Incidence of orbital forcing on calcareous nannofossil assemblages new insights from lower-middle Eocene successions of the Basque-Cantabrian Basin (Spain)

The Paleogene experienced the most profound climatic shift of the past 65 million years of Earth history, Earth’s climate changing from a ‘greenhouse’ in the early Paleocene, to an ‘icehouse’ in the Oligocene. This transition resulted in significant re-organization of oceanic circulation, marine communities, and biogeochemical cycles. Particularly, climate changes occurred during the Eocene are forced (or thought to be forced) by variations of orbital parameters, especially by precession (21ky) and eccentricity (100ky), which can be detected by biotic proxies, like planktonic foraminifera, calcareous nannofossils and dinoflagellates (e.g., variation in their diversity, abundance and size), in hemipelagic and shallow waters. The high abundance of calcareous nannofossils in pelagic marine sediments, makes this group one of the most useful tools to perform biostratigraphic, paleoecologic and paleoclimatic studies. Calcareous nannofossils, showing high evolutionary rate, result to be a good biostratigraphic marker, particularly for Cenozoic, when they reach high diversification. In this PhD thesis, I investigate the relationships between orbital parameters and the variation of calcareous nannofossil abundances, expressed in eccentricity cycles of three different stratigraphic successions in the Basque-Cantabrian Basin (Spain): the Sopelana (early Ypresian), the Gorrondatxe (upper Ypresian) and the Oyambre sections (upper Lutetian). Most of the studies applying such an approach, are focused on the Neogene and Quaternary, and only few studies have been carried out in older Cretaceous and Jurassic records. Up to now, no detailed studies have been performed with the aim to investigate precession and eccentricity cycles and their forcing on calcareous nannofossil aassemblages during the Eocene. The three analyzed sections are constituted by rhythmic alternations of marls and limestones. A depositional-sedimentological model has been previously proposed for the three studied sections, on the base of geochemical proxies variation (CaCO3 content, Oxygen and Carbon isotopes). For the Oyambre and Gorrondatxe sections, a detailed biostratigraphy was already present, and the role of orbital forcing on sedimentation had been already highlighted by previous studies, while, for the Sopelana section there was a lack of this information. Concerning the Sopelana section (Chapter 2), before analysing the influence of precession and eccentricity forcing on calcareous nannofossil abundances, I provide a detailed nannofossil biostratigraphy, and, by means of spectral analysis on CaCO3 record, I highlight the role of the precession cycle in driving the deposition of marl-limestone couplets, and the effect of the eccentricity cycles on the sedimentation of bundles (each of which is composed by five marl- limestone couplets). The variation of calcareous nannofossil abundances induced by orbital forcing has been investigated in Chapter 3. The statistical analyses (implemented with Principal Component Analysis and Cluster methods), based on the variation of calcareous nannofossil abundances, carried out in the Sopelana, Oyambre and Gorrondatxe sections, confirm the previous sedimentological models, adding new information regarding the sedimentary environments during the Eocene in the Basque-Cantabrian Basin. The performed analyses indicate that the major calcareous nannofossil assemblages variations occur at the maximum eccentricity, in correspondence of which we observe a decrease in oligotrophic and stable environments taxa and an increase in eutrophic and low salinity taxa. These observations confirm that the maximum eccentricity corresponds to the maximum seasonality, leading to an increase in nutrients supply from continents and a higher water mixing (upwelling). A further work has been performed in the framework of the studies for the definition of the GSSP, considering that for the Paleogene two GSSPs are still pending (i.e., the base of Bartonian, middle Eocene, and the Priabonian, middle-upper Eocene). In our work, we focus on the definition of the Bartonian Stage. In Chapter 4, I report the results of a multidisciplinary study based on calcareous nannofossil, dinoflagellate, larger and smaller benthic foraminifera and magnetostratigraphy to assess potential correlations and placements for the Lutetian/Bartonian boundary. The Bartonian unit stratotype is located at the Barton coastal section (Hampshire Basin, UK). In this work, I study the parastratotype section, located 7 km away at Alum Bay, since it shows a better exposure. Basing on the obtained results, the Alum bay section is within the nannoplankton Zones CNE14 and CNE15. The Base of Rhombodinium draco, (dinoflagellate), an important and correlatable event in the Tethys domain, is recognized within the section, which also coincides with the acme of N. prestwichianus (formerly indicated as the base of the Bartonian Stage in the Barton area), both of which are correlated with nannoplankton Zone NP16, spanning the upper Lutetian and lower Bartonian. The palaeomagnetic correlation with calcareous nannofossil data allows to assign the normal Chron at the base of the section to C19n, considered an approximation for the base of the Bartonian Stage according to the Geological Time Scale. The remaining portion of the section correlates to C18n and C18r. All analyses, therefore, support that the section is lower Bartonian in age, with the palaeomagnetic data indicating that the first 5 m of the section also contains the Upper Lutetian.