Mid-late Holocene Asian monsoon evolution indicated by peat deposits in the source area of the Yellow River, northeastern Tibetan Plateau.

Study of climatic evolution in the source area of the Yellow River (SAYR) and its mechanisms is of significance for understanding and predicting the climate and environmental changes and permafrost evolution on the northeastern Tibetan Plateau (TP). This study reconstructed the Asian monsoon and climatic evolution in SAYR since 6.1 cal (cal = the calendar year, as below) ka BP in the mid-late Holocene using geochemical parameters (i.e., SiO₂/[RO+R₂O], SiO₂/Al₂O₃, SiO₂/TiO₂, SiO₂/[Al₂O₃ + Fe₂O₃], chemical index of alteration (Al₂O₃/[Al₂O₃ + CaO^* + Na₂O + K₂O]), chemical index of weathering (Al₂O₃/[Al₂O₃ + CaO^* + Na₂O]), [CaO + K₂O + Na₂O]/Al₂O₃ and K/Na ratios) of peat deposits and AMS ¹⁴C chronology. The climate was warm and humid between 6.1 and 5.4 cal ka BP, with weak winter monsoon intensity and strong summer monsoon intensity, and accompanied by strong chemical weathering and leaching. From 5.4 to 1.5 cal ka BP, the climate was relatively cold and dry with enhanced winter monsoon and weakened summer monsoon intensities. During 1.5 and 0.8 cal ka BP, with the winter and summer monsoon changing rapidly, lower winter monsoon intensity, higher summer monsoon intensity, and higher chemical weathering and leaching since 6.1 cal ka BP indicated a warm and humid regional climate. It has been relatively cold and dry as a whole, with a trend of being warm and wet, since 0.8 cal ka BP, accompanied by gradually weakened winter monsoon intensity, gradually enhanced summer monsoon intensity, and gradually enhanced chemical weathering and leaching. The Asian monsoon and climatic evolution process in SAYR is highly unstable and has undergone centennial-millennial scale oscillations since 6.1 cal ka BP. Also, six phases at 6.1-5.9, 4.5-4.2, 2.7-2.4, 1.9-1.7, 1.5-1.4, and 0.6-0.4 cal ka BP correspond with cold events recorded by peat and lacustrine deposits and ice cores on the northeastern and eastern TP and with deep-sea sediments in high-latitude regions in the Northern Hemisphere. The results show that climate change in SAYR on the northeastern TP has been consistent with global climate change since the mid-late Holocene. [ABSTRACT FROM AUTHOR]