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This PhD thesis focuses on ecological and evolutionary dynamics of calcareous nannoplankton communities during the early-middle Eocene, when the extreme warmth condition of the Early Eocene Climatic Optimum progressively switched towards the long-term middle Eocene cooling. This time interval represents a crucial transitional phase in the long-term global Paleogene climate evolution, but the relationship between climate and calcareous nannoplankton are poorly constrained due to the lack of high resolution records. We present calcareous nannofossil data combined with stable isotope of bulk sediments and carbonate records from the deep-sea succession recovered at Site U1410 (IODP Exp. 342, northwest Atlantic). The continuous stratigraphic record and the exceptional preservation of calcareous nannofossils at Site U1410 enabled us to improve the biostratigraphic framework and to document the temporal relationship between changes in paleoenvironmental trends and modification in calcareous nannofossil assemblage. Throughout the study interval, the general long-term evolution of calcareous nannofossil assemblages is characterized by the onset and successively the dominance of family Noelaerhabdaceae. In particular, genus Reticulofenestra emerged during the EECO and became established as the prominent component of Eocene assemblages by the early Lutetian. The prominent increase in abundance observed during the latest Ypresian was concomitant with the decline of other important components of the assemblage such as Zygrhablithus, Discoaster and Sphenolithus and appears to have been favoured by a combination of different biotic and abiotic factors, such as the long-term effect of EECO coupled with minor environmental changes recorded as a positive step of δ18O. On a short-term perspective, the early Lutetian at Site U1410 is characterized by a distinctive short-term negative perturbation in the stable isotope records that have been tentatively interpreted as a warming episode that interrupted the initial post-EECO cooling phase (Chapter 2). Calcareous nannofossil assemblages seems to respond to this transient event with changes in abundance observed both in Sphenolithus and Discoaster. In addition, the exceptionally preservation of middle Eocene calcareous nannofossils enabled a comprehensive taxonomic revision of middle Eocene Coccolithus-like placoliths; the SEM analyses highlighted the development of cross or X-shaped structures in the central area (Chapter 3), which eventually lead to the description a new genus Pletolithus and a new species (Pletolithus giganteus). Finally, a taxonomic reassessment of the middle Eocene Sphenolithus furcatolithoides group was the case study of Chapter 4. Based on the morphology and extinction pattern we described the successive steps observed in this lineage that led to comprehensive understanding of the stepped evolution of this group. More importantly, these taxonomic studies improve the stability of middle Eocene biostratigraphic schemes and highlight that the appearance of reiterative characters, a apical spine within sphenoliths and a robust structure in the central area within coccolithaceans, have occurred during a phase of relatively stable conditions during the Lutetian. |
