<p>Reconstructing deep ocean temperature is important to infer deep water mass structure and hence ocean circulation patterns in the past. The late Paleocene-early Eocene experienced the warmest climates of the Cenozoic, with highly elevated CO<sub>2</sub> levels and no ice sheets on the continents [1,2]. Benthic foraminiferal δ<sup>18</sup>O records suggest relatively stable deep ocean conditions on long time scales (>100 kyr) in this hothouse [2–4]. However, interpretations from benthic δ<sup>18</sup>O records are complicated by influences of factors other than temperature, such as the isotope composition of the seawater (δ<sup>18</sup>O<sub>sw</sub>), pH, and species-specific physiological effects [5,6]. Carbonate clumped isotope thermometry (Δ<sub>47</sub>) has the major advantage that it is independent of the isotope composition of the fluid source, and is not measurably affected by other non-thermal influences [7–10]. Early Cenozoic clumped isotope reconstructions from the North Atlantic have revealed surprisingly large deep-sea temperature swings under hothouse conditions [11]. Extreme warming is recorded at the onset of the Early Eocene Climatic Optimum (EECO) [11]. To explore the spatial extent of these deep-sea temperature changes, we reconstructed early Eocene Δ<sub>47</sub>-based deep-sea temperatures from the South Atlantic Ocean, a location that is considered to capture a global signal [2–4]. We find similar deep-sea temperatures as those from the North Atlantic. Cooler temperatures of ~12 °C stand out in the interval (54–52 Ma) before the peak warmth of the EECO (52–50 Ma) of ~20 °C. This result overthrows the classic view of a gradual early Eocene warming trend based on benthic δ<sup>18</sup>O records, at least for the deep Atlantic Ocean. Our findings raise new questions on the regions of deep water formation, changes in deep ocean circulation, and the driving mechanisms in the early Cenozoic hothouse.<br><br><strong>References</strong><br>[1] Anagnostou, E. <em>et al</em>. (2016). <em>Nature</em>, <em>533</em>(7603), 380-384.<br>[2] Zachos, J. <em>et al</em>. (2001). <em>Science</em>, <em>292</em>(5517), 686-693.<br>[3] Lauretano, V. <em>et al</em>. (2018). <em>Paleoceanography and Paleoclimatology</em>, <em>33</em>(10), 1050-1065.<br>[4] Westerhold, T. <em>et al</em>. (2020). <em>Science</em>, <em>369</em>(6509), 1383-1387.<br>[5] Ravelo, A. C., & Hillaire-Marcel, C. (2007). <em>Developments in marine geology</em>, <em>1</em>, 735-764.<br>[6] Pearson, P. N. (2012). <em>The Paleontological Society Papers</em>, <em>18</em>, 1-38.<br>[7] Ghosh, P. <em>et al</em>. (2006). <em>Geochimica et Cosmochimica Acta</em>, <em>70</em>(6), 1439-1456.<br>[8] Tripati, A. K. <em>et al</em>. (2015). <em>Geochimica et Cosmochimica Acta</em>, <em>166</em>, 344-371.<br>[9] Guo, W. (2020). <em>Geochimica et Cosmochimica Acta</em>, <em>268</em>, 230-257.<br>[10] Meinicke, N. <em>et al</em>. (2020). <em>Geochimica et Cosmochimica Acta</em>, <em>270</em>, 160-183.<br>[11]<strong> </strong>Meckler, A. N.<em> et al</em>. (in revision).</p>