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The Baodun Culture (4600–3700 a BP) is currently the earliest archeological culture found at the Chengdu Plain in southwestern China. Archeological evidence has shown that the Baodun Culture likely originated from the upper reaches of the Minjiang River, with two possible routes for human migration into the lower Chengdu Plain proposed: (1) At about 5000 a BP, favorable climatic conditions in the upper reaches of the Minjiang stimulated the growth of populations and the development of agriculture in the region. Due to this shift, some groups expanded from the upper Minjiang River to sites in the northern area of the Chengdu Plain, such as the Guiyuanqiao site; (2) during 4800–4300 a BP, a drying event occurred in the upper regions of the river valley causing a reduction in agricultural harvests and the migration of populations into sites – including the Baodun site – from this area of the Minjiang to the southwest piedmont of the Chengdu Plain. A multi-facetted study of the T3321 profile at Baodun site, including pollens, grain size, and geochemistry, demonstrates that after 6000 a BP, the climate of the Chengdu Plain shifted from cool, toward warmer and dryer conditions. However, as the Chengdu Plain is located in an alluvial fan, hydrothermal conditions here are still generally better than those in the upper reaches of Minjiang River. A change in the proportion of Concentricystes seen in the profile T3321 also indicates that after 4700 a BP, the lake and marsh areas associated with the Baodun site further decreased. However, as a result of this reduction, more terrestrial surfaces were exposed, providing favored space for Baodun Cultural groups in the area of the site. Due to such conditions, by about 4600 cal. a BP, people in the upper reaches of the Minjiang River had primarily moved to the southwest piedmont regions of the Chengdu Plain. [ABSTRACT FROM AUTHOR]

Reconstructing the vegetation history of southwest China since the Last Glacial Maximum is crucial for understanding the evolution of the Indian summer monsoon. In this study, we present a high-resolution palynological investigation from a 5 m core from the Ganchi peatland (southwest margin of the Sichuan Basin, southwest China) in order to reconstruct regional vegetation and climate over the last 25 kyrs. The succession, from bottom to top, comprises lake, wetland, and peat deposits, and wetland/aquatic pollen types are found to be a sensitive indicator of palaeoenvironmental change. Deciduous broad leaf forests dominated by Betula were present between 25-18 cal kyrs BP, suggesting a cold and dry climate during the Last Glacial Maximum. After 18 cal kyrs BP, temperature and summer insolation increased gradually, marking the Lateglacial transition. Thermophilous and hygrophilous species expanded between 11-8 cal kyrs BP, indicating a progressively warmer and wetter climate. From 8-4 cal kyrs BP, evergreen forests developed, while Tsuga reached its maxim abundance, implying a warm and humid climate coincided with the mid-Holocene climatic optimum. Results indicate that residual ice sheets may have impeded the northward shift of the Intertropical Convergence Zone, resulting in peak Holocene warmth lagging behind peak summer insolation forcing. Vegetation change was primarily controlled by climate, with no obvious evidence of anthropogenic disturbance until the last 1 cal kyrs BP. Our study shows that vegetation and climate in the southwest margin of the Sichuan Basin were controlled by summer insolation and ice sheet dynamics, and also influenced by sea surface temperature in the Indian Ocean. • High-resolution pollen records revealed vegetation and climate history since 25 cal kyrs BP • Variations in proxies were highly correlated with depositional processes • Paleovegetation records suggest that the Holocene climate optimum occurred during 8-4 cal kyrs BP • The most conspicuous landscape transformation caused by human impact in the Sichuan Province began at ∼1 cal kyrs BP [ABSTRACT FROM AUTHOR]