全新世时期形成的连续性古洪积层是追溯古环境信息的可靠记录.重庆市丰都县玉溪遗址存在多期连续的文化层与洪积层,但已有研究并未系统分析该遗址区的古洪水发生机制及其气候背景,并且缺乏基于多期古洪积层的周期性研究.(1)本文基于动物骨屑AMS14C年代数据,用Bacon程序拟合玉溪遗址剖面的沉积-年代关系,判定玉溪剖面古洪积层形成时段为约6.4 ka BP～7.3 ka BP.(2)古洪水沉积序列的粒度和端元分析结果显示,玉溪古洪积层由细粉砂-粉砂组成,表明古洪积层为溢岸憩流和滞水缓流堆积而成.(3)孢粉组合、磁化率、Rb/Sr和Si/Al比值等环境指标显示,玉溪剖面的古洪水沉积经历了早期湿热、中期温干、晚期暖湿三个阶段,其中古洪积层在温干阶段的沉积速率最大(30 cm·(100a)-1),而且古洪水发生的频率亦高于湿热期.(4)古洪水沉积序列的小波功率谱分布特征表明,玉溪剖面的古洪水存在约30 a的短周期和约350 a的长周期;其长周期与区域性气候冷事件相关、短周期与ENSO事件导致的夏季风异常有关.本文提出的搬运洪积物的三种动力类型、干湿期古洪水的沉积速率差异以及古洪水泛滥的长短周期,对研究新石器早期重庆地区的人地关系和长江上游的洪水发生机制有参考意义.
Periodicity of Multi-Period Paleofloods and Paleoclimatic Context Recorded by the Layers at the Yuxi Site,Southwestern China
Continuous paleo-diluvia formed in Holocene epoch have been proved to be one of the reliable carriers for tracing paleoenvironmental information.In the Yuxi site in Fengdu county,Chongqing city,China,there found multiple-period consecutive cultural and paleo-diluvial layers;unfortunately,previous studies had not systematically analyzed the layers for paleo-floods mechanism and paleoclimatic context,particularly short of knowledge of periodicity of the layers.(1)In this study,it examined AMS14C dating of animal bone fragments collected in the cultural layers of the T0403 profile at the Yuxi site;then it obtained a profile of deposition-chronology relationship fitted by Bacon program.It concluded that the formation period of paleo-diluvia in the Yuxi site was in the range of 6.4 ka BP-7.3 ka BP.(2)In terms of granularity analysis and end-members analysis of the paleo-diluvial layers,the paleoflood sediments were mainly composed of fine-silt and silt,which suggests that the paleo-diluvia were formed by gradual accumulation inflicted by overbank slack flow and perched slow water.(3)According to environmental indexes such as synsporopollen assemblage,magnetic susceptibility,Rb/Sr and Si/Al ratios,the paleo-diluvia in the Yuxi site experienced three stages:early hot-wet,middle cool-dry and late warm-dry.The deposition rate of paleo-diluvia was the highest(30 cm·(100a)-1)in warm-dry stage,and the frequency of paleoflood was higher than those in the hot-wet stage.(4)In view of the wavelet power spectrum distribution of the paleo-diluvium sedimentary sequence,the paleoflood events speculated by the Yuxi profile exhibited a short cycle of ca.30 years and a long cycle of ca.350 years.The long cycle was related to climate cooling events,and the short one was related to summer monsoon anomalies caused by ENSO events.By this study,it proposed the three dynamic types of paleo-flood deposit transportation,the difference in accumulation rate of paleo-diluvia in dry and wet periods,and the long or short cycles of paleofloods,which are of reference significance for studying the man-land relationship in Chongqing area in the early Neolithic Age and the flood mechanism in the upper reaches of the Yangtze River.
paleoflood depositionenvironmental evolutionend-member analysispaleoflood periodicityclimatic anomalythe Yuxi site
万方数据
维普
