沙漠/黄土过渡带6 ka B.P.以来气候环境变化及其对人类活动的影响

CLIMATE AND ENVIRONMENT CHANGE OF DESERT/LOESS TRANSITION ZONE AND ITS IMPACT ON PREHISTORIC HUMAN ACTIVITIES SINCE 6 ka B. P.

文章选择沙漠/黄土过渡带的榆林市白河庙村钻孔（38°35＇9.89″N，109°24＇0.71″E）泥炭沉积，通过建立良好的年代序列和高分辨率花粉记录分析，重建研究区内植被演替和气候环境变化过程。在此基础上，结合考古学资料，探讨适宜的气候环境阶段和极端气候事件对人类活动的影响。结果表明，6 ka B.P.以来研究区植被类型经历典型草原、荒漠草原、干草原、荒漠草原4个阶段，指示6.0～4.2 ka B.P.气候较为暖湿，为全新世适宜期；4.2 ka B.P.时发生极端干旱事件，之后（4.2～1.1 ka B.P.）气候向干旱化发展；1.1～0.5 ka B.P.，气候经历短暂的湿润期；0.5 ka B.P.之后，气候再次转干。另外，通过统计沙漠/黄土过渡带6～3 ka B.P.考古遗址数量及其空间分布变化信息，发现史前人类活动与气候、生态系统变化紧密相关。东亚夏季风带来的降水量控制干旱/半干旱地区水资源变化，从而影响依赖水资源的旱作农业发展、人类活动强弱和考古学文化兴衰。6.0～4.2 ka B.P.降水较多，植被盖度高，毛乌素沙地固定，沙漠/黄土过渡带发育古土壤，并向西北移动。适宜的气候环境和良好的生态条件有利于旱作农业生产，导致人口增加，遗址点数量增多，人类活动范围扩大，仰韶时期和龙山时期的考古学文化繁荣发展。而4.2～3.0 ka B.P.遗址点大幅减少，人类活动规模减小，考古学文化衰退。4.2 ka B.P.极端干旱事件及之后持续的干旱，导致植被退化，毛乌素沙地沙丘活化，沙漠/黄土过渡带向东南方向摆动。水资源短缺和生态环境破坏，对以粟为主的旱作农业生产和人类生存造成严重影响，可能是导致该地区史前考古学文化大范围衰退的重要原因。

Baihemiao core（38°35＇9.89″N, 109°24＇0.71″E） was drilled in desert/loess transition zone. The core is 619 cm in length, and from top to bottom, mainly consists of yellowish brown eolian sand（0～88 cm）, grey brown sand（88～150 cm, 155～235 cm, and 472～615 cm）, black silty peat（150～155 cm, 235～366 cm, and 615～619 cm） and sandy silt（366～472 cm）. The bottom of the core（619 cm） was dated to 6 cal.ka B.P. by precise radiocarbon date. Then high resolution fossil pollen data was used for reconstructing vegetation succession and climate change. In addition, ancient archaeological sites in 6～3 ka B.P. were counted, and their spatial and temporal distribution information were analysed in order to explore how optimum climate and extreme climate event affected human activities. The result shows that 12921 pollen grains in total were counted and identified in 42 samples, mainly including Pinus, Picea, Abies, Betula, Quercus, Tamarix, Artemisia, Chenopodiaceae, Poaceae, Leguminosae, Thalictrum, Cyperaceae, Ephedra, Humulus, Myriophyllum and Selaginellaceae. Herbaceous pollen dominated by Artemisia and Chenopodiaceae account for 75%to 99%of total pollen percentage content, and tree pollen content was 1%to 24%. Vegetation types experienced four phases since 6 ka B.P., including typical steppe, desert grassland, dry steppe and desert grassland. Vegetation succession presents four phases of climate change. 6.0～4.2 ka B.P., climate was warm and moist. 4.2～1.1 ka B.P., extreme arid climate event occurred and climate distinctly became dry. After 1.1 ka B.P., the moist climate resumed and then turned to dry condition. Climate change and archaeological data indicate ancient human activities were closely related to climate and ecosystem change. East Asian summer monsoon brought rainfalls and controlled water sources change in semi-arid and arid region, and then affected rain-fed agriculture, human activities and archaeological culture succession. 6.0～4.2 ka B.P., relatively sufficient rainfall and high vegetation cover fixed Mu Us Desert. Desert/loess transition zone expanded northwest. Moist climate condition and good ecological environment contributed to agriculture development and population growth. Therefore archaeological sites increased rapidly and spatial space of human activities expanded. However, 4.2～3.0 ka B.P., archaeological sites decreased and human activities fade. The development of archaeological culture was interrupted because extreme dry climate event of 4.2 ka B.P. and continuous dry climate conditions. Water sources were lack and made a severe impact on rain-fed agricultural production and human subsistence. Therefore, climate change was testified as a significant influencing factor of archaeological culture succession.