Surface Ocean Cooling in the Eocene North Atlantic Coincides With Declining Atmospheric CO₂

The Eocene (56–34 million years ago) is characterized by declining sea surface temperatures (SSTs) in the low latitudes (∼4°C) and high southern latitudes (∼8–11°C), in accord with decreasing CO₂ estimates. However, in the mid-to-high northern latitudes there is no evidence for surface water cooling, suggesting thermal decoupling between northern and southern hemispheres and additional non-CO₂ controls. To explore this further, we present a multi-proxy (Mg/Ca, δ¹⁸O, TEX₈₆) SST record from Bass River in the western North Atlantic. Our compiled multi-proxy SST record confirms a net decline in SSTs (∼4°C) between the early Eocene Climatic Optimum (53.3–49.1 Ma) and mid-Eocene (∼44–41 Ma), supporting declining atmospheric CO₂ as the primary mechanism of Eocene cooling. However, from the mid-Eocene onwards, east-west North Atlantic temperature gradients exhibit different trends, which we attribute to incursion of warmer waters into the eastern North Atlantic and inception of Northern Component Water across the early-middle Eocene transition.