重庆永川汉东城遗址地层记录的长江上游唐代洪水事件

FLOOD EVENT RECORDED BY THE LAYER OF HANDONGCHENG SITE AND ITS SIGNIFICANCE TO THE UPPER REACHES OF YANGTZE RIVER

历史文献中关于长江洪水的记录比较粗略,对于了解长江水体的历史变化带来很多不确定性.长江两岸考古遗址地层中的事件地层为研究长江水体的历史变化提供了新的视野.汉东城遗址位于重庆市永川区朱沱镇汉东村,地处长江上游西岸一级阶地上,是川南黔北渝西水上交通要道.本文对遗址地层样品进行了粒度、地球化学元素、酸碱度等的测试与分析.结果显示,T0104剖面土壤以粉砂质壤土为主,但是与同剖面其他层位的样品相比,第7层样品粉砂含量明显偏高;地层中样品的地球化学元素含量,以SiO2、Al2 O3、Fe2 O3为主;与UCC相比,本地区土壤SiO2、Al2O3、K2O等元素含量相对稳定,但是Na2O明显偏低,P2O5明显偏高.地层中的CIA值,在汉代及以前,在65~75之间,Rb/Sr值在65~80之间,处于中等化学风化作用阶段,气候相对温湿;在汉宋之间,CIA值和Rb/Sr值均有一个明显降低及逐渐回升的过程,之后CIA值和Rb/Sr值一直在60左右徘徊,说明气候在降温之后虽有所回升,但整体比前期冷干.地层中铷锶含量及其比值的变化显示,在汉宋之间的某个时段内,该区域发生强降水的可能性非常大.在遗址发掘出土遗存和测年的基础上,对遗址土壤粒度、地球化学元素等分析,结合气候变化背景资料综合分析认为:1)汉东城遗址所在区域在唐中后期确实发生过一次大规模的洪水事件;2)本次洪水事件可能与强降水有关;3)通过与现代洪水记录的对比分析认为,遗址地层中记录的洪水事件可能比1998年和2018年洪水规模还要大.

The flood records of the Yangtze River in historical documents are relatively sketchy,which poses a large degree of uncertainty for understanding the historical changes in the river's water body of the Yangtze River,but the event strata in the archaeological sites along the Yangtze River have provided a new perspective for the study of ancient water and climate change.The Handongcheng site is located in Handong Village,Zhutuo Town,Yongchuan District,Chongqing City,on the first terrace of the west bank of the upper reaches of the Yangtze River(29°00′27.6″N,105°50′57.1″E)at an elevation of 223 meters.It is also a main waterway connecting southern Sichuan,northern Guizhou and western Chongqing.The particle size,geochemical elements and PH value of the stratum samples are measured and analyzed in this paper.The particle size analysis showed that the soil in the site's strata is mainly silt,while the content of sand and clay is relatively low,especially in the seventh layer,which exhibited a significant increase in silt.The geochemical composition of the strata is mainly SiO2(61.29%on average),Al2O3(14.47%on average)and Fe 2O3(4.92%on average);compared with UCC values,the content of Na 2O in the soil is markedly low,while the figures for P2O5 are slightly higher.During the Han Dynasty and before,the CIA value in the strata was between 65~75 and the Rb/Sr value was between 65~80,indicating that these strata were in the middle stage of chemical weathering,and the climate at that time was relatively warm and humid.Between the Han and Song dynasties,there was both an observable decrease and a gradual increase in CIA and Rb/Sr values.After that,the two values hovered around 60,showing that although temperatures and precipitation recovered after a period of cooling and drying,the weather was generally drier and colder than previously.Also,from the content and ratio variation of Rb/Sr in the strata,it can be seen that there was a high likelihood of heavy precipitation in this area in a certain period between Han Dynasty and Song Dynasty.The particle size and geochemical elements of the site soil was analyzed on the basis of excavated remnants and radiocarbon dating.Furthermore,coupled with a comprehensive analysis of background data on climate change,it can be concluded that:1)In the middle and late Tang Dynasty,there was indeed a large-scale flood event in the area where the Handongcheng site is located;2)this flood event may be related to heavy precipitation;3)according to a comparative analysis with modern flood records,the flood event recorded in the site strata should be larger in scale than those in 1998 and 2018.