青藏高原东北部甘加盆地古梯田地层与年代初探

梯田是人类几千年来利用和改造自然能力的象征,在人类社会发展中发挥了重要的作用。甘加盆地位于青藏高原东北部甘肃省夏河县,盆地内的丘陵、山麓地带分布大面积的层状梯形景观,疑似弃耕的古代梯田。本文在实地考察的基础上,基于卫星遥感资料,通过GIS手段分析发现古梯田面积为42.2 km²（约63000亩）,主要分布在盆地内央曲河及其支流两侧海拔2936~3326 m间的山坡上。通过对3个古梯田剖面样品的磁化率、总有机碳/氮（TOC、TN）含量、孢粉、粒度等环境代用指标分析,同时对比具有准确年代控制的自然剖面磁化率与粒度。研究结果显示：梯田开垦于晚全新世（距今3000 a）古土壤,梯田剖面顶部25~35 cm不同于自然剖面而呈均一化的指标证实了耕作层的存在（距今约1000 a以下层位）;但耕作层之上TOC、TN含量的增多,以及梯田剖面与自然剖面整体一致的指标变化趋势说明了梯田被短期利用后长期废弃;结合历史文献资料推断夏河古梯田是在北宋“弓箭手屯田”制度（1074—1125年）下开垦的;气候重建资料显示该时段温度较高、降水增加,整体有益于农业生产。本文提供了过去人类社会适应气候变化的典型案例。

A preliminary study of the strata and age of ancient agricultural terraces in the Ganjia Basin,northeastern Tibetan Plateau

Agricultural terraces are a manifestation of man's ability to transform nature during the past thousands of years, and they have played an important role in the development of human society. The Ganjia Basin in Xiahe County, Gansu Province, is located in the northeastern Tibetan Plateau. A large area of banded landscape is distributed on the hills and piedmonts in the Ganjia Basin, which is claimed to be composed of large-scale abandoned ancient terraces. Based on fieldwork, satellite remote sensing data and GIS analysis in the present study, we found that the ancient terraces cover an area of 42.2 km² and are mainly distributed on slopes between 2936 and 3326 m a.m.s.l. (meters above mean sea level) on both sides of the Yangqu River and its tributaries in the basin. Additionally, we analyzed various environmental proxies, including magnetic susceptibility (MS), grain size, the contents of total organic carbon (nitrogen) (TOC, TN), and palynological assemblages, from three representative sections of ancient agricultural terraces (GJAT-1, GJAT-1, and GJAT-3), and compared them with the MS values and clay content of a natural profile with accurate chronological control in the Ganjia Basin. The results indicate that the terraces are reclaimed paleosols dating to the Late Holocene (3000 kyr BP), and that the homogeneous MS values of the top 25-35 cm of the terrace profiles differ from those of the natural soil section, which confirms the existence of a cultivation layer in the terraces. However, the increases in the contents of TOC and TN above the cultivated layer, and the fact that the trend of MS in the terrace profile is broadly in line with the natural profile, indicate that the terraces were only used for a short interval and have been abandoned for a relatively long time. Combined with historical documentary evidence, we infer that the ancient Xiahe agricultural terraces were constructed as a result of the "Gongjianshou (bowman) militia farming" policy during the Northern Song China (1074-1125 CE). Climate reconstructions for the Northern Hemisphere and northern China demonstrate that the temperature was higher and that the precipitation increased during this interval, providing a favorable environment for agricultural production. Overall, our results provide a typical example of societal adaptation to climate change in the past.