CO2- and orbitally- driven oxygen isotope variability in the Early Eocene
Abstract. Paleoclimate reconstructions of the Early Eocene provide important data constraints on the climate and hydrologic cycle under extreme warm conditions. Available terrestrial water isotope records have been primarily interpreted to signal an enhanced hydrologic cycle in the Early Eocene associated with large-scale warming induced by high atmospheric CO2. However, orbital-scale variations in these isotope records have been difficult to quantify and largely overlooked, even though orbitally driven changes in solar irradiance can impact temperature and the hydrologic cycle. In this study, we fill this gap using water isotope-climate simulations to investigate the orbital sensitivity of Earth’s hydrologic cycle under different CO2 background states. We analyze the relative difference between climatic changes resulting from CO2 and orbital changes and find that the seasonal climate responses to orbital changes are larger than CO2-driven changes in several regions. Using terrestrial δ18O and δD records from the Paleocene-Eocene Thermal Maximum (PETM), we compare our modeled isotopic seasonal range to fossil evidence and find agreement between empirical and simulated isotopic compositions. Our conclusions consider systematic impacts on the proxy records, including the preservation state of the proxy, analytical uncertainty, and the relationship between δ18O or δD and environmental context, as well as illustrate the utility of fully coupled, isotope-enabled climate models when interpreting proxy records in times of extreme warmth.
CESM1.2 simulation data for "CO2 and orbitally driven oxygen isotope variability in the Early Eocene" https://doi.org/10.5281/zenodo.7971738
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