关键词: 环境变化, 人类活动, 孢粉分析, 稻作农业

Abstract:

Based on pollen analysis of sediment samples collected from the profiles at the Youzhougang site in Liyang Plain, we explored the climatic and hydrological changes, vegetation history, and human activities during 5500-4000 a B.P. The results showed that the climate was relatively warm and humid during the Late Daxi cultural period (5500-5200 a B.P.), and then it became warmer and wetter during the early and middle Qujialing cultural stage which spanned approximately from 5200 to 4800 a B.P. A gradual climatic transition from warm and humid condition to cool and dry condition occurred in the late Qujialing cultural stage, and early and middle Shijiahe cultural stage (4800-4200 a B.P.). However, the climate was still relatively favorable to humans on the whole. The climate deteriorated significantly during the late Shijiahe cultural period (4200-4000 a B.P.), which was characterized by unstable cool and dry condition. Moreover, our pollen analysis revealed hydrological changes in our study site. We found small wetlands around the Youzhougang site, and from the significant increase in the proportion of Cyperaceae, Typha, ferns, Concentricystes, and Zygnema we determined two stages of water level rise and water body expansion, which corresponded with climate fluctuations occurring around 4800 a B.P. and 4200-4000 a B.P., respectively. The relatively favorable climate during 5500-4200 a B.P. promoted the consecutive development of the Daxi culture, the Qujialing culture and the Shijiahe culture. Meanwhile, we found a high proportion of Gramineae, a certain amount of hygrophyte herbaceous plants (e.g., Cyperaceae and Typha), and a large amount of charcoals in the deposition, which indicated the damage of mixed deciduous and evergreen broad-leaved forest as a result of excessive logging and rice agriculture expansion. An increase in the proportion of large Gramineae pollen with a size of about 35-40 μm in diameter from the Daxi cultural stage to the early and middle Shijiahe cultural stage reflected the continuous development of rice agriculture. However, the proportion of large-size Gramineae decreased significantly in the late stage of the Shijiahe culture, which indicated that rice agriculture greatly reduced. The decreased rice agriculture together with environmental changes around 4000 a B.P. might contribute to the collapse of prehistoric civilization of the middle reaches of the Yangtze River.

Key words: environmental change, human activity, pollen analysis, rice agriculture