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

The Asian monsoon affects the natural patterns of climate in China and its origin and evolution have been a matter of much debate in palaeoclimatology. Results from recent studies indicate that the Asian monsoon had reached the subtropical zone by at least c. 41 Ma and expanded to the central Tibetan Plateau during the Late Oligocene, although more geological evidence is required to confirm its spatial and temporal evolution. The well-developed Late Oligocene palaeosol in the Lunpola Basin, central Tibetan Plateau is an ideal material for addressing the issue. Observations of various climatic indicators suggest that these palaeosols were forest cinnamon soils, as shown by the compound Bt and Bk horizons, abundant clay coatings and carbonate nodules, and the diagnostic chemical composition of clays in the Bt horizon. A high chemical weather index, the Rb/Sr ratios and high contents of illite/smectite mixed-layer minerals show that these palaeosols experienced intense weathering and leaching during pedogenesis. The mean annual temperature and mean annual precipitation during the developmental period of the palaeosols were 10.4-14.8 degrees C and 615-1128 mm, respectively, as estimated by empirical formulas, which are comparable with those of cinnamon soils in current monsoon climates. This study provides important independent evidence from palaeosols for studies of the evolution of the Asian monsoon.