Planktic foraminifera-bound nitrogen isotopes across the Middle Eocene Climatic Optimum (MECO, ~40 Ma): implications for photosymbiosis and community change in the Atlantic Ocean

The Cenozoic Era provides a key framework for investigating how ocean oxygenation and marine productivity responded to past global warming events, offering valuable analogues for ongoing and future climate change. Here, we integrate foraminifera-bound nitrogen isotopes (FB-δ¹⁵N) with stable carbon and oxygen isotopes (δ¹³C, δ¹⁸O) to reconstruct nitrogen-cycle dynamics, water-column oxygenation, and photosymbiotic behaviour in planktic foraminifera across the Middle Eocene Climatic Optimum (MECO; ~40 Ma), a major greenhouse interval lasting ~500–600 kyr. The dataset is based on planktic foraminifera from two Atlantic sites spanning contrasting latitudes: subtropical ODP Site 1051 and subantarctic ODP Site 702. FB-δ¹⁵N records from both sites show a marked and coherent decrease during the MECO, reaching minimum values at peak warming. This trend indicates a general reduction in water-column denitrification, a process that generally occurs under extremely low oxygen conditions, suggesting that prolonged warming was not associated with widespread deoxygenation in the global ocean . These results are consistent with patterns observed during other Cenozoic hyperthermals (e.g. PETM, EECO, MCO) and imply that enhanced deep-water ventilation and/or reduced biological productivity counteracted warming-driven oxygen loss. Paired δ¹³C and δ¹⁸O data confirm persistent vertical habitat partitioning among planktic foraminiferal taxa, despite partial convergence in δ¹⁸O values during the MECO, indicating upper-ocean thermal homogenization and temporary niche compression. This preservation of depth-related ecological structure supports the interpretation of interspecific FB-δ¹⁵N offsets as reflecting distinct symbiotic strategies. Lower δ¹⁵N values in Acarinina and Globigerinatheka relative to Subbotina confirm their photosymbiotic nature, while systematic differences between the two symbiotic genera suggest dinoflagellate symbionts in Acarinina and non-dinoflagellate algae (e.g. diatoms or coccolithophorids) in Globigerinatheka. The contrasting evolutionary trajectories of these taxa, recording a decline for Acarinina and expansion for Globigerinatheka during and after the MECO, likely reflect differences in symbiont flexibility and sensitivity to photic-zone environmental change. Overall, this study provides the first reconstruction of the nitrogen cycle across the MECO and demonstrates the value of FB-δ¹⁵N, combined with δ¹³C–δ¹⁸O constraints, as a dual proxy for local and global denitrification and planktic foraminiferal ecology during sustained greenhouse warming.