Published in "Newsletters on Stratigraphy 51 (3): 343–380, 2018" which should be cited to refer to this work. Age and synchronicity of planktonic foraminiferal bioevents across the Cenomanian– Turonian boundary interval (Late Cretaceous) Francesca Falzoni1*, Maria Rose Petrizzo1, Michèle Caron2, R. Mark Leckie3, and Khalifa Elderbak3,4 With 11 figures and 3 tables Abstract. The upper Cenomanian – 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 organ- isms 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 high- ly-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 eco- logical 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 “Globigerinelloides” bentonensis in the latest Cenomanian are reliable bioevents for correlation. In addition, our analysis highlights other promising lowest occurrences http://doc.rero.ch (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 “Heterohelix” shift are likely diachronous across low to mid-latitude localities. Finally, our study suggests that different species con- cepts among authors, different sample size and sampling resolution, as well as species paleoecology are im- portant factors that control the stratigraphic position at which bioevents are identified. Key words. Cenomanian–Turonian, stratigraphy, mid-low latitude correlations, planktonic foraminifera, Pueblo, Eastbourne Authors’ addresses: 1 Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, via Mangiagalli 34, 20133 Milano, Italy. 2 Département de Geosciences, Institut de Géologie, Université de Fribourg, Pérolles, 1700 Fribourg, Switzerland. 3 Department of Geosciences, University of Massachusetts-Amherst, Amherst, Massachusetts 01003, USA. 4 ALS Oil& Gas, 6510 Guhn Road, Houston, TX 77040, USA. * Corresponding author: [email protected] 1 1. Introduction and their contribution to implement the accuracy and resolution of the Geologic Time Scale has been par - The Cenomanian–Turonian boundary interval (Late ticularly important since the Early Cretaceous (e. g., Cretaceous) represents one of the most interesting Bralower et al. 1995, Premoli Silva and Sliter 1995, case-studies for investigating the evolution of the ma- Caron et al. 2006, Coccioni and Premoli Silva 2015). rine biota under the intense environmental perturba- However, despite the wide distribution of this group of tions that occurred during Oceanic Anoxic Event 2 pelagic organisms, each living/fossil species possesses (e. g., Schlanger and Jenkyns 1976, Scholle and Arthur ecologic preferences that may control its geographic 1980, Schlanger et al. 1987). In fact, OAE 2 is globally and stratigraphic distribution. Consequently, plank- recognized as a time of increased sea-surface produc- tonic foraminiferal bioevents cannot be assumed to be tivity under greenhouse climate conditions interrupted globally synchronous and their reliability for correla- by a brief cooling episode (i. e., the “Plenus Cold tion requires testing with other relative dating tech- Event”, see Gale and Christensen 1996, Forster et al. niques. For instance, the identification of the Ceno- 2007, Sinninghe Damsté et al. 2010, Jarvis et al. 2011, manian/Turonian boundary based on planktonic fora- Jenkyns et al. 2017, Kuhnt et al. 2017) that may cor - miniferal events only is problematic. In fact, the base respond to rising sea level and to a re-oxygenation of the Turonian Stage is formally defined by the lowest event of bottom waters in the Western Interior Seaway occurrence (LO) of the ammonite Watinoceras devo- (WIS) (i. e., the “Benthonic Zone”: Eicher and Wor- nense at the GSSP section at Rock Canyon, Pueblo, stell 1970, Eicher and Diner 1985, Leckie 1985, Elder- Colorado (Kennedy et al. 2000, 2005). However, am- bak and Leckie 2016). Across OAE 2, planktonic fora - monites are often rare or absent in hemipelagic and miniferal assemblages underwent a substantial turn- pelagic successions, thus the identification of the C/T over due to the extinction of the single-keeled rotali- boundary in the absence of the primary marker is porids with umbilical supplementary apertures (genera based on secondary bioevents, including the LO of Rotalipora and Thalmanninella) and to the appearance Helvetoglobotruncana helvetica among planktonic and progressive diversification of double-keeled taxa foraminifera. However, the appearance of H. helvetica (genera Dicarinella and Marginotruncana), that dom- is known to be an unreliable event to approximate the inated the assemblages until the Santonian (Robaszyn- base of the Turonian because of its diachronous occur- ski et al. 1990, 1993, Premoli Silva and Sliter 1999, rence, rarity in the lower part of its stratigraphic distri- Leckie et al. 2002, Petrizzo 2002, Falzoni et al. 2013, bution, very transitional evolution from its ancestor 2016a, Petrizzo et al. 2017). However, correlating Helvetoglobotruncana praehelvetica, and absence or stratigraphic sequences, discriminating global from very rare occurrence in epicontinental margin settings local signals, and reconstructing the cause and effect (e. g., Hart and Carter 1975, Carter and Hart 1977, Hart relationships between environmental changes and or- and Weaver 1977, Hart and Bigg 1981, Leckie 1985, ganism response require a reproducible and highly- Hilbrecht et al. 1986, Jarvis et al. 1988, Lipson-Beni- resolved stratigraphic framework. Unfortunately, the tah et al. 1988, Robaszynski et al. 1990, Kuhnt et al. C–T boundary interval lacks magnetostratigraphic 1997, Keller et al. 2001, Luciani and Cobianchi 1999, http://doc.rero.ch control, since it is within the Cretaceous Normal Su- Tur et al. 2001, Petrizzo 2001, Holbourn and Kuhnt perchron (e. g., Gradstein et al. 2012). Nevertheless, 2002, Caron et al. 2006, Mort et al. 2007, Desmares et this interval is accompanied by a ~ +2‰ to 4‰ excur- al. 2007, Hart 2008, Gebhardt et al. 2010, Huber and 13 13 sion in both the δ Ccarb and δ Corg resulting from the Petrizzo 2014, Elderbak and Leckie 2016). Further burial of organic matter during OAE 2 (e. g., Jenkyns complication is introduced by inconsistencies in the 2010). The shape of the δ13C profile with its typical stratigraphic position of planktonic foraminiferal peaks and troughs represents one of the most repro- events, including the identification of LOs (lowest oc- ducible features of this stratigraphic interval, as it is currences) and HOs (highest occurrences) of marker synchronously registered in the marine and continental taxa (e. g., Rotalipora cushmani, H. helvetica) when records, and it represents a powerful tool for global the same section is studied by different authors correlation (e. g., Pratt and Threlkeld 1984, Arthur et (Pueblo: Eicher and Diner 1985, Leckie 1985, Leckie al. 1987, Tsikos et al. 2004, Jarvis et al. 2006, 2011, et al. 1998, Keller and Pardo 2004, Caron et al. 2006, Jenkyns 2010, Joo and Sageman 2014). Desmares et al. 2007, Elderbak and Leckie 2016; East- Planktonic foraminiferal bioevents are routinely ap- bourne: Paul et al. 1999, Keller et al. 2001, Hart et al. plied to correlate pelagic and hemipelagic successions, 2002, Tsikos et al. 2004). 2 The aim of this study is to select the most reliable 2. Materials and Methods and reproducible sequence of planktonic foraminiferal bioevents across the C–T boundary interval by distin- To document the sequence of planktonic foraminiferal guishing between the most trustworthy isochronous bioevents across the C–T boundary interval, we have bioevents from those