Impacts of the Early Eocene Climatic Optimum (EECO, ∼53-49 Ma) on Planktic Foraminiferal Resilience

The Early Eocene Climatic Optimum (EECO) is an interval of prolonged warmth that occurred similar to 53 to 49 million years ago. Planktic foraminifera are important (alongside coccolithophores) for understanding the carbon cycle and determining export production in the ocean. To understand how foraminifera can be impacted by extreme heat, we analyzed samples from the Pacific Ocean through measuring changes in (a) the relative abundance, that is, the count of different taxa in a sample, and (b) body (shell) size of planktic foraminifera. At the start of the EECO, the abundance of the genera Morozovella and Chiloguembelina decreased. Despite this decline, the number of foraminifera being buried and the size of the largest shells in a sample does not change. We attribute this to the increased abundance of the genus Acarinina. In general, the accumulation of foraminifera remains stable while the relative abundance of foraminifera to coccolithophores decreases. Together, this implies that coccolithophores are increasing in abundance, and/or mass. During the EECO, one group of foraminifera was able to counterbalance the decrease in abundance of other genera. This highlights the resilience of open-ocean carbonate production and the base of the marine food web. The Early Eocene Climatic Optimum (EECO; similar to 53 and 49 Ma) records the warmest long-term global average temperature and highest CO2 levels of the Cenozoic. Multiple transient global warming events occur within the EECO, offering an opportunity to investigate the impact of extreme heat on planktic foraminifera. Pacific Ocean Drilling Program (ODP) Sites 1209-1210 (Shatsky Rise) provide an excellent age model and stable isotope records to link biotic data with the carbon cycle across the interval here analyzed (55.6 and 49.93 Ma). We combine carbonate production proxies with changes in planktic foraminiferal assemblages and test-size. Our data show that during the EECO planktic foraminiferal assemblages were permanently modified, besides transient changes. At the EECO onset, abundance of the genera Morozovella and Chiloguembelina decreased at 53.28 and 52.85 Ma, respectively, confirming published Atlantic Ocean data and thus the global decline of these genera. Given the dominance and large size of Morozovella in early Eocene tropical assemblages, we postulated that this change would have reduced foraminiferal production and the assemblage test-size. In contrast, we record a slight increase in test-size within assemblages, controlled by the now dominant genus, Acarinina. The decrease in coarse fraction weight, partially controlled by dissolution, during times of stable foraminferal mass accumulation rate indicates enhanced calcareous nannofossil productivity reducing the foraminiferal contribution to the sediment. During the EECO, despite the decrease in abundance of some genera, species replacement within communities highlights the resilience of pelagic carbonate production, emphasizing the critical role of planktic foraminifera in regulating the marine food web and global carbon cycling. At the EECO onset, the abundance of the planktic foraminiferal genera Morozovella and Chiloguembelina decreased and never recovered Species replacement and test-size increase within assemblages sustained production of pelagic carbonate by foraminifera Greater adaptability and flexibility of the genus Acarinina led to its dominance over the more specialized morozovellids