Morozovella and Acarinina contrasting response to the EECO (Early Eocene Climatic Optimum; 53-49 Ma) in Tropical Pacific Site Shatsky Rise 1209-1210

The EECO represents the long-term highest global average temperature and CO2 levels of the Cenozoic. This past warming event provides the opportunity to explore the interplay between extreme warmth and marine ecosystems, of which planktic foraminifera are a crucial component. We present here relative abundance, test-size analysis, foraminiferal mass accumulation rate (FMAR) and δ13C data from this group at the tropical Pacific sites 1209-1210. Our study records a marked decline in Morozovella abundance and a parallel increase in Acarinina abundance at the EECO onset, as previously documented from the Atlantic Ocean. Given the dominance and large size of Morozovella in the Late Paleocene-earliest Eocene tropical assemblages, we postulated that this change would have reduced foraminiferal production and the assemblage test-size. In contrast, we record a relatively stable FMAR and a slight increase in test-size within assemblages. In addition, our analysis shows that the acarininids increase in abundance is coupled with a test size decrease and lower δ13C values with respect to Morozovella, which unexpectedly records a test-size increase. This suggests a deepening in the mixed layer of Acarinina with a reduction in symbiosis activity as a possible strategy to survive the warming conditions. By reducing body size, acarininids may have increased their chances of survival and reproductive success in the face of environmental challenges, contrary to Morozovella. Despite individual taxa responses, the overall stability of the foraminiferal assemblage's carbonate production implies complex ecological interactions and compensatory mechanisms at play.