Towards the global record of planktic foraminiferal resilience to the Early Eocene Climatic Optimum (EECO, ~ 53-49 Ma): the assemblage response from Sites 1209-1210 (Shatsky Rise, Pacific Ocean) and 762 (Exmouth Plateau, Indian Ocean)

The EECO (~53-49 Ma) is a key interval to investigate the planktic foraminiferal resilience on a long-term perspective as it records the peak temperature and pCO2 of the entire Cenozoic [1]. Following the evidence of the EECO marked impact on planktic foraminifera at the Atlantic Oceans [2];[3], we investigate the Pacific Sites 1209-1210 and eastern Indian Ocean Site 762 to extend the record towards a global scenario. At Sites 1209-1210 we detect abrupt and permanent abundance decline (more than one-third) of the symbiont-bearing genus Morozovella at the EECO beginning (J event, ~53 Ma) whereas Acarinina concomitantly increased, as from the Atlantic sites. In addition, the Morozovella morphospecies display change in coiling direction (ability to add chambers in clock- or counter-clockwise) from dominantly dextral below the EECO to sinistral at the EECO, similarly to the Atlantic record [4], although ~ 200 kyr later (~400 kyr after K/X event, ~52.8 Ma). The morozovellid crisis can thus be read mainly as the dextral forms decline. Our stable isotope data performed on sinistral and dextral morozovellids morphotypes (or criptic species) show that the survivor sinistral forms generally record lower δ13C values, as for the Atlantic Ocean. This evidence suggests their reduced symbiosis relationship and/or a slightly deeper habitat. Conversely, acarininids that do not show preferential coiling, reveal δ13C values that imply major ecological flexibility possibly enabling them to proliferate. The increased temperature of at least 1°C [Mg/Ca (LA)-ICP-MS] recorded by sinistral morozovellid within the EECO may have acted in the reduced photosymbiotic activity, probably allowing them to sustain the EECO stressors. The EECO also induced the Chiloguembelina virtually disappearance after the K/X event, possibly due to the recorded thermocline warming and Oxygen Minimum Zone enhanced oxygenation [5]. Our derived paleobiology gives new insights on planktic foraminiferal strategies adopted under global warmings. The Site 762 records similar changes but only after the K/X event: decrease of Morozovella, increase of Acarinina (though much less marked than elsewhere), virtual disappearance of Chiloguembelina and change from dominantly dextrally to sinistrally coiled Morozovella.