The evolution history of Asian monsoon and westerly winds over the past 14,000 years: Evidences from grain size of ash in the Hongyuan peat on the eastern Qinghai-Tibetan Plateau

The relationship between the Asian monsoon and westerly winds (WWs) on various temporal scales remains a topic of debate. The Zoige wetland, situated to the east of the Qinghai-Tibetan Plateau, is the largest highland wetland in China and highly sensitive to environmental changes. While several paleoenvironmental reconstructions have been conducted in this region, comprehensive investigations into grain size are still limited. In this study, we conducted a systematic investigation of grain size in ash from the sediments of the Hongyuan peat HY2014 profile in this area. The ash composition in this profile was primarily clay and silt particles, with a very low sand content. The grain size frequency curves generally exhibited a bimodal modality, except for the sediments in the bottom layers which displayed a single-peak pattern. By employing the end-member (EM) model and considering previous research findings, we reconstructed the atmospheric circulation since the last deglaciation. Three distinct EMs were identified. EM1 indicated long-distance suspended materials primarily transported by WWs. EM2 was associated with low-altitude circulation and linked to a decrease in the Asian summer monsoon (ASM). EM3 represented materials transported into the wetland via the Asian winter monsoon (AWM). Our results suggested that during the late deglacial period, characterized by strong AWM intensity, the Zoige wetland experienced cold and arid conditions. Moreover, the WWs and ASM exerted dominant influences, while the AWM gradually weakened during the Holocene, with variations in their intensities across different time intervals. The results also revealed a positive correlation between WWs and ASM, as well as a negative correlation between WWs and AWM.