Mg/Ca surface-water paleotemperatures during the Early Eocene Climatic Optimum from the Pacific Ocean: impact on planktic foraminiferal assemblages

The Early Eocene Climatic Optimum (EECO; ~53–49 million years ago, Ma), that represents the Cenozoic peak of temperature and atmospheric CO₂ concentrations, significantly affected planktic foraminiferal (PF) assemblages. The main change documented is the permanent decline in abundance and diversity of the symbiont-bearing mixed-layer dweller genus Morozovella, coupled with an increase in abundance and diversity of the genus Acarinina at the beginning of the EECO in the tropical Pacific Ocean (Sites 1209-1210), similar to the previously documented record from the Atlantic Ocean. A second significant variation is the change in coiling direction of morozovellids that moved from dominantly dextral to sinistral close to the K/X event (52.85 Ma), in contrast to Acarinina which does not show any preferential coiling direction throughout. In addition, the deep-dweller genus Chiloguembelina virtually disappeared at the K/X event. Even though a link between these PF changes and EECO climatic change appears evident, the driving causes are still unknown. With the aim of evaluating whether a possible temperature increase may have impacted the observed PF changes, we performed Mg/Ca analysis to derive paleotemperatures from diverse species of Morozovella and Acarinina, and on the thermocline-dweller Subbotina from tropical Pacific sites 1209-1210 using laser ablation (LA)-ICP-MS. Our B/Ca and Sr/Ca results in all the examined samples/species, along with the observed low PF test-fragmentation allow us to exclude a significant influence of contamination or diagenesis on the reconstructed temperatures. As uncertainties in the interpretation of Mg/Ca data remain when working with extinct species, the temperatures were evaluated with both a ‘Trilobatus sacculifer-like’ calibration (no pH correction) and with a ‘Globigerinoides ruber-like’ calibration (pH-correction).In both cases a mixed-layer mean temperature increase of at least 1°C is recorded, with much warmer absolute temperature resulting from the former calibration approach.We hypothesize that the temperature rise may have impacted the morozovellid symbiotic relationship that, in turn, can represent a reason for their decline in abundance, given the many examples of the evolutionary benefits of symbiosis in modern oligotrophic mixed-layer habitats. Although there may have been several contributing factors resulting in photosymbiont bleaching at this time, increased temperature is considered a primary factor of bleaching in modern tropical larger benthic foraminifera. Our assumption appears supported by the lower δ¹³C values exhibited by the surviving sinistral morozovellids (Luciani et al. 2021 GloPlaCha) while the new dominant genus, Acarinina that does not record lower δ¹³C values displays greater ecological adaptability. Our evidence appears in line with the hypothesis of Davis et al. (2022 PlosOne) that acarininids changed their symbiotic associations in response to the extreme warming of the PETM (~56 Ma) (but not the subsequent smaller hyperthermals), resulting in long term evolutionary success.