澧阳平原优周岗遗址孢粉记录的环境变化与人类活动

通过对澧阳平原优周岗遗址剖面沉积样品孢粉分析,重建了该区5500 a B.P.~4000 a B.P.气候、水文、植被变化和人类活动历史。结果表明：距今5500~5200年大溪文化晚期,气候环境较为适宜;距今5200~4000年的屈家岭—石家河文化时期,气候暖湿程度先升高后降低,转折点在4800 a B.P.左右。其中,距今4200~4000年的石家河文化晚期,气候向干凉化发展。孢粉组合显示优周岗遗址周边一直有湿润生境存在,且在屈家岭文化中晚期和石家河晚期出现湿生草本、蕨类孢子和藻类的明显增加,反映两次水文变化过程。优周岗遗址大溪文化时期常绿和落叶阔叶林已遭破坏,稻作农业有一定发展,屈家岭—石家河文化早中期稻作规模扩展。然而石家河文化晚期,稻作农业规模明显收缩,可能与区域文化衰落有一定关系。

Environmental changes and human activities around the Youzhougang site in Liyang Plain based on pollen analysis

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.