Late Oligocene paleosols in the Haltang Basin of northeastern Tibetan Plateau and their implications for the early extent of the Asian monsoon

Abstract

The Asian monsoon is thought to have established its present-day pattern and extent by ∼26 Ma. In this paper, we report a succession of Late Oligocene (∼23.94 Ma) luvic calcisols in the Haltang Basin, northeastern Tibetan Plateau, which allow us to evaluate the early history of the monsoon. The paleosols contain calcareous nodules and iron‑manganese cutans, and magnetic mineral content fluctuates in different horizons. These features indicate that paleosols formed in a humid climate, characterized by seasonal precipitation and eluviation. Analysis of the goethite/hematite ratio, burial depth and thickness of the Bk horizons, yield a paleoprecipitation estimate of 398–748 mm with a seasonal fluctuation of 88 mm. The mean annual temperature is estimated as ∼12 °C based on carbonate clumped isotope data from a carbonate nodule. This probably indicates a relatively lower elevation at that time. These palaeotemperature estimates, in combination with paleosol data and other proxies in the northeastern Tibetan Plateau, indicate that the Asian monsoon had onset in the Haltang Basin at ∼23.94 Ma. This suggests that the Asian monsoon was more widespread during the late Oligocene than it is today.