Evaluating the decline in size of brachiopod assemblages during the Pliensbachian/Toarcian boundary event (Eastern Morocco)

Abstract

The size structure of macrobenthic assemblages exhibits a major shift across the Pliensbachian/Toarcian boundary event on the NW European shelf, with the appearance of common micromorphic brachiopods (Koninckella community). This community type then abruptly disappears near the base of the main negative carbon isotope excursion associated with the Toarcian Oceanic Anoxic Event. Several studies documented that micromorphic brachiopods become common already in the Spinatum Zone in the NW Algeria (Traras Mountains) or in Tunisia (Jebel Zaghouan), indicating that the community shift took place earlier on the southern Tethyan margin. Therefore, to assess temporal changes in the composition and size structure of macrofaunal community on the southern Tethyan margin, we investigated a Pliensbachian-Toarcian succession in the Beni Snassen Mountains. New chemostratigraphic data document the presence of the initial negative carbon isotope excursion at the Pliensbachian/Toarcian boundary, with δ13C declining from -24.5 to‰ -26‰ just at the base of a marl that overlies the Beni Hammad Formation. δ13C values decline to -23.5‰ in the middle part of the Beni Amyir Formation, and abruptly decline to -26‰ and -27‰ just in the uppermost part of the Beni Amyir Formation, documenting the onset of the main negative carbon isotope excursion. We find that koninckinid brachiopods and Nannirhynchia become common and appear already in the Spinatum Zone where they co-occur with larger brachiopods (Prionorhynchia, Phymatothyris, Cisnerospira), similarly as in Algeria and Tunisia. Interestingly, in the lowermost Toarcian (Polymorphum Zone), the assemblage of micromorphic brachiopods is more diverse than in the Lusitanian Basin. With the exception of Nannirhynchia, all micromorphic genera went extinct near the onset of the main carbon isotope excursion. This work was supported by the Slovak Research and Development Agency (APVV22-0523).