Biostratigraphic study and paleoecological response of calcareous nannoplankton assemblages during the Middle Eocene Climatic Optimum (MECO) at ODP Site 702 (South Atlantic Ocean)

This Master'sthesis presents a biostratigraphic study and paleoecological reconstruction of calcareous nannoplankton assemblages across the Middle Eocene Climatic Optimum (MECO) at ODP Site 702, located in the central area of the Islas Orcadas Rise (50°56.760′S, 26°22.122′W, 3,083.4 m water depth) in the South Atlantic Ocean. The MECO represents a transient yet significant global warming event that occurred approximately 40 million years ago and is associated with major perturbations in ocean temperature, carbonate chemistry, and biological communities. Calcareous nannoplankton, due to their high evolutionary rates, widespread distribution, and well-preserved fossil record, offer valuable tools for tracking biotic responses to such environmental changes and for refining stratigraphic frameworks. The primary goal of this thesis is to develop a robust age model for the sedimentary succession encompassing the MECO at Site 702. High-resolution qualitative and quantitative analyses were conducted on calcareous nannofossil assemblages prepared through smear slide methodology. A total of 48 samples were analyzed from Hole 702B, covering a stratigraphic interval of approximately 26.8 meters. Nannofossil assemblages were examined under polarized light microscopy to evaluate taxonomic composition, preservation state, and fluctuations in abundance. semi-quantitative and relative abundance counts were performed to estimate the distribution of selected taxa per mm2 and and percentage abundance of taxa. The biostratigraphic zonation applied follows the schemes of Martini (1971), Okada & Bukry (1980), and Agnini et al. (2014), allowing integration with previously established bioevents and enabling temporal calibration based on magneto- and cyclostratigraphic age models. The examined interval falls within Zone NP16 (Martini, 1971), Subzone CP14a (Okada & Bukry, 1980), and Zone CNE14 (Agnini et al., 2014). Compared to deeper sites in the South Atlantic or equatorial regions (e.g., Sites 1260 and 929), the intermediate paleodepth of Site 702 (~2200–2300 m) appears to have allowed for better preservation of calcareous nannofossils across the MECO interval. This relatively favorable preservation has facilitated the recognition of discrete bioevents and paleoecological signals, including subtle changes in assemblage composition and relative abundance. The succession at Site 702 confirms the broad- scale reliability of MECO-associated bioevents previously identified at other locations, supporting their applicability as global stratigraphic markers. In summary, the integration of biostratigraphic, paleoecological, and chronostratigraphic data at Site 702 provides a detailed account of calcareous nannoplankton dynamics during a critical episode of global climatic change. The findings contribute to our understanding of biotic responses to extreme warmth in Earth's history and enhance the resolution of stratigraphic correlation across different ocean basins during the Middle Eocene.