Significato biostratigrafico e biocronologico delle associazioni a nannofossili calcarei durante l'Optimum climatico dell'Eocene Medio: risultati dal Site ODP 1260 (Demerara Rise, Atlantico Equatoriale)

This Master degree thesis aims to perform biostratigraphic and biocronological results based on chages observed in calcareous nannofossils assemblages across the Middle Eocene Climatic Optimum (MECO). The MECO is a global and relatively long-lasting (ca. 500 kyr) warming event, occurred at about 40 Ma (Sexton et al. 2006; Bohaty et al., 2009). Since they enter in the geological in the Late Triassic, calcareous nannoplankton show high abundances, wide biogeographic distributions and high rates of evolution. These characteristics make them one of the most powerful tools for dating marine sedimentary successions. The intergration with other chronological disciplines, such as magnetostratigraphy and cyclostratigraphy, allows the construction of reliable age models. As a side result, absolute dating techniques provide estimates for the absolute ages of biohorizons based on calcareous nannofossil bioevents, which can be used either to evaluate the reliability of any single biohorizons or to use the calibrated ages when absolute chronologic data are not available. Sediments studied here were recovered during ODP Leg 207, in particular, the material comes from ODP Site 1260 (Hole A,B). The main scientific objectives of this expedition were to construct a depth transect of coring sites distributed down the north flank of Demerara Rise (western equatorial Atlantic), in order to better constrain the Cenozoic history of deep-water circulation and chemistry. A high resolution calcareous nannofossil biostratigraphy is provided and this results in precise biostratigraphic classification of the studied interval at ODP Site 1260. The investigated succession can be ascribed to Zone NP16 (Martini, 1971) or Subzone CP14a (Okada & Bukry, 1980). According to the biozonation of Agnini et al. (2014), the succession belongs to Zone CNE14. Calcareous nannofossil biohorizons recognized during this study are calibrated using the two different age models available for the study section (Edgar et al., 2010; Westerhold & Röhl, 2013). Biochronological results are then comprehensevely discussed in view of literature data and published stable oxygen and carbon isotope profiles. At ODP Site 1260, biostratigraphic results are generally consistent with data available from other areas and depositional setting (Fornaciari et al., 2010; Agnini et al., 2014). Overall, these results confirm the validity of the additional events recently proposed for the middle Eocene (Fornaciari et al., 2010; Agnini et al., 2014), though some possible discrepancies are suggested for the Base of D. bisectus if the age models available are used. The study interval spans the MECO, which is a hyperthermal event characterized by a prominent shoaling of the CCD documented by the carbonate dissolution of sediments. Data gathered for the equatorial Atlantic at ODP Site 1260 and 929 suggest that deeper ODP Site 929 is much affected by dissolution than the shallower ODP Site 1260, where processes of dissolution are less evident. The integration of these data with others coming from different areas and depositional settings could serve to depict the history of the global CCD evolution across the MECO.