Eocene climate and hydrology of eastern Asia controlled by orbital forcing and Tibetan Plateau uplift

Abstract

The relationship between the uplift of the Tibetan Plateau and the aridification of eastern Asia during the middle to late Eocene is still controversial, primarily due to the lack of high-resolution chronological frameworks for global isochronous comparisons. Here, we present a new high-resolution astrochronology for the Eocene Suonahu Formation in the Qiangtang Basin on the Tibet Plateau, which has been precisely dated using a secondary ion mass spectrometry U‒Pb age of 46.57 ± 0.30 Ma. This astrochronology establishes a detailed and continuous timescale of 7.61 ± 0.5 Myr duration, spanning the period from 42.08 ± 0.5 Ma to 50.36 ± 0.5 Ma. Sedimentary noise modeling revealed cycles of 1.2 Myr that are anti-phase to the 1.2 Myr obliquity cycle of the obliquity/total power (O/T) curve. This suggests that variations in lake level are modulated by these 1.2 Myr obliquity cycles. The change in lake level is inversely related to global sea level trends, reinforcing the hypothesis that groundwater dynamics are a significant driver of lake level fluctuations. The data further highlight the importance of orbital forcing in controlling hydrological cycling in eastern Asia. The new astrochronology further confirms the onset of aridification of in eastern Asia around 45.5 Ma, which is evidenced in the Suonahu Formation by the cyclic deposition of gypsum and an increase in chlorite content. Cyclostratigraphic analysis indicated that gypsum-siltstone/mudstone alternations were paced by 100 kyr short-eccentricity cycles. Thus, the late Eocene aridification of eastern Asia was driven both by the uplift of the Tibetan Plateau, which restricted northwards moist transport, and by orbital forcing and global cooling, which affected evaporation and precipitation patterns and intensities. This study highlights the complex interplay between tectonic processes and orbital forcing as key drivers of hydrological cycling and climate during the Eocene greenhouse-to-icehouse transition.