Schizosphaerella size and abundance variations across the Toarcian Oceanic Anoxic Event in the Sogno Core (Lombardy Basin, Southern Alps)

• Published in: Palaeogeography, Palaeoclimatology, Palaeoecology
• Main Authors: Faucher G., Visentin S., Gambacorta G., Erba E.
• Other Authors: G. Faucher, S. Visentin, G. Gambacorta, E. Erba
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier B.V. 2022
• Subjects: Calcareous nannoplankton; Jurassic; Morphometry; Nannofossil; T-OAE; Settore GEO/01 - Paleontologia e Paleoecologia; Ocean acidification
• Online Access: http://hdl.handle.net/2434/923593; https://doi.org/10.1016/j.palaeo.2022.110969

Abundance and size variations of nannofossil Schizosphaerella punctulata were quantified in the uppermost Pliensbachian–Lower Toarcian succession recovered with the Sogno Core (Lombardy Basin, Northern Italy). High-resolution nannofossil biostratigraphy and C-isotopic chemostratigraphy identified the Jenkyns Event within the Toarcian oceanic anoxic event (T-OAE) interval. Absolute abundances and morphometric changes of “small S. punctulata” (< 7 μm), S. punctulata (7–10 μm; 10–14 μm; > 14 μm) and “encrusted S. punctulata” (specimens with a fringing crust) show large fluctuations across the negative δ13C Jenkyns Event. The Schizosphaerella crisis is further characterized by a decrease in average valve size in the early–middle Jenkyns Event. The abundance fall was caused by the failure of S. punctulata specimens >7 μm and “encrusted S. punctulata” that along with the increased relative abundance of small specimens, produced the reduction of average dimensions also documented in the Lusitanian and Paris Basins, although with a diachronous inception. The average valve size from the Lombardy Basin is ~2 μm smaller than in these other basins. Hyperthermal conditions associated with excess CO2 and ocean acidification possibly forced the drastic reduction of S. punctulata abundance/size. In the pelagic succession of the Sogno Core there is a strong positive correlation between the S. punctulata (> 7 μm) absolute abundance/size and the CaCO3 content, with a negligible contribution by small specimens (< 7 μm). Encrusted specimens testify selective neomorphic processes: the diagenetic crust seems diagnostic to separate S. punctulata from S. astraea.