South Atlantic ∆47-based deep sea temperatures reveal underestimation of Early Cenozoic warming and lack of understanding in application of δ18O-temperature proxy

Abstract

Climate evolution during the late Paleocene (60 - 56 Ma) might be the best analogue to near-future climate change. At over a 1,000 ppm atmospheric CO2 levels were similar to what we expect to reach by the end of the century under continued emission and kept rising until the reversal of CO2 and temperature at the enigmatic Early Eocene Climatic Optimum (EECO, 50 - 52 Ma). Applying clumped isotope thermometry (∆47) to deep sea sediments in the South Atlantic this study finds robust deep sea temperatures that are 4 to 8 °C higher than previously estimated. These findings put our understanding of the commonly used δ18Opaleothermometer to test. Furthermore, reconstructed δ18OSW values cast doubt on the assumption that the Cenozoic earth was free of ice until large scale glaciation started at 34 Ma. Elevated South Atlantic deep sea temperatures are corroborated by other deep sea clumped isotope studies that consistently report higher temperatures than previous estimates throughout the entire Cenozoic and across different ocean basins. Because deep sea temperatures are linearly related with global mean surface temperatures this increased temperature puts new estimates of Earth System Sensitivity at the high end (6 to 8 °C temperature increase per doubling of CO2) of previous projections.