Evaluating planktic foraminiferal resilience during the Middle Eocene Climatic Optimum (MECO) in the Atlantic Ocean

Abstract

The Middle Eocene Climatic Optimum (MECO), centered around ∼40 Ma, is characterized by a steady decline in marine bulk and benthic carbonate δ¹⁸O values by approximately ∼1 ‰ over ∼400 kyr. This is typically interpreted as a 3–6 °C increase in global temperatures, followed by a rapid return to pre-event conditions. This event is increasingly attracting scientific attention, as it represents a natural experiment of the temperatures and pCO₂ levels that Earth may reach by the end of this century if anthropogenic greenhouse gas emissions are not reduced. The δ¹³C signal, along with biotic and paleoceanographic changes across the MECO, exhibits significant geographic heterogeneity, making this event still enigmatic. In particular, the biotic response remains poorly constrained. Here, we aim to address this gap by focusing on planktic foraminifera, which are highly sensitive to the physical and chemical state of the oceans and can offer a valuable long-term perspective on marine ecosystem resilience to global warming. We selected Ocean Drilling Program Sites 1051, 1263, and 702, which cover different latitudinal settings across the Atlantic Ocean and provide established age models and stable isotope constraints. Planktic foraminifera display a pronounced assemblage turnover across the MECO, primarily related to an increase in surface-water temperature that altered pelagic food webs. The intense warming caused a southward migration of warm-index taxa at Site 702, as also recorded for calcareous nannofossils. The warm-index “Large Acarinina” (>150 μm) shows a marked and permanent decline within ∼250 kyr during the late stage of the MECO at Sites 1051 and 702, approximately 2 Myr before their evolutionary disappearance at the Bartonian-Priabonian boundary. This decline is widespread, being also recorded in the Tethys. We speculate that changes in microalgal symbionts may have impacted the success of this group. We also document a drop in the abundance of the genus Chiloguembelina, possibly related to enhanced oxygenation of its ecological niche, the oxygen deficient zone (ODZ). The planktic foraminiferal assemblages, though demonstrating some degree of plasticity by absorbing periodic stress extremes through community modifications and latitudinal migration, did not recover their pre-disturbance state. This indicates low stability during the MECO event and ultimately lack of resilience.