Planktic foraminiferal changes and the Early Eocene Climatic Optimum (EECO, ~ 53-49 Ma): crisis or resilience strategy to global warming?

Modern studies on marine ecosystems are limited in time so the evaluation of their stability under the ongoing CO₂ emissions and global warming remains uncertain and necessarily requires a long-term perspective. The dynamic early Paleogene climate offers the crucial opportunity to detect relationships among calcareous plankton, past carbon cycle perturbations and climate. Specifically, the EECO (~53-49 Ma) represents 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. We investigated the Pacific Sites 1209-1210 and eastern Indian Ocean Site 762 following the evidence that the EECO marked impacted planktic foraminiferal assemblages at the Atlantic Oceans. Abrupt and permanent abundance decline (more than one-third) of the symbiont-bearing genus Morozovella occurred at the EECO beginning (J event, ~53 Ma) at sites 1209-1210, whereas Acarinina concomitantly increased, as from the Atlantic sites. Site 762 recorded the southern high-latitude migration of the warm Acarinina species coupled with the decline of the ‘cold’ subbotininds. Another major change documented at the Pacific and Indian Oceans revealed to be similar to the Atlantic record and involved the coiling direction (ability to add chambers clock- or counter-clockwise) of the genus Morozovella. Indeed, the morozovellid coiling direction is dominantly dextral below the EECO but became sinistral within the EECO, although this change is registered ~ 200 kyr later at the Pacific Ocean and ~ 200 kyr before at Site 762 where it occurred at the K/X event (~52.8 Ma). Therefore, the morozovellids crisis observed in the Atlantic and Pacific Oceans can be mainly read as the dextral forms decline. Searching for the driving causes of the observed modifications, we performed stable isotope analysis on sinistral and dextral morozovellids morphotypes (possibly cryptic species) from sites 1209-1210 and 762. Results show that the sinistral forms generally record lower d¹³C values, once again as recorded for the Atlantic Ocean. This evidence suggests a reduced symbiosis relationship and/or a slightly deeper habitat, probably a strategy to sustain the stressors induced by the EECO. Our record advises on a causal relationship to chemical-physical modifications in the surface waters, such as the temperature increase. The increased temperature of at least 1°C [Mg/Ca (LA)-ICP-MS] recorded by sinistral morozovellids within the EECO may have acted in the reduced photosymbiotic activity. Conversely, acarininids do not show preferential coiling nor below neither within the EECO and they reveal d¹³C values that imply major ecological flexibility, possibly enabling them to proliferate. The EECO also induced the virtual disappearance of the genus Chiloguembelina after the K/X event at all the Atlantic, Pacific and Indian sites investigated. This disappearance appears to be related to thermocline warming and Oxygen Minimum Zone enhanced oxygenation. Our records demonstrate the wide geographic and possibly global character of the striking modifications occurred in the planktic foraminiferal assemblages during the first ~800 kyr of the EECO. Our derived paleobiology gives new insights into planktic foraminiferal strategies adopted under long-term global warming.