Warm deep ocean temperatures from clumped isotopes suggest high climate sensitivity in early Cenozoic hothouse

The early Cenozoic was characterized by the warmest climates and highest atmospheric CO2 levels of the past 85 Myrs. Reconstructions of deep ocean temperatures based on benthic foraminiferal oxygen isotope records are typically used to infer Earth’s global climate state during this hothouse world. However, this approach requires uncertain assumptions, regarding the seawater isotope composition and pH, and species-specific vital effects. Here we use clumped isotope thermometry, a proxy not complicated by non-thermal influences, to reconstruct early Cenozoic deep ocean temperatures on million-year time scales from the South Atlantic Ocean. We find on average warmer temperatures than previously derived from oxygen isotopes, consistent with clumped isotope reconstructions from the North Atlantic. These results challenge our understanding of seawater isotope composition in an ice-free world and invoke potential pH effects on the benthic oxygen isotopes. Deep ocean temperatures reached maximum values of up to 20 °C during the Early Eocene Climatic Optimum, which is more than 5 °C warmer than previously thought. These findings imply that (past) climate sensitivity to pCO2 forcing was much higher (6 to 8 °C) than currently used in climate models that simulate future warming scenarios of the IPCC (2.5 to 4.0 °C).