Pronounced warming negatively impacts ecosystem resilience in modern oceans. To offer a long-term geological perspective of the calcareous plankton response to global warming, we present an integrated record, from two Tethyan sections, Madeago and Terche (north-eastern Italy), spanning the Eocene Thermal Maximum 2 hyperthermal (ETM2, ~54 Ma). We quantify abundances (%) of planktic foraminiferal and calcareous nannofossil assemblages and changes in their body size (% for calcareous nannofossils and 95 percentile µm for planktic foraminifera). To characterize environmental background conditions, we analyse multiple geochemical and dissolution proxies. Geochemical proxies include: bulk Carbon and Oxygen stable isotope (VPDB ‰), CaCO3 content (%), trace-element (TE) concentration (ppm and ppb), and indices determined by the Rock-Eval pyrolysis such as total organic carbon (TOC, wt.%) content, mineral carbon (MinC, wt.%) content, hydrogen index (HI, mg HC/g TOC, HC=hydrocarbons), oxygen index (OI, mg CO2/g TOC), and Tmax (°C). Dissolution proxies are the same adopted by D'Onofrio et al. (2016). We place our foraminiferal size data into a wider context by comparing it to size data (95 percentile µm) from the southern Atlantic Ocean ODP (Ocean Drilling Program) Site 1263 and tropical Pacific ODP Site 1209. Our study reveals pronounced changes in assemblage composition and a striking dwarfing of planktic foraminiferal tests up to 40% during the event. The environmental pressure impacted both surface and deeper dwellers. We find the smallest sizes in close temporal association with peaks in Hg/Th-Hg/Rb recorded during the ETM2. The foraminiferal size reduction lasted several thousand years. Calcareous nannofossils as well display increased abundance of small placoliths. Our foraminiferal size data from Site 1263 (Atlantic Ocean) and Site 1209 (Pacific Ocean) and literature records highlights that the pronounced dwarfism is restricted to the Tethyan area.

Further funding: FAR (Fondo Ateneo Ricerca, Ferrara University) 2021 R.D.O. fundings (ex 60%) of the University of Padova