Human activities flexibility under volatile environment conditions around 4000 a BP: Insights from the Jinsha site in the Chengdu Plain, SW China

Adapting to and navigating a dynamic environment has become a hot topic in recent years. Despite the Chengdu Plain (CDP) in southwestern China being long regarded as a cradle of Chinese civilization, scant exploration has left critical aspects of human-environment interactions unresolved. This study leverages high-resolution sporepollen and charcoal records from the IT8007 profile excavated at the esteemed Jinsha Site Museum, complemented by other materials of the CDP. This profile dating from approximately 4000 to 3650 cal. a BP, which is related to the transition and collapse of the Baodun Culture in the CDP. A prevalent mixed coniferous and broadleaved forest in the site region at 4000 cal. a BP was developed under a warm and wet climate, with subdued regional fire activity. Notably, a high concentration of large-size charcoal indicates the migration of Baodun people to the hinterland since the late Baodun Culture period. Between 4000 and 3900 cal. a BP, sporepollen concentration and cold-resistant plants clearly increased under a deteriorating climatic condition. The changed environment pushed the Baodun people to diversify their lifestyles and food sources for adaptation. However, regional fire activity remained weak, with humidity decreasing after 3900 cal. a BP. From 3900 to 3650 cal. a BP, the forest vegetation transformed into sparse woody and xerophytic herbs and regional fire activity intensified under a colder and drier climate, likely contributing to the decline of the Baodun Culture due to the dynamic environment and frequent flooding events. A consistent rise in Concentricystes indicates the formation and expansion of lakes and swamps due to recurrent floods, aligning with the higher site elevation during this period. Around 3650 cal. a BP, the forest vegetation recovered to mixed coniferous and broad-leaved forests, accompanied by a resurgence and subsequent rapid decline in regional fire activity. The slightly increase in temperature and humidity created a stable environment with increased living resources, facilitating the large-scale human livelihood during the Sanxingdui Culture period. Examining the IT8007 profile reveals nuanced dynamics of the 4.2 ka event, and the high-resolution sporepollen and charcoal records provide valuable insights into the impacts of this event and the adaptability of human activity in the CDP.