Climate system asymmetries drive eccentricity pacing of hydroclimate during the early Eocene greenhouse

The early Eocene Climatic Optimum (EECO) represents the peak of Earth's last sustained greenhouse climate interval. To investigate hydroclimate variability in western North America during the EECO, we developed an orbitally resolved leaf wax delta H-2 record from one of the most well-dated terrestrial paleoclimate archives, the Green River Formation. Our delta H-2(wax) results show similar to 60 parts per thousand variation and evidence for eccentricity and precession forcing. iCESM simulations indicate that changes in the Earth's orbit drive large seasonal variations in precipitation and delta H-2 of precipitation at our study site, primarily during the summer season. Our findings suggest that the astronomical response in delta H-2(wax) is attributable to an asymmetrical climate response to the seasonal cycle, a "clipping" of precession forcing, and asymmetric carbon cycle dynamics, which further enhance the influence of eccentricity modulation on the hydrological cycle during the EECO. More broadly, our study provides an explanation for how and why eccentricity emerges as a dominant frequency in climate records from ice-free greenhouse worlds.