Abstract

Our understanding of past climate conditions largely comes from paleoclimate proxies, such as oxygen isotope ratios (delta O-18(c)) in marine fossils. The marine delta O-18(c) signal primarily reflects a mixture of seawater temperature and oxygen isotopic composition of seawater (delta O-18(w)) a... Show moreOur understanding of past climate conditions largely comes from paleoclimate proxies, such as oxygen isotope ratios (delta O-18(c)) in marine fossils. The marine delta O-18(c) signal primarily reflects a mixture of seawater temperature and oxygen isotopic composition of seawater (delta O-18(w)) at the time of calcification. Knowledge of delta O-18(w) is critical for the interpretation of marine delta O-18(c) records but remains poor for past hothouse climates. Here, we conduct water isotope-enabled simulations of the early Eocene using CO2 levels of 1x, 3x, 6x, and 9x the preindustrial value. We calculate model delta O-18(c) using simulated delta O-18(w) and ocean temperature, and make direct comparison with proxy records. Model delta O-18(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 delta O-18(w) in the model exhibits strong variation depending on states of the hydrological cycle and ocean circulation. Differences in the mean delta O-18(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 O-18-enriched deep water into the mixed layer, increasing sea-surface delta O-18(w) near the ventilation site and in certain remote regions through fast upper ocean currents. delta O-18(w) variations and the linkage to both hydrological cycle and ocean circulation bring challenges for an accurate interpretation of marine delta O-18(c) records. Our study illustrates the value of using water isotope-enabled simulations and model-data comparison for learning past climate changes. (C) 2020 Elsevier B.V. All rights reserved. Show less