Transient temperature asymmetry between hemispheres in the Palaeogene Atlantic Ocean

During the Late Palaeogene between similar to 40 and 23 million years ago (Ma), Earth transitioned from a warm non-glaciated climate state and developed large dynamic ice sheets on Antarctica. This transition is largely inferred from the deep-sea oxygen isotope record because records from independent temperature proxies are sparse. Here we present a 25-million-year-long alkenone-based record of surface temperature change from the North Atlantic Ocean. Our long temperature record documents peak warmth (similar to 29 degrees C) during the middle Eocene, a slow overall decline to the Eocene/Oligocene transition (EOT, similar to 34 Ma) and high-amplitude variability (between similar to 28 and 24 degrees C) during the Oligo-Miocene. The overall structure of the record is similar to that of the deep-sea record, but a distinct anomaly is also evident. We find no evidence of surface cooling in the North Atlantic directly coinciding with the EOT when Antarctica first became cold enough to sustain large ice sheets and subantarctic waters cooled substantially. Surface ocean cooling during the EOT was therefore strongly asymmetric between hemispheres. This transient thermal decoupling of the North Atlantic Ocean from the southern high latitudes suggests that Antarctic glaciation triggered changes in ocean circulation-driven heat transport and influenced the far-field climate response