The role of paleogeography and CO2 on Eocene deep-sea temperatures: A model-proxy comparison study

We present new analysis of climate model simulations for the Eocene (~56 – 34 Ma) and investigate the relative role of atmospheric pCO₂ and changes in paleogeography on ocean circulation and deep-sea temperatures. The Early Eocene experienced warm greenhouse conditions, followed by cooling towards the Late Eocene, leading to the formation of land-based ice sheets near the Eocene–Oligocene Transition. The cooling was largely controlled by decreasing atmospheric pCO₂ but was also likely influenced by changes in ocean circulation caused by paleogeographic changes, including the opening and closing of oceanic gateways. Changes in ocean circulation influence the distribution of heat in the surface ocean but also the storage of heat in the deep ocean and are crucial to account for in order to reproduce the Eocene climatology. Reconstructed deep-sea temperatures can thereby provide crucial benchmark constraints on ocean circulation simulated by climate models.
In this study, we analyze a series of simulations using the Norwegian Earth System Model (NorESM-F), run with different paleogeographies, pCO₂, and realistic oceanic gateway configurations. Our results show that changes in deep sea temperatures caused by CO₂ perturbations are sensitive to oceanic gateway configurations and corresponding ocean circulation patterns. Specifically, reducing pCO₂ in simulations where the paleogeography allow for an active AMOC yields less changes in mean deep-sea temperature than simulations without AMOC, which show significant mean deep-sea cooling. This is related to changes in ventilation and deep-water formation. The modelled changes vary on regional and basin scale, and we compare the model simulations to new clumped isotope temperature reconstructions from a variety of drill sites in the global ocean with the aim to understand the mechanisms causing the observed deep sea temperature changes.