Impact of the Middle Eocene Climatic Optimum (MECO) on Foraminiferal and Calcareous Nannofossil Assemblages in the Neo-Tethyan Baskil Section (Eastern Turkey): Paleoenvironmental and Paleoclimatic Reconstructions

Featured Application A challenging issue for humankind is the understanding of the future biota state within the context of global change (anthropogenic CO2 emissions, global warming, ocean acidification, eutrophication, anoxia). Studies on modern biomaterials reveal that the climate-induced environmental stress is affecting the stability of ecosystems. However, modern analyses are limited to decades/historical times whereas the paleontological record can reconstruct the impact of past global climate analogues on a thousand/hundreds/million-year perspective. This work fits the goal to deciphering how marine calcareous nannofossils, benthic and planktic foraminifera, unicellular algae and protozoan, reacted to the global warming known as Middle Eocene Climatic Optimum (MECO), centered at ~40 Ma ago and lasting ~500-600 kyr. Our detailed integrated reconstruction from the Neo-Tethyan geological setting (eastern Turkey) with thousand years' resolution (~100 m/My) reveals that the MECO warming peak induced marked water eutrophication, reduction in oxygen availability at the sea floor and carbonate dissolution. Part of the analyzed marine biota proved to be resilient to the MECO perturbation through marked modifications within assemblages whereas the most specialized planktic foraminifera declined permanently their abundance permanently. Biota and environment only partially recovered the pre-event conditions after the MECO warming peak. Our study, recording transient and permanent changes, allows to understand how paleoenvironment and marine biota reacted to the global MECO perturbation. Abstract 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 (pre-MECO) to eu-mesotrophic conditions, possibly related to accelerated hydrological cycle, during the initial MECO and MECO delta C-13 negative excursion phases. The MECO WARMING PEAK 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 Acarinina and Morozovelloides, and peak in eutrophic deep dwellers Subbotina. Benthic foraminifera suggest in this phase an improvement in the quality of organic matter to the seafloor. The post-MECO phase shows only a partial recovery of the pre-event conditions. Large Acarinina and Morozovelloides 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.