Late Oligocene Vegetation Shifts and Paleoelevation Reconstruction in the Tuotuohe Basin, NE Tibet: Evidence from Palynostratigraphy

The Hoh Xil Basin, located along the northern margin of the Tibetan Plateau, preserves key sedimentary archives for deciphering Cenozoic plateau uplift, paleoenvironmental change, and atmospheric circulation. However, the age of its sedimentary successions and associated paleoelevation reconstructions remain highly debated due to limited biostratigraphic constraints. Here we present new palynological records from the Tongtianhe (TTHP) and Wudaoliang (WDLP) sections in the Tuotuohe Basin to constrain depositional age, vegetation evolution, paleoclimate, and paleoelevation during the late Oligocene. Pollen assemblages were analyzed using plant functional types (PFTs), the Coexistence Approach (CA), and a recently developed pollen-based conifer ratio proxy (TP/TPAP). High abundances of Pinaceae pollen (>65% in TTHP and >91% in WDLP Zone I), persistently low Tsuga content, and the absence of Miocene indicator taxa such as Fupingopollenites constrain the studied strata to the interval ~36–23 Ma. Comparative assemblage characteristics indicate that the TTHP section represents an older phase (~36.5–34 Ma), whereas the WDLP section postdates ~34 Ma but predates the Miocene. Pollen assemblages dominated by conifers and xerophytic taxa indicate a cool and predominantly arid regional climate, with a modest increase in humidity after ~34 Ma. Pollen-based paleoaltimetry suggests a moderate paleoelevation of approximately 2.0–2.4 km above sea level during the late Oligocene, implying that significant surface uplift of the northern Tibetan Plateau predated the Miocene. These results provide robust biostratigraphic constraints for the Yaxicuo and Wudaoliang formations and highlight the value of palynological data for resolving long-standing debates on Tibetan Plateau uplift and climate–tectonic interactions.