High resolution upper Cenomanian to Turonian paleoenvironmental changes: Inferences from calcareous nannofossils at the Oued Ettalla section (Central Tunisia)

Abstract

Based on the distribution patterns of seventy calcareous nannofossil species from one hundred and seventy samples from the Oued Ettalla section (Central Tunisia), the upper Cenomanian to Turonian calcareous nannofossil biostratigraphy (CC10-CC12 zones) and paleoenvironmental changes are inferred. The present study identified globally comparable δ13C isotope trends and events such as Monument, OAE2, Holywell, Lulworth, and Round Down. Watznaueria barnesae forms a major fraction of the total population (average: 55%) and reflects increased dissolution, however, moderate to good preservation of calcareous nannofossil species suggests that the primary signal is good for paleoenvironmental analyses. Based on changes in species distribution patterns, diversity, % Eutrophic taxa, Productivity Index and % Delicate taxa, five intervals of change are noted. Additionally, based on coeval assemblage comparisons with the neighboring Oued Bahloul section, the data from the Ettalla section is interpreted. Interval 1 (depth 0–6.9 m) encompasses the carbon isotope Monument Event and at the Ettalla section is characterized by high fertility indicator species such as Eiffellithus gorkae, Retecapsa ficula and Eprolithus floralis, suggesting cool mesotrophic conditions, whereas at the Bahloul section, Zeugrhabdotus and Biscutum dominate. Interval 2 (7.2–18.3 m) encompasses the OAE2 isotope peaks A–C and also reflects cool and mesotrophic conditions, characterized by increased abundances of R. ficula, E. gorkae and E. floralis, with the highest relative abundance of W. barnesae. Lower values for W. barnesae and higher relative abundances of Zeugrhabdotus spp. and Biscutum spp. are noted at the Bahloul section, suggesting similar cool mesotrophic conditions. Interval 3 (depth 18.6–26.4 m; including Peak D of OAE2) is also associated with relatively cool surface waters and mesotrophic conditions with E. gorkae, E. turriseiffelii and R. ficula and the dominance of Zeugrhabdotus and Biscutum at the Bahloul section. The boundary between intervals 2 and 3 marks the C/T boundary near the base CC11 Zone. Interval 4 (26.7–36.3 m) is marked by cooler surface waters with increased abundances of Zeugrhabdotus spp. and Z. erectus whereas at the Bahloul section, similar conditions are recorded with increased relative abundances of Zeugrhabdotus and Biscutum and decreased values of W. barnesae. Interval 5 (36.6–50.7 m) is marked by warm, oligotrophic and stressed conditions and characterized by the maximum average relative abundance of W. barnesae (68%).