Eccentricity Forcing of the Hydrological Cycle in East Asia During the Early Eocene Climatic Optimum (EECO)

The Early Eocene Climatic Optimum (EECO) may be a potentially useful analog for future global warming under high CO2 concentrations. However, a paucity of orbital-scale terrestrial records limits our understanding of how the hydrological cycle responded during this protracted (similar to 4 Myr) interval of global warmth. In this study, we combine zircon U-Pb dating and cyclostratigraphy to establish a high-resolution astronomical timescale spanning the EECO (similar to 52.9 Ma to similar to 49.9 Ma) through a >1 km fluviolacustrine succession from the Gonjo Basin, Southeast Tibet. Our results suggest that hydroclimate variability in the region during this interval was strongly controlled by eccentricity forcing (similar to 405 Kyr, similar to 135-100 Kyr, and possibly similar to 200 Kyr cycles). The dominance of eccentricity forcing in our record is consistent with coeval marine records, and indicates that modulation of low-latitude summer insolation through nonlinear interactions with the global carbon cycle likely controlled hydroclimate and paleolake level in the Gonjo basin during the EECO. Our study offers new perspectives for the forcing mechanisms of terrestrial hydroclimate changes of East Asia in response to subtle changes in insolation during the EECO. Plain Language Summary High resolution and relatively long-duration terrestrial paleoenvironmental records across an interval of extreme warmth similar to 50 million years ago (called the EECO) are rare, limiting our understanding of how the hydrological cycle operated during this time interval. In this study, we present high-resolution geochemical data through a succession of continental sedimentary rocks from the Gonjo Basin (Southeast Tibet) spanning the EECO. Our data suggest that EECO hydroclimate was dominated by climate forcing controlled by variations in Earth's eccentricity, emphasizing the importance of eccentricity modulation of low-latitude summer insolation through nonlinear interactions with the global carbon cycle. This finding is consistent with coeval marine records. Our study provides new insights into the forcing mechanisms of terrestrial paleoclimate change in East Asia under warm climate conditions and high CO2.