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

• Published in: Geophysical research letters 2023-12, Vol.50 (24), p.n/a
• Main Authors: Inglis, Gordon N., Bhatia, Rehemat, Evans, David, Zhu, Jiang, Müller, Wolfgang, Mattey, David, Thornalley, David J. R., Stockey, Richard G., Wade, Bridget S.
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Carbon dioxide control; cenozoic; Climate; Cooling; Cooling water; Decoupling; Deep water; Eocene; Eocene climates; foraminifera; Fossils; Greenhouses; Hemispheres; Latitude; Liquid cooling; Marine chemistry; Mathematical models; multi‐proxy; North Atlantic; Oceans; Sea surface; Sea surface temperature; Sediments; Surface temperature; Surface water; Temperature; Temperature changes; Temperature gradients; Water masses
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL105448

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. Plain Language Summary Over the past 541 million years, the Earth has oscillated between warm (greenhouse) and cold (icehouse) climates. The most recent transition between a greenhouse and icehouse climate state occurred during the Eocene (56–34 million years ago). This transition shows a gradual cooling, previously suggested to be driven by a decline in atmospheric carbon dioxide (CO2). However, we know little about this transition in the North Atlantic Ocean. Previous studies show limited cooling of surface waters in this region. This suggests that changes in North Atlantic temperatures are not driven by CO2. To understand how sea surface temperature changes in the western North Atlantic, we analyzed the chemistry of microscopic marine fossils in sediments. Our results show a 4°C decline in temperature from the early (∼53 Ma) to the middle Eocene (∼42 Ma). This matches computer simulations of Eocene climate and confirms CO2 was responsible for the transition. The lack of cooling observed in previous work is probably due to the development of an ancient water mass known as Northern Component Water (observed today as North Atlantic Deep Water) and changes in how the Eocene ocean transported heat. Key Points Long‐term (∼4°C) decline in North Atlantic sea surface temperatures (SSTs) between the early (∼53–49 Ma) and middle (∼44–41 Ma) Eocene This indicates that CO2 was likely responsible for the onset of long‐term Eocene cool