Age and synchronicity of planktonic foraminiferal bioevents across the Cenomanian–Turonian boundary interval (Late Cretaceous)

The upper Cenomanian \u2013 lower Turonian is a key-stratigraphic interval, as it encompasses the Late Cretaceous supergreenhouse and a major perturbation of the global carbon cycle (i. e., Oceanic Anoxic Event 2) as evidenced by a global positive carbon isotope excursion and by the nearly world-wide deposition of organic-rich marine facies. A turnover in planktonic foraminiferal assemblages and in other marine organisms is documented across this stratigraphic interval, but reconstruction of the timing and identification of the cause and effect relationships between environmental perturbations and organism response require a highly- resolved stratigraphic framework. The appearance and extinction levels of planktonic foraminiferal species generally allow 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 ecological preferences exhibited by each taxon and controlled by oceanic circulation, often resulting in earlier or delayed events in certain geographic areas (i. e., diachronous datums). The aim of this study is to test the synchronicity of the planktonic foraminiferal bioevents recognized across the C/T boundary and to provide the most reliable sequence of events for correlation of low to mid-latitude localities. For this purpose, we have compiled a highly-resolved biostratigraphic analysis of the European reference section for the C/T boundary at Eastbourne, Gun Gardens (UK), and core S57 (Tarfaya, Morocco), and correlated the sequence of bioevents identified with those recorded in other coeval sections available in the literature, including the GSSP section for the base of the Turonian Stage at Rock Canyon, Pueblo (Colorado), where we calculated reliable estimates of planktonic foraminiferal events that are well-constrained by radioisotopically and astrochronologically dated bentonite layers. Results indicate that the extinctions of Thalmanninella deeckei, Thalmanninella greenhornensis, Rotalipora cushmani and \u201cGlobigerinelloides\u201d bentonensis in the latest Cenomanian are reliable bioevents for correlation. In addition, our analysis highlights other promising lowest occurrences (LOs) that need to be better constrained by bio- and chemostratigraphy, including the LO of Marginotruncana schneegansi falling close to the C/T boundary. By contrast, the appearance of Helvetoglobotruncana helvetica and of some Dicarinella species, the extinction of anaticinellids and the onset of the \u201cHeterohelix\u201d shift are likely diachronous across low to mid-latitude localities. Finally, our study suggests that different species concepts among authors, different sample size and sampling resolution, as well as species paleoecology are important factors that control the stratigraphic position at which bioevents are identified

Local versus seaway-wide trends in deoxygenation in the Late Cretaceous Western Interior Seaway

The sedimentary record of the Late Cretaceous North American Western Interior Seaway (WIS) is characterized by periods of enhanced organic carbon burial, including an interval that has been identified as Oceanic Anoxic Event (OAE) 3. The timing and duration of this event has proved difficult to constrain, calling into question whether a widespread organic carbon burial event actually occurred. In order to evaluate the nature of OAE 3 better, different localities are studied within the WIS to investigate whether a single anoxic, carbon burial event exists within Upper Cretaceous strata above the global OAE 2. New trace metal and Fe speciation records from the Niobrara Formation within the Sweetgrass Arch region in southern Alberta, Canada, highlight that there were several intervals of deoxygenation. Comparison of these new results with similar records from the southern portion of the seaway (Colorado, USA), allows seaway wide versus local changes in biogeochemical cycling to be differentiated. These results suggest that while enhanced organic carbon burial is favored in the WIS during the Niobrara transgression, there is no evidence for a single, discrete, seaway-wide anoxic event during the Coniacian-Santonian. In the Sweetgrass Arch region, enhanced organic carbon accumulation and the development of anoxia is controlled by the incursion of Tethyan waters into the region and may have been influenced by significant nutrient addition from ash deposition. A sea level control on the incursion of southern sourced water masses and, subsequent deoxygenation and enhanced organic carbon burial in the WIS, may explain why "OAE 3" is only identified in shallow and restricted marine environments that would be sensitive to changes in sea level. A combination of local and eustatic c