Impact of the Middle Eocene Climatic Optimum on Planktic Foraminiferal Resilience in the Atlantic Ocean

The Middle Eocene Climatic Optimum (MECO), occurring around 40 million years ago, is marked by a gradual decline in marine bulk and benthic carbonate δ¹⁸O values by approximately 1‰ over a span of ~400,000 years. This is typically interpreted as a global temperature increase of 3–6 °C, followed by a rapid return to pre-event conditions. The MECO event is garnering increasing scientific interest, as it serves as a natural experiment for the temperatures and pCO2 levels Earth could reach by the end of this century if anthropogenic greenhouse gas emissions are not reduced. The MECO's δ13C signal, along with biotic and paleoceanographic changes, exhibits significant geographic variability, leaving many aspects of the event unresolved. Specifically, the biotic response remains poorly understood. This study addresses this gap by focusing on planktic foraminifera, which are sensitive to oceanic physical and chemical conditions and can provide insights into marine ecosystem resilience to global warming. We analyzed Ocean Drilling Program Sites 1051, 1263, and 702, spanning different latitudes across the Atlantic Ocean. These sites provide robust age models and stable isotope data. Our results show a marked turnover in planktic foraminiferal assemblages during the MECO, mainly driven by increased surface-water temperatures affecting pelagic food webs. The warming prompted a southward migration of warm-water taxa at Site 702, also observed in calcareous nannofossils. Notably, the warm-water taxon Large Acarinina (>150 μm) showed a significant, permanent decline within ~250,000 years during the late MECO stage at Sites 1051 and 702, well before its evolutionary disappearance at the Bartonian-Priabonian boundary. This decline was also observed in the Tethys. We hypothesize that changes in microalgal symbionts may have contributed to this decline. Additionally, a drop in Chiloguembelina abundance suggests increased oxygenation in its ecological niche, the oxygen-deficient zone (ODZ). While the foraminiferal assemblages exhibited some plasticity through community shifts and latitudinal migration, they did not recover their pre-disturbance diversity, indicating low stability and a lack of resilience during the MECO.