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). However, from the mid-Eocene onwards, east-west North Atlantic temperature gradients exhibit different trends. This is attributed to inception of Northern Component Water during the early-middle Eocene transition and incursion of warmer waters into the eastern North Atlantic, but additional data sets are required to test this further. We also demonstrate that the onset of long-term Eocene cooling in the western North Atlantic (∼49–48 Ma) occurs synchronously in other ocean basins (e.g., N. Atlantic vs. SW Pacific) and across different latitudinal bands, implying that CO₂ was likely responsible for the onset of long-term Eocene cooling.