Independent constraints on deep-sea temperatures across two early Eocene hyperthermals from clumped isotope thermometry

The early Eocene hothouse experienced the occurrence of transient global warming events, so-called hyperthermals. The deep ocean constitutes a stable and vast heat reservoir, and hence arguably provides a good estimate of the global mean climate state. However, available proxy deep-sea temperature reconstructions rely on uncertain assumptions of non-thermal influences associated with seawater chemistry and species-specific effects. Here, we apply carbonate clumped isotope thermometry on benthic foraminifera to evaluate South Atlantic deep-sea temperatures across two early Eocene hyperthermal events. Our reconstructions indicate deep-sea temperatures of 13.5±1.8 °C (95% CI) for the background state, and average hyperthermal peak temperatures of 16.9±2.2 °C (95% CI). Absolute temperatures are approximately three degrees warmer than estimates from benthic oxygen isotopes. These findings imply a necessary reassessment of the isotope composition of ocean water during the Eocene, of deep-sea pH in hothouse climates, and of a potential pH effect on benthic foraminiferal oxygen isotope records.