Integrated Bio-Chemostratigraphy and Paleoenvironmental Responses Across the Mid-Cenomanian Event 1 (mce 1) in the Tethyan Vergons and Boreal Folkestone Sections

Mid-Cenomanian Event 1 (MCE 1) represents a globally documented positive carbon-isotope (δ¹³C) excursion of approximately +1‰ during the middle Cenomanian (Late Cretaceous), characterized by a distinctive double-peaked δ¹³C pattern (peaks a and b) in both carbonate and organic carbon archives, and is considered a precursor to the Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE 2). MCE 1 has been identified across multiple ocean basins, including in marine carbonates throughout Europe and in terrestrial organic matter from Japan. MCE 1 signals significant disturbances in the global carbon cycle, as well as changes in ocean circulation, biological productivity, and oxygenation levels. This study focusses on a new record of MCE 1 in an expanded succession at Vergons, Alpes-de-Haute-Provence, in the Vocontian Basin of SE France, with an emphasis on changes on the planktonic foraminifera assemblages and their paleoenvironmental significance. The stratigraphy of the middle Cenomanian interval is constrained by correlation to another Vocontian Basin section at Blieux using macrofossils (ammonites, inoceramids) and stable isotopes, and to the Folkestone section in SE England that has yielded comparable planktonic foraminifera, stable isotope and macrofossil data. Integrated biostratigraphic and quantitative analyses of planktonic foraminifera from Vergons and Folkestone, combined with δ¹³C and δ¹⁸O data, elucidate the expression and paleoenvironmental impact of MCE 1 across the Tethyan and Boreal realms. In both sections the analyzed interval encompasses the upper Thalmanninella globotruncanoides, T. greenhornensis, and lower Rotalipora cushmani Zones. Planktonic foraminifera are moderately well preserved, continuously present, and account for 40–50% of the total foraminiferal assemblage. Both records show the emergence of double-keeled, meso-eutrophic Dicarinella species and a diversification trend within the Rotalipora lineage from R. montsalvensis to R. cushmani, pointing to rapid turnover among single-keeled, oligotrophic taxa. Despite these similarities, notable regional differences occur. At Folkestone, MCE 1 is associated with a temporary decline of oligotrophic Rotalipora species and a rise in the abundance and diversity of mesotrophic taxa, indicating weakened water-column stratification and disruption of the thermocline. Conversely, at Vergons, rotaliporids persist throughout the interval, and the earlier appearance of R. cushmani suggests accelerated evolutionary change without significant ecological stress. The integrated analysis of planktonic foraminifera, stable isotope records, and sedimentary facies highlights contrasting paleoenvironmental responses. At Folkestone, cooling phases corresponding to the lower part of peak a and to immediately below peak b, align with Boreal macrofaunal horizons (Chlamys arlesiensis and Praeactinocamax primus) and positive δ¹⁸O excursions. These features indicate the influx of cold, low-salinity Boreal waters from the Norwegian Sea into the Anglo-Paris Basin. This incursion likely disrupted stratification and enhanced primary productivity through mixing of nutrient-rich Boreal waters with warmer Tethyan waters, contributing to the δ¹³C increase in a middle–outer shelf environment. At Vergons, comparable cooling signals are recorded in the δ¹⁸O data, but without associated Boreal macrofauna or clear evidence of strong vertical mixing. Due to its upper-slope setting along the western Tethyan margin, the site likely experienced only a distal influence of Boreal waters, resulting in a more subdued increase in productivity and limited ecological impact.