Reassessing planktonic foraminiferal biostratigraphy across the Cenomanian–Turonian boundary interval (middle Cretaceous)

The upper Cenomanian–lower Turonian is a key-stratigraphic interval of the mid-Cretaceous, as it encompasses a hot greenhouse phase and a major perturbation of the global carbon cycle (i.e, the Oceanic Anoxic Event 2) testified by a globally registered positive carbon isotope excursion and by the nearly world-wide deposition of dark organic-rich facies, pinched in between the normal whitish biogenic carbonates in open marine environments. A turnover in planktonic foraminiferal assemblages (extinction of single-keeled rotaliporids replaced by double-keeled dicarinellids and marginotruncanids) and in other marine organisms has been related to these environmental perturbations. However, the reconstruction of the timing of environmental changes and of the cause and effect relationships between ecological forcing and organism response requires a highly-resolved stratigraphic framework. The appearance and extinction levels of planktonic foraminiferal species represent a powerful tool to biostratigraphically constrain Cretaceous pelagic and hemipelagic sequences allowing accurate intra- and supra-basinal correlations. However, bioevents cannot be assumed to be globally synchronous, because the stratigraphic and geographic distribution of species is modulated by the ecological preferences exhibited by each taxon. The aim of this study is to assess the reproducibility and reliability of planktonic foraminiferal bioevents across the C–T boundary interval by correlating each bioevent to the carbon isotope profile, in order to test their synchronicity across mid-low latitude localities. To perform this study, we have completed a highly-resolved biostratigraphic analysis of the European reference section for the C/T boundary at Eastbourne, Gun Gardens (UK), and of core S57, drilled in the Tarfaya Basin (Morocco). The sequence of bioevents identified has been compared to those recorded in other coeval sections previously studied (the GSSP section for the base of the Turonian Stage at Rock Canyon, Pueblo, Colorado; wadi Bahloul in Tunisia; Clot Chevalier and Pont d’Issole in SE France; and Gongzha in Tibet) that satisfy the condition of lacking major unconformities and of yielding a highly-resolved planktonic foraminiferal biostratigraphy and δ13C record. Results indicate that the extinctions of Thalmanninella deeckei, Thalmanninella greenhornensis, Rotalipora cushmani and “Globigerinelloides” bentonensis in the latest Cenomanian are extremely reliable bioevents for correlation. In addition, our analysis highlights other promising lowest occurrences (LOs) within the uppermost Cenomanian and lowermost Turonian that, however, need to be better constrained by bio- and chemostratigraphy, including the LOs of Praeglobotruncana oraviensis, and of Marginotruncana schneegansi, the latter event falling close to the C/T boundary. Other bioevents, including the appearances of Helvetoglobotruncana helvetica, the secondary marker for the base of the Turonian, and of most Dicarinella species, appear to be diachronous. The ‘Heterohelix shift’ is a regional diachronous ecologic response not recorded in all sections. Although the stenotopic ecological behavior of these species might explain these results, we believe that evolutionary transition between species, different species concepts among authors, and rarity of the species might partially account for the discrepancies observed in the identification of extinction and appearance levels in the sections compared in this study.