Holocene grain size end-member characteristics of the aeolian and fluvio-lacustrine sediments and paleoenvironmental evolution in the eastern sandy land of Qinghai Lake

The Qinghai Lake Basin, located in the marginal zone of the East Asian monsoon, contains extensively distributed sediments that preserve abundant paleoclimatic and paleoenvironmental information. These records are of great significance for revealing regional environmental evolution and its response to monsoon variability. However, the pattern of climate and environmental evolution in this region since the Holocene remains highly controversial. To clarify the sedimentary dynamics processes and the patterns of climatic-environmental evolution, this study selected the aeolian-fluvial interplay sedimentary profile (36°45'15″N, 100°53'30″E) at Aobao North in the eastern lakeside sandy land (with a section thickness of 482cm) as the research object. A reliable chronological framework was established using 15 optically stimulated luminescence (OSL) data (at intervals of 30cm). Combined with analyses of grain-size parameters and magnetic susceptibility (at intervals of 2cm, with a total of 241 samples), a parametrized end-member model was applied to decompose the grain-size components. The results indicate that the sediment grain-size distribution of the Aobao North profile can be decomposed into four end-member components: EM1 (modal size 14.5 μm) reflects weathering and pedogenesis, indicating variations in summer monsoon intensity; EM2 (modal size 51.8 μm) represents fluvial transport of sandy material; EM3 and EM4 (modal sizes 111.5 μm and 163.5 μm, respectively) jointly indicate the intensity of aeolian activity.Based on multiple environmental proxy indicators and regional comparisons, the Holocene environmental evolution in the study area can be divided into five stages: (1) Early-Middle Holocene (9.9~6.9 ka), Fluvial processes dominated, but weathering and pedogenesis were weak, and aeolian activity was relatively strong. The hydrodynamic forces were inferred to be driven mainly from mountain snow and ice meltwater rather than monsoon precipitation. (2) Mid-Holocene Climatic Optimum (6.9~5.7 ka), monsoon precipitation significantly increased, pedogenesis intensified, and aeolian activity was suppressed, representing the warmest and wettest phase of the Holocene. (3) Middle-Late Holocene transition (4.5~3.6 ka), monsoon precipitation began to weaken, and weakly developed paleosols formed. (4) Early Late Holocene (3.6~2.2 ka), monsoon precipitation further declined, aridity intensified, aeolian activity increased, and centennial-scale dry-wet fluctuations occurred. (5) Since 2.2 ka, aeolian sand and loess deposits recorded regional-scale climatic events such as the Medieval Warm Period and the Little Ice Age. This study demonstrates that the grain-size end-member modeling can effectively identify complex sedimentary processes in the Qinghai Lake region. The reconstruction supports the interpretation that Holocene environmental evolution in the area was primarily controlled by the intensity of the East Asian Summer Monsoon. Furthermore, it also reveals a unique Early Holocene pattern characterized by "intense meltwater and active aeolian activity", providing important insights for understanding the interaction between climate and surface process in the monsoon marginal zones.