Simulation of early Eocene water isotopes using an Earth system model and its implication for past climate reconstruction

Our understanding of past climate conditions largely comes from paleoclimate proxies, such as oxygen isotope ratios (δ¹⁸O_c) in marine fossils. The marine δ¹⁸O_c signal primarily reflects a mixture of seawater temperature and oxygen isotopic composition of seawater (δ¹⁸O_w) at the time of calcification. Knowledge of δ¹⁸O_w is critical for the interpretation of marine δ¹⁸O_c records but remains poor for past hothouse climates. Here, we conduct water isotope-enabled simulations of the early Eocene using CO₂ levels of 1×, 3×, 6×, and 9× the preindustrial value. We calculate model δ¹⁸O_c using simulated δ¹⁸O_w and ocean temperature, and make direct comparison with proxy records. Model δ¹⁸O_c matches the proxy values well for the early Eocene and Paleocene–Eocene Thermal Maximum with root-mean-squared errors approaching the standard error in individual records. Eocene δ¹⁸O_w in the model exhibits strong variation depending on states of the hydrological cycle and ocean circulation. Differences in the mean δ¹⁸O_w between regions of net evaporation and precipitation increase monotonically with the magnitude of the net atmospheric moisture transport that connects them; however, this relationship breaks at the regional scale due to ocean circulation changes. In particular, an increase in ocean ventilation brings more ¹⁸O-enriched deep water into the mixed layer, increasing sea-surface δ¹⁸O_w near the ventilation site and in certain remote regions through fast upper ocean currents. δ¹⁸O_w variations and the linkage to both hydrological cycle and ocean circulation bring challenges for an accurate interpretation of marine δ¹⁸O_c records. Our study illustrates the value of using water isotope-enabled simulations and model-data comparison for learning past climate changes.