Orbital eccentricity pacing of hydroclimate variability in Qinghai-Tibet Plateau during the middle–late Eocene

The middle–late Eocene climate evolution and its orbital forcing mechanisms remain poorly constrained for Qinghai-Tibet Plateau. We present a radiometrically anchored astrochronological framework and orbital-scale hydroclimate reconstruction from the Niubao Formation in central Tibet, spanning the interval between the Middle Eocene Climatic Optimum (MECO) and the Eocene–Oligocene Transition (EOT). A key tuff bed yields a zircon U–Pb age of 36.50±0.21 Ma, providing an independent tie point for stratigraphic calibration and sedimentation rate assessment. High-resolution elemental geochemistry and carbon isotope stratigraphy were analyzed using MTM spectral methods and cyclostratigraphic approaches. Proxy ratios sensitive to aridity/humidity (Sr/Cu, Al/Mg, K/Al, Sr/Ba), weathering and hydrology (Rb/Sr, Ti/K), and redox conditions (Fe/Mn) display persistent orbital pacing, with dominant periodicities at 405 kyr and 100 kyr, consistent with long and short eccentricity forcing. Across the studied interval we observe an overall trend toward more arid conditions, while eccentricity-band variability modulates hydroclimate and redox states. Carbon isotope variations facilitate correlation to coeval global records, linking central Tibetan environmental change to global Eocene climate transitions.