively increasing abundance and diversity of calcareous nannoplankton remain unclear. We have investigated the early Jurassic nannofossil assemblages of the Paris Basin across the mid-Sinemurian to lowermost Toarcian. Comparisons to geochemical proxy data let us decipher evolutionary trends from paleoenvironmental responses, test paleoecological affinities of coccolith species and improve our understanding of paleoenvironmental changes. Despite a strong evolutionary trajectory of the coccolith assemblages, multivariate analyses allowed to identify taxa indicating cold (Parhabdolithus, Cru-cirhabdus, Crepidolithus pliensbachensis), open ocean (Mitrolithus elegans, Mitrolithus lenticularis, Crepidolithus crassus sensu lato, Crepidolithus granulatus (both variants), Crepidolithus crucifer) and high fertility conditions (placoliths: Bussonius,Lotharingius, Similiscutum novum, Similiscutum finchii). Our data support that: (1) stable cool conditions persisted through the Upper Sinemurian, (2) progressive warming and high sea-levels onsetting across the Sinemurian/Pliensbachian transition, persisting to the lower margaritatus ammonite zone (AZ) may have favored the evolution of placoliths, (3) a first significant increase in abundance of placoliths was triggered in the stokesi ammonite subzone (ASz) by elevated fertility levels which preceded immediately the late Pliensbachian organic matter preservation interval (OMPI), (4) the OMPI and remaining subnodosus ASz were characterized by warm temperatures but low fertility surface waters, (5) another large increase in abundance of placoliths sug-gests again higher fertility levels across the uppermost margaritatus AZ, spinatum cold event and lowermost Toarcian.

Automated summary

This study investigates the early Jurassic nannofossil assemblages in the Paris Basin and explores the links between climate change, ocean nutrient concentrations, and the abundance and diversity of calcareous nannoplankton. The results suggest that factors such as cold conditions, open ocean conditions, and high fertility levels influence the evolution and distribution of calcareous nannoplankton.