LATE ALBIAN-CENOMANIAN PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY, TAXONOMY AND PALEOCEANOGRAPHIC INFERENCES

The mid-Cretaceous period (about 120 Ma to 80 Ma) was characterized by unusually warm climate, low thermal gradient between latitudes and by a series of Oceanic Anoxic Events (OAEs) promoting deposition of black shales (e.g. Bréhéret, 1988; Gale et al., 1996; Coccioni, 2001; Bornemann et al., 2005; Gambacorta et al., 2014, among others). During the late Albian-Cenomanian (~105-93 Ma), a major radiation of planktonic foraminifera did occur for the first time in their evolutionary history and was marked by the appearance of novel morphologies (biserial taxa and single-keeled trochospiral taxa with supplementary apertures) and by the development of new and more complex wall textures (macroperforate, muricate and costellate) on planispiral and trochospiral taxa that lasted until the end of the Cretaceous (González-Donoso et al., 2008; Lipson-Benitah, 2008; Georgescu and Huber, 2006, 2009; Huber and Leckie, 2011). However, the often imprecise usage of the key taxonomic criteria for species identification (morphology vs. wall texture) and/or broad species concept used for the identification of some upper Albian-Cenomanian species have lead to uncertainties in the planktonic foraminiferal biozonation. This study is aimed 1) to review the upper Albian-Cenomanian planktonic foraminiferal species in order to clarify the key taxonomic features of some often misidentified species (e.g., Th. reicheli, R. cushmani, R. montsalvensis); 2) to refine the planktonic foraminiferal biozonation according to the revised taxonomy; 3) to describe the planktonic foraminiferal preferred trophic regime in the upper Albian-Cenomanian interval through the observation and comparison of relative abundances and diversity of species with inferred similar depth ecology; and 4) to clarify the planktonic foraminiferal response to the upper Albian Oceanic Anoxic Event 1d (OAE1d) (Erbacher and Thurow, 1997) and to the Mid Cenomanian Event (MCE) (Ernst et al., 1983). The upper Albian-Cenomanian samples analyzed were selected from different sedimentary basins: 1) the Umbria-Marche Basin (Monte Petrano and Le Brecce sections) and 2) the Mazagan Plateau (Deep Sea Drilling Program - DSDP Leg 79, Sites 547A and 545). All the studied sections include the Albian/Cenomanian boundary and the OAE1d (Leckie, 1984; Nederbragt et al., 2001; Gambacorta et al., 2014, 2015, 2016). In addition, the Monte Petrano section also records the MCE (Coccioni and Galeotti, 2003; Gambacorta et al., 2014, 2015, 2016). Samples from the Umbria-Marche Basin were processed using a technique with acetic acid, while those from the Mazagan Plateau were prepared with the standard methodology. A scanning electron microscope (SEM) was used to document the species and to observe the wall texture at higher magnification. Finally, quantitative analysis were performed for the Monte Petrano and Site 547A to document variations in abundance of species and genera respect to the total planktonic foraminifera, and, consequently, to evaluate the evolutionary trends and the changes in the assemblages composition. Five Biozones and one Subzone (Robaszynski and Caron, 1995) were identified, specifically from base to top: Pseudothalmanninella ticinensis Zone, Thalmanninella appenninica Zone, Planomalina buxtorfi Subzone, Thalmanninella globotruncanoides Zone, Thalmanninella reicheli Zone and Rotalipora cushmani Zone. Moreover, the LO (lowest occurrence) of Th. appenninica, the LO of P. buxtorfi, the LO of P. libyca, the LO of Th. brotzeni, the LO of Th. globotruncanoides, the LO of Th. greenhornensis, and the LO of R. cushmani are identified to be potentially useful bioevents for regional and global correlations by comparing the results from this study with data reported from the literature (Mont Risou section, Vocontian Basin: Gale et al., 1996; Kennedy et al., 2004; Petrizzo et al., 2015; and Blake Nose ODP Site 1050C, northwestern Atlantic: Petrizzo and Huber, 2006a; Petrizzo et al., 2008; Ando et al., 2010; Petrizzo et al., 2015). Thalmanninella reicheli that in the current planktonic foraminiferal biozonation (Robaszynski and Caron, 1995) defines the base of the nominal Zone, resulted to be a problematic marker, as it is a rare taxon and/or displays a scattered stratigraphic distribution. Therefore, the lowest occurrences of Th. greenhornensis and of Whiteinella sp. are proposed to be potentially useful bioevents to approximate the base of the Th. reicheli Zone in the absence of the zonal marker. The lowest occurrence of Th. globotruncanoides, primary criterion for the definition of the Albian/Cenomanian boundary (Kennedy et al., 2004), falls close to the interval of maximum values of the δ13C in all the studied sections and at Site 1050C (Blake Nose, northern west Atlantic Ocean). On the contrary, in the Mont Risou section, (stratotype section for the Cenomanian GSSP), Th. globotruncanoides first appears below the interval of higher δ13C values (Gale et al., 1996). Although the observed discrepancies may rely on different causes (e.g., sampling resolution, diagenesis) new integrated stratigraphy studies of the stratotype section are certainly needed to verify the reproducibility of the primary and secondary criteria for the definition of the base of the Cenomanian Stage in localities from different paleogeographic areas. According to the composition of the planktonic foraminiferal assemblages, both sedimentary sections record a late Albian time interval characterized by a dominant oligo-mesotrophic regime with a rather stable column water stratification, sometimes disrupted by perturbations possibly related to the late Albian OAE1d. In fact, as confirmed also at Blake Nose (Wilson and Norris, 2001; Petrizzo et al., 2008), in the stratigraphic interval inferred to be coincident with the OAE1d, the extinction of the warmest planktonic foraminiferal genera (Biticinella, Paracostellagerina and Ticinella) is documented, indicating a reduced thermal gradient of the water column. The low abundance and diversity of warmer taxa (Protoheterohelix) and the increase in abundance and diversity of colder taxa (Praeglobotruncana, rotaliporids, Muricohedbergella), suggest a mesotrophic to meso-eutrophic regime with a low temperature gradient in the stratigraphic interval comprised between the OAE1d and the MCE. The variation in the composition of the planktonic foraminiferal assemblages across the MCE at Monte Petrano reveals a remarkable decrease in relative abundance of warm taxa (Whiteinella), which might be associated to a minimum of temperature in agreement with the data reported in the Anglo-Paris Basin (Voigt et al., 2004; Wilmsen et al., 2007), where the MCE is correlated to a cold event, the so-called primus Event (LO of the belemnite Praeactinocamax primus).