| Summary: | The Eocene (56-34 million years ago) is characterized by declining sea surface temperatures (SSTs) in the low latitudes (similar to 4 degrees C) and high southern latitudes (similar to 8-11 degrees 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, delta O-18, TEX86) SST record from Bass River in the western North Atlantic. Our compiled multi-proxy SST record confirms a net decline in SSTs (similar to 4 degrees C) between the early Eocene Climatic Optimum (53.3-49.1 Ma) and mid-Eocene (similar to 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. 2202777
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