Planktonic foraminiferal productivity and test-size changes at the early Eocene climatic optimum (EECO, ~53–49 Ma; Pacific Ocean, sites 1209–1210)

The early Eocene climatic optimum (EECO; c.53–49 Ma) records the highest temperatures and CO2 levels of the entire Caenozoic and is marked by several transient global warming events, offering an opportunity to investigate the impact of global warming on the marine carbonate production. Pacific ODP sites 1209–1210 (Shatsky Rise) provide an excellent age model and stable isotope records thus allowing to link data acquired to climate and carbon cycle. We compare the Fragmentation index (as dissolution proxy), the weight percent coarse fraction (CF; index of foraminiferal production and preservation) and foraminiferal mass accumulation rate (FMAR, foraminiferal production) with changes in the composition of planktonic foraminiferal assemblages and their test-size, a significant character yet poorly investigated linked to physiological performance and species composition. At the beginning of the EECO (J Event, 53.28 Ma) we find a significant abundance drop of the symbiont-bearing genus Morozovella, along with an increase of Acarinina, confirming observations from Atlantic Ocean indicating that this is a global event. We postulated that this change would have impacted foraminiferal production and potentially also the assemblages test-size range. In contrast, we record a long-term slight increase in test-size within assemblages, controlled by the dominant Acarinina. The lower CF and FMAR from the ETM2 (54.05 Ma) to J Events appear related to the stronger dissolution during the hyperthermals. After the J Event up to the top section (c.49.8 Ma), the slightly lower CF values in contrast to a relatively stable FMAR may be linked to fragmentation and enhanced of calcareous nannofossil productivity reducing foraminiferal relative contribution to the sediment. Our new data suggest that during the EECO the foraminiferal assemblages were markedly altered even though the whole communities were able to replace species going to extinction and to balance the assemblage test-sizes.