New calcareous nannofossil and carbon isotope data for the Pliensbachian/Toarcian boundary (Early Jurassic) in the western Tethys and their paleoenvironmental implications

Several studies have focused on the response of calcareous nannofossils to the Toarcian anoxic event (T-OAE), one of the most dramatic events of environmental perturbations of the Mesozoic Era. Nevertheless, the impact of marked environmental perturbations on calcareous nannofossils during the Pliensbachian/Toarcian transition (Pl/To; similar to 183 Ma), slightly before the T-OAE, remain comparatively less well documented. Here, we present a high-resolution biostratigraphical and chemostratigraphic study of two sections where the Pl/To is well represented. One section is located in Central Spain (La Almunia) and the second one is in North-Western France (Anse St. Nicolas). Although the two localities were paleogeographically close to each-other during the Early Jurassic, dissimilarities in the nannofossil assemblages suggest different paleoenvironmental conditions. Across the Pl/To boundary, absolute abundances (nannofossils/gram of rock) and placolith-coccoliths increased substantially in both sections. Equally, carbon isotopes show a negative excursion at the boundary followed by a trend to more positive values. The comparison of our new data to seven sections located in both Northern and Southern Tethyan margins, shows that the Pl/To corresponded to a major diversification event of coccoliths. The Pl/To speciation caused a pronounced shift from muroliths to placoliths. Some differences are observed from one section to another, but several first occurrences (FO) of species and a reorganization of the nannofossil community are documented in all the localities. Some of these FOs are biostratigraphically useful for the definition of the Pl/To boundary, like the FOs of Biscutum brief-medium, Zeugrhabdothus erectus and Discorhabdus ignotus. The carbon cycle perturbation at the Pl/To occurred during a transgressive event after a pronounced cooling event and a sea-level drop that triggered hiatuses in many localities. The origination event of calcareous nannofossils is closely linked to all these phenomena. Thus, the Pl/To event has to be seen as the onset of profound paleoenvironmental perturbations that culminated with the T-OAE.