Stepwise intensification of human activities over the past two millennia in Southwest China

As human activities accelerate landscape change, it is crucial to evaluate the potential impacts of natural climate fluctuations and anthropogenic disturbances. Accordingly, a continuous sediment record covering the past two millennia was collected from Beihai Wetland, Yunnan Province. The geochemical elemental compositions and organic material content were subsequently analyzed to evaluate the interactions between climate and human activities on the sedimentation process. The geochemical elements, such as K, Ti, Fe and Al, illustrate an overall increase in detrital inputs following catchment erosion. The total organic carbon content and C/N ratio suggest that organic matter deposition gradually decreased while terrestrial contribution increased. The geochemical indices mainly respond to regional moisture conditions before 1000 CE, and show a broadly consistent pattern with the intensity of Asian Summer Monsoon, suggesting that climate variation was a predominant factor in the catchment erosion and deposition process. Accordingly, the collapse of Nanzhao Kingdom in southwest China in 902 CE is supposed to be closely associated with an exceptionally weak monsoon phase between 900 and 1000 CE. During the Medieval Warm Period and the Little Ice Age, the terrestrial contributions (indicated by compositions of Ti, Fe, K, Al, and C/N ratio) increased significantly, which cannot be solely attributed to the gradual decline in monsoonal precipitation. The substantial population growth and implementation of production policies in southwest China provide a plausible explanation for such phenomenon. Since the Ming Dynasty, the impact of human activities on the landscape evolution and subsequent sedimentation process gradually increased and overcame the impact of natural climate fluctuations. In addition, various sediment records and historical documents have also supported the gradual intensification of human activity in southwest China over the past two millennia. The results revealed the interactive influence of climate change and human activities on the sedimentation process, providing a scientific basis for understanding the mechanisms of environmental changes during historical periods.