Hydroclimatic and anthropogenic evolution revealed by grain size end-members of the Yangzong Lake sediments since Last Deglaciation

Based on the reliable chronological sequences established using Accelerator Mass Spectrometry AMS14C dating techniques of sediment core YZH-1 collected from Yangzong Lake, a tectonic freshwater lake in Yunnan Province, the grain size end-members (EM) and their environmental significance were analysed. Then the climatic and hydrological evolution and human activities of this region were reconstructed since 13300 cal. a B.P.. The results showed that clayey silt dominated the central lake sediments since the Last Deglaciation. Non-parametric end-member modeling decomposed the grain size into three components: the character of EM1 (5.0 μm) is similar to regional red soil’s, reflecting the intensity of basin erosion. EM2 (15.1 μm) is the most abundant fraction, representing deep lake sedimentation. EM3 is the coarsest component, suggesting shallow lake sediment. The hydroclimatic evolution of the Yangzong Lake catchment since 13300 cal. a B.P. can be divided into 5 stages. Climate of Period 1 (13300 —10000 cal. a B.P.) was relatively cold and humid with the high-water level in Yangzong Lake. Strong catchment erosion coincided with vegetation degradation in the basin, facilitating the transport of iron-rich red soils into the lake. Erosion intensified further, accompanied by a slight decline in lake level in the Period 2 (10000—8200 cal. a B.P.). During Period 3 (8200—3400 cal. a B.P.) and Period 4 (3400—650 cal. a B.P.), the lake experienced gradual contraction, with erosion intensity initially decreasing before a minor rebound. From Period 1 to Period 4, the lake exhibited an overall trend of shrinkage and dropped lake level, consistent with reduction in humidity of this region, as the result of decreased precipitation and increased evaporation. The overall intensity of human activities and catchment erosion was relatively weak, decreasing initially and then increasing, with a turning point at 5500 cal. a B.P., coinciding with a shift in climate from warm-humid to warm-dry conditions. Since 1300 CE the content of EM1 sharply increased (averaging >75%), becoming the dominant end-member in the sediments of Yangzong Lake, which reflected the rapid growth of population and agricultural activities, resulting in vegetation destruction and intensified catchment erosion. This aligns with the historical context following the establishment of the Yunnan provincial administration by the Yuan Dynasty in 1276 CE, which implemented a land reclamation policy and encouraged immigration and agricultural production. Consequently, since the Yuan Dynasty, the environmental evolution of the Yangzong Lake watershed has diverged from natural state, and humans had became to be the primary geological force shaping the landscape and influencing erosion intensity.