Early Eocene Climatic Optimum Pacific deep ocean temperatures from clumped isotope thermometry

Past greenhouse climates provide crucial insights into how the climate system operates under conditions with high atmospheric carbon dioxide concentrations and smaller or absent continental ice sheets, and thus inform projections of future climate. When studying past greenhouse climate states, deep ocean temperatures are often used to estimate global mean temperature (Westerhold et al., 2020), which is needed to determine climate sensitivity and assess the performance of climate models. Additionally, deep ocean temperatures provide insights into past ocean circulation patterns. The Early Eocene Climatic Optimum (EECO; 53.3 to 49.1 Ma) was characterized by extreme global warmth and elevated atmospheric CO2 concentrations (Hollis et al., 2019a). Currently, our understanding of deep ocean temperatures during the EECO relies primarily on benthic foraminiferal stable oxygen isotopes and Mg/Ca records. These proxies, however, are influenced by factors other than temperature, making robust deep-time temperature reconstructions challenging. Carbonate clumped isotope thermometry, on the other hand, is largely independent of past seawater chemistry and can therefore provide essential new constraints. Recent clumped isotope-based deep ocean temperature reconstructions from the Atlantic Ocean are substantially warmer during the peak EECO than previous estimates based on stable oxygen isotopes and Mg/Ca records (Meckler et al., 2022). However, it remains to be tested whether these warmer temperatures are a regional signal restricted to the Atlantic Ocean or found globally in the deep ocean. Here we present a record of deep ocean temperatures using clumped isotopes in benthic foraminifera from the Pacific Ocean (ODP Site 1209, ~2300 m paleo-water depth). Our new record spans the interval between 52 Ma to 50.3 Ma, covering the peak EECO and the major shift in benthic foraminiferal stable carbon isotopes around 51 Ma which is observed globally in the deep ocean. Warmer than expected Atlantic as well as Pacific deep ocean temperatures could indicate that EECO global mean temperature was warmer than previously assumed, which would have implications for existing estimates of climate sensitivity. References Hollis, C. J., Dunkley Jones, T., Anagnostou, E., Bijl, P. K., Cramwinckel, M. J., Cui, Y., ... Lunt, D. J. (2019a). The DeepMIP contribution to PMIP4: Methodologies for selection, compilation and analysis of latest Paleocene and early Eocene climate proxy data, incorporating version 0.1 of the DeepMIP database.Geoscientific Model Development,12(7), 3149-3206. Meckler, A. N., Sexton, P. F., Piasecki, A. M., Leutert, T. J., Marquardt, J., Ziegler, M., ... Bernasconi, S. M. (2022). Cenozoic evolution of deep ocean temperature from clumped isotope thermometry.Science,377(6601), 86-90. Westerhold, T., Marwan, N., Drury, A. J., Liebrand, D., Agnini, C., Anagnostou, E., ... Zachos, J. C. (2020). An astronomically dated record of Earths climate and its predictability over the last 66 million years.Science,369(6509), 1383-1387.