Planktic foraminiferal resilience to the Early Eocene Climatic Optimum (EECO, ~53-49 Ma) at the Atlantic Ocean

There is consensus since the last IPCC report that the rate of climate warming related to high CO₂ pressure is generating a strength of the greenhouse state thus the comprehension of marine ecosystems resilience is a pressing humankind issue. The long-term response of the biota under elevated temperatures and CO₂ concentrations remains uncertain because modern studies are limited in time.  The geological archive offers the key opportunity to evaluate the resilience of planktic foraminifera on a long-term perspective. Planktic foraminifera are important marine calcifiers, regulating biogeochemical cycles, abundant in marine sediments since the Cretaceous, and largely adopted for paleoecological and paleoceanographic reconstructions as they are extremely sensitive to environmental parameters. The Early Eocene Climatic Optimum (EECO, ~ 53-49 Ma) is a crucial interval of Earth’ history to investigate as recording the maximum temperature and pCO₂ of the entire Cenozoic (Zachos et al., 2001, Sciences; Anagnostou et al. 2016, Nature; Inglis et al., 2020 Clim. Past). Our recent research shows that the EECO markedly impacted the planktic foraminifera communities by inducing an important turnover as recorded from the latitudinally spaced Atlantic sites 1051, 125 and 1263. Specifically, the mixed-layer symbiont-bearing genus Morozovella, that dominated tropical-subtropical early Paleogene assemblages, abruptly and permanently declined its abundance (up to one-third), size, and diversity at the EECO beginning, close to the carbon isotope excursion known as the J event (~ 53 Ma) whereas the abundance and diversity of Acarinina markedly increased (Luciani et al., 2016 Clim. Past; Luciani et al. 2017 Paleoceanogr.; Luciani et al. 2017 GloPlaCha; D’Onofrio et al., 2020 Geosciences). In addition, the Morozovella morphospecies (or criptic species) display different coiling direction (the ability to add chambers in clock- or counter-clock wise) that was dominantly dextral below the EECO and becoming dominantly sinistral at the EECO, within ~ 200 kyr after the carbon isotope excursion known as K/X event (~ 52.8 Ma) (Luciani et al. 2021 GloPlaCha). Therefore, the sinistral morphotypes represent the main survivors at the expense of dextral forms, that were less resilient to the EECO stressors. Our stable carbon and oxygen data on dextral and sinistral morphotypes performed below and above the major coiling shift, show that sinistral morphotypes typically have lower δ¹³C values. The lower δ¹³C signatures of the sinistral specimens, suggests that they were less dependent on their photosymbiotic partnerships, possibly migrating slightly deeper in the mixed-layer. This strategy could have made them able to better tolerate the pressure occurring during the EECO. Our record strongly advises on a causal relationship to chemical-physical modifications in the surface waters, possibly to the temperature increase. Preliminary Mg/Ca derived paleotemperatures through (LA)-ICP-MS from Site 1263 reveal that Morozovella crater and M. subbotinae record a greater warming at the EECO than Acarinina coalingensis and A. soldadoensis. The higher rise in temperature recorded by morozovellids may explain the reduced symbiotic relationship, even though further geochemical analyses are in progress to explore the influence of other potential stressors such as pH decrease.