La risposta dei nannofossili calcarei al Middle Eocene Climatic Optimum (MECO): dati dalla Kuma Formation (Belaya River, Russia) = The response of calcareous nannofossils to the Middle Eocene Climatic Optimum (MECO): clues from the Kuma Formation (Belaya River, Russia)

In this Master thesis, the biostratigraphic and paleoecological analysis of the Belaya River section, an on-land Eocene succession, belonging to the Kuma Formation, an outcropping along the Belaya River (south-west Russia) is presented. This study is based on calcareous nannofossil assemblages. The studied succession documents a portion of the middle Eocene. The results allowed to biostratigraphically frame the section and analyze the response of calcareous nannofossils assemblages to the Middle Eocene Climatic Optimum (MECO), a prominent global warming event, occurred during the middle Eocene. The biostratigraphic analysis carried out on the calcareous nannofossil assemblages allowed to date the succession, based on its the paleontological content, using appearance and disappearance events. To this aim, the standard biozonations of Martini (1971) and Okada & Bukry (1980) have been applied. Furthermore, to improve the biostratigraphic resolution obtained using the traditional biozones, two additional bioschemes have been integrated (Fornaciari et al., 2010; Agnini et al., 2014). According to the traditional biozonations, the analyzed section belongs to Zone NP16 (Martini, 1971), or Subzone CP14a (Okada & Bukry, 1980). Following Agnini et al. (2014), the succession extends between Zone CNE13 and Zone CNE15. Finally, according to Fornaciari et al. (2010), the section comprises an interval between Subzone MNP16A and Subzone MNP17A. However, due to the coincidence/inversion of some bio-horizons, a complete application of this biozonation was not possible. An age model has been derived, based on GTS (2012), which provides an age of 42.62 Ma for the base of the section and of 39.67 Ma for the top, and a total duration of 2.95 Myr. From the chronostratigraphic point of view, the succession spans from the Lutetian to the Bartonian (middle Eocene). The age-depth graph has evidenced for three sedimentation rates along the studied section: 10.3 m/Myr for the lower part of the succession, 5.1 m/Myr for a short interval in the middle part, and 42.1 m/Myr for the upper part. Biostratigraphic and biochronological data have been integrated with oxygen and carbon isotopic curves (δ 18 O and δ 13 C) data as well as with the bulk carbonate content (%). Consistent with the literature (Bohaty & Zachos, 2003; Zachos et al., 2001; Zachos et al., 2008), the δ 18 O curve displays a lightening during the the MECO, while the δ 13 C curve is characterized by changes related to perturbation in the carbon cycle (Dickens et al., 1995); the carbonate content profile is consistent with a progressive increase in productivity. Finally, the paleoecological analysis indicates that temperature and trophic conditions are the two main factors affecting the changes observed in the calcareous nannofossil assemblages. In particular, the carbonate content, the changes the relative abundances of taxa and the interpretation of the Principal Component Analysis harmonize perfectly with a gradual trend toward more eutrophic conditions, with a paroxysmal phase coinciding with the acme phase of the MECO. Therefore, despite the lack of chronological constraints based on magnetostratigraphic or cyclostratigraphic data, the use of biochronological data has allowed to classify more precisely the section. Moreover, the paleoecological analysis allowed to observe the response of the calcareous nannofossils assemblages to the Middle Eocene Climatic Optimum.