Late Oligocene monsoonal climate in the Lunpola Basin, central Tibetan Plateau: evidence from paleosol records

The Asian monsoon affects the natural environment pattern in China, and its origin and evolution have been a debated issue in paleoclimatology. Recent studies indicate that the Asian monsoon reached the subtropical zone at least ~ 41 Ma and expanded to the central Tibetan Plateau during the Late Oligocene, but more geological evidence is still required to confirm its spatial evolution. The well-developed Late Oligocene paleosols in the Lunpola Basin, central TP, provide excellent material to address the above issue. In this paper, various climatic proxy indicators suggest that the late Oligocene LPL paleosols were forest cinnamon soils, as shown by the significant compound Bt and Bk horizons, abundant clay coating and carbonate nodules, and diagnostic clay chemical composition in Bt horizons. High CIA value, Rb/Sr ratio, and high content of illite/smectite mixed layer mineral show that these paleosols experienced intense weathering and leaching pedogenesis. Furthermore, the mean annual temperature and mean annual precipitation of the Late Oligocene LPL Basin were 10.4~14.8 ℃ and 615~1128 mm estimated by the empirical formulas, respectively, which are comparable to the monsoonal climate parameter of Chinese modern cinnamon soils. So the development of these paleosols in the LPL Basin indicate that the Asian monsoon has reached the central TP at least during the Late Oligocene, providing important independent evidence for the study of the evolution of the Asian monsoon.