Orbital forcing of rainfall belt and vegetation evolution in the Middle Jurassic on the northeastern Tibetan Plateau

Terrestrial vegetation dynamics offer a sensitive record of climate variability. However, how external orbital dynamics modulate internal climate processes to drive vegetation evolution remains unclear. Focusing on two deep boreholes in the northern Qaidam Basin at the northeastern margin of the Tibetan Plateau, we integrate palynology, multivariate statistics, and cyclostratigraphy to link Middle Jurassic vegetation evolution to orbital forcing. Palynological assemblages indicate fern-gymnosperm dominance, followed in the late Middle Jurassic by a sharp decline in ferns and a marked increase in Classopollis, signaling a shift from humid to more arid conditions. High-resolution natural gamma spectra show a pronounced decrease in obliquity/total power from the early to the late Middle Jurassic. Together with published belemnite δ18O records and climate simulations, these observations support a mechanism whereby reduced obliquity weakens the Hadley circulation, displaces rainfall belts, and thus restructures terrestrial communities. Our synthesis identifies obliquity as the primary driver of Middle Jurassic climate dynamics and vegetation change.