Intensification of aridity in the eastern margin of the Tibetan Plateau since 300 ka BP inferred from loess-soil sequences, western Sichuan Province, southwest China

The climate in the eastern margin of the Tibetan Plateau (TP) is mainly controlled by the Indian Ocean summer monsoon and the TP monsoon. Widely distributed aeolian deposits in this region are one of the best terrestrial materials for studies on the evolutionary history of the Indian summer monsoon and the climatic response to uplift of the TP. In this study, two loess-palaeosol sequences from the western Sichuan Province in southwest China were studied in detail using geochemical and sedimentological methods. Carbon isotope compositions of organic matter, major element geochemistry and grain-size records of these two loess-palaeosol sequences indicate that the extent of aridity in the eastern margin of the TP increased rapidly between 300 and 150 ka BP and stabilized at a relatively higher level after 150 ka BP. Marine records from the Indian Ocean indicate that the Indian summer monsoon did not experience obvious weakening since 300 ka BP. The deep-sea δ18O record, which reflects changes in global ice volume, also lacks any obvious changes during this period. Therefore, we interpret this palaeoclimate event to reflect the decreased influence of the Indian Ocean summer monsoon on this region, which is related to the uplift of the TP since 300 ka BP. This process of increasing aridity is a local palaeoclimate event, which is not recorded in the loess-palaeosol sequences in the Chinese Loess Plateau in northern China where the climate is mainly controlled by the East Asian monsoon. This indicates that the climate response to the uplift of the TP differs in different monsoon regions.