Calcareous nannoplankton response to Oceanic Anoxic Event 2 on the southern Tethyan margin (Bahloul Formation, Tunisia)

Oceanic Anoxic Event 2 (OAE2; latest Cenomanian–earliest Turonian) represents a major perturbation of the global carbon cycle and marine ecosystems, yet its expression along the southern Tethyan margin remains incompletely constrained. We present new quantitative calcareous nannofossil data from the Oued Kharroub section (central Tunisia), integrated with carbon-isotope stratigraphy (δ¹³C), carbonate content, and total organic carbon (TOC), to investigate surface-water environmental changes across OAE2. The Oued Kharroub section spans nannofossil zones UC3 to UC8 and records a positive δ¹³C excursion that allows identification of the main phases of OAE2. Calcareous nannofossil assemblages display pronounced variations in abundance, diversity, and composition through the event. Species richness and Shannon diversity index values decrease significantly during the main build-up and plateau of the δ¹³C excursion, coinciding with reduced CaCO₃ content and increased TOC. Assemblages during this interval are dominated by the opportunistic taxon Watznaueria barnesiae, whereas meso- to eutrophic indicators such as Biscutum constans and Zeugrhabdotus erectus show strong short-term fluctuations, suggesting unstable surface-water conditions. Morphometric analyses of W. barnesiae reveal a statistically significant reduction in coccolith size during the core of OAE2, with minimum values coinciding with peak TOC levels, followed by partial size recovery in the post-OAE2 interval. This pattern indicates subtle but detectable calcification stress affecting even ecologically tolerant taxa under peak anoxic conditions. A short-lived increase in Eprolithus floralis near the onset of the event, together with the decline of warm-water taxa, may reflect a weakly expressed cooling episode tentatively linked with the Plenus Cold Event.