历史时期塔里木盆地周缘绿洲演化和人类活动与气候控制的水文条件变化密切相关.罗布泊西岸地区是古塔里木河尾闾三角洲区,曾是东西方交往的重要通道和著名的楼兰古国所在地,而如今却变为不毛之地.自然与人文环境的剧烈变化,使罗布泊地区成为研究干旱区人类活动-气候-水文三者之间相互关系最具代表性的区域.文章对楼兰地区大量自然植物遗存和古代人类活动遗址进行系统的AMS 14C年代学分析,结合塔里木盆地人类活动历史记录和已有的高分辨率气候记录,对其所指示的古绿洲环境格局和人类活动进行初步研究.结果表明,1260~1450 cal.A.D.是罗布泊地区自古楼兰之后的又一次绿洲湿润期,在罗布泊西岸三角洲区生长大量的植被,对应中世纪暖期向小冰期转变的过渡阶段.14C年龄的统计结果显示湿润期内存在多次洪水事件,表现为多次洪水期与枯水期脉冲式交替出现.同时多处古代遗址的14C年代结果显示元明时期楼兰地区再次出现人类居住,并修建引水渠、灌溉耕地.气候变化导致区域性水文条件变好应该是驱动元明时期人类再次定居楼兰地区的主要因素.
The oasis evolution and human activity strongly related to hydrological conditions in Tarim Basin,Xinjiang,China,in historic time.The Lop Nur(Loulan) region,southern Xinjiang,was the terminal lake of the Tarim River and the famous ancient Loulan kingdom located on the west bank of the Lake Lop Nur.However,the area has been experienced dramatic changes from oasis to wasteland.In this paper,we conducted systematical radiocarbon dating (AMS 14C) with 30 specimens of subfossil Populus euphratica lying on ancient riverbeds,which may be driftwoods transported by floods or be dead trees grew on both sides of the river,10 specimens of Tamarix chinensis and reed in Yardang area between ancient rivers,and 5 specimens sampled from three archaeological sites(two settlements and an artificial canal).The statistical characteristics of 14C ages were also analyzed.Finally by compared with historical documents and reported palaeoclimatic records,the relation between oasis environment and human activity was discussed.Results show good consistency between natural plant remains and archaeological sites,corresponding with the Yuan-Ming Dynasty(1271~ 1644 A.D.).It was found that the difference on age distribution of two aforementioned sources of subfossil P.euphratica is obvious.The presence of three age peaks of P.euphratica,corresponding with our driftwood samples,implies that the Loulan area experienced at least three flood events and re-experienced oasis environment from 1260 cal.A.D.to 1450 cal.A.D.,which is the climate transition stage from the Medieval Warm Period to the Little Ice Age.Specifically,results show that reed and tamarisk samples appear to be dated to the end of flood events,indicating a decrease in groundwater level during intermittent periods.Therefore,it is concluded that several pulse-like flood event-featured wet-dry alternative cycles constitute the wet period.In the meantime,human re-settled in the Loulan area and built canals to irrigate farmlands.Buried reed layers were found on the artificial bank because the people who lived in the Loulan area at this time dredged the canal and placed sediments directly onto growing reeds in the banks of the canal during the flooding periods.The presence of reddish sinteredclay breccia in the artificial bank can also be interpreted as evidence of human activity at this time,which may be related to people maintaining the canal after flooding events.By compared with high-resolution palaeoclimatic records(ice core,tree ring,and stalagmite)from Kunlun Mountains,Qilian Mountains,and Tianshan Mountains,around Tarim Basin,it was believed that human resettlement in the area during the Yuan-Ming Dynasties was primarily dominated by an increase in runoff resulted from mountain precipitation.Furthermore,Lop Nur region is the core arid area of Eurasia,the enlargement of Loulan oasis strongly suggests the wet climate and a prosperous oasis environment in the whole central Asia,that may be a major environtnental factor for the three times of Mongol westward conquest in 13th century.