| Summary: | The Middle Eocene Climatic Optimum (MECO; ~40 Ma), which interrupted for ~500–600 kyr the long-term cooling trend culminating at the Eocene/Oligocene boundary, still requires a comprehensive understanding of the biotic resilience. Here we present a high-resolution integrated foraminiferal and calcareous nannofossil study across the MECO from the expanded and continuous Tethyan Baskil section (eastern Turkey) that offers a complete magneto-biostratigraphic and geochemical framework. The five MECO phases identified reveal a transition from oligotrophic (<i>pre-MECO</i>) to eu-mesotrophic conditions, possibly related to accelerated hydrological cycle, during the <i>initial MECO</i> and MECO δ<sup>13</sup>C negative excursion phases. The <i>MECO WARMING PEAK</i> phase, marking the highest carbonate dissolution interval, records the most striking biotic changes, such as peak in warm and eutrophic nannofossils, virtual disappearance of the oligotrophic planktic foraminiferal large <i>Acarinina</i> and <i>Morozovelloides</i>, and peak in eutrophic deep dwellers <i>Subbotina</i>. Benthic foraminifera suggest in this phase an improvement in the quality of organic matter to the seafloor. The <i>post-MECO</i> phase shows only a partial recovery of the pre-event conditions. Large <i>Acarinina</i> and <i>Morozovelloides</i> did not recover their abundance, possibly due to cooler conditions in this phase. Our reconstruction reveals how paleoenvironment and marine biota from the studied Neo-Tethyan setting reacted to the MECO perturbations.
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