近百年来长江中游牛轭湖沉积物铅元素富集记录及其环境意义——以长江荆江段牛轭湖群为例

通过对长江荆江段天鹅洲、中洲子和尺八3个牛轭湖钻孔沉积物进行短寿命放射性核素、粒度、磁学、铅元素测试与史志资料分析,在利用²¹⁰Pb比活度计算平均沉积速率,并结合史志资料与沉积物特征指示的事件进行沉积物年代校核基础上,运用地表物质非自然富集系数(EF值)方法,分析了牛轭湖沉积物铅元素富集状况。结果表明:三钻孔铅元素非自然因素富集特征基本一致,均起始于20世纪80年代初,并于2000年后呈下降状态,与我国铅污染历史及禁用含铅汽油的时间基本吻合;利用沉积物中铅元素非自然因素富集事件对沉积物进行断代分析,具有一定的可行性;牛轭湖河/湖沉积相转变未对沉积物铅元素的非自然因素富集产生影响,说明铅污染具有较广的“暴露面”。

SEDIMENT LEAD ENRICHMENT RECORDS AND ITS ENVIRONMENT SIGNIFICANCS OF OXBOW LAKES IN JINGJIANG SECTION OF THE YANGTZE RIVER DURING LAST 100 YEARS

High-resolution and highly precise lead record for recent lake sediments (last 50-100 years) are essential for quantitative paleoenvironment research. Sample cores TE with a length of 85 cm, ZZ-2 with a length of 89cm and CB-1 with a length of 91cm were drilled in the deepest part of Tian'ezhou, Zhongzhouzi and Chiba oxbow Lakes by the Gravity Impact Core Sampler in Jingjiang section of the Yangtze River during November 2012. They were divided into sediment samples in which a set interval is applied to pick about every 1 cm. They were selected for understanding the Pb enrichment conditions and its environment significancs of oxbow lake sediments by using enrichment factor analysis. Dating of the ²¹⁰Pb, characteristics of the grain size, frequency dependent susceptibility and element contents (Al and Pb) were analyzed. By the analyses of grain-size parameters and magnetic parameters of cores TE and ZZ-2 two stages of oxbow lake sediments,which named layer A (lower) and layer B (upper) can be distinguished. According to the average deposition rate and the diagnostic characteristics of the grain size and frequency dependent susceptibility, the possible sediment depth of the river/lake transformation event layer was found. After combining local chronicles with environmental events such as river/lake transformation events and flood events, which indicated by sediment characteristics, the high-resolution chronological sequence were set up. Three cores disclosed the lead enrichment records and its environment significancs of oxbow lakes. The results of the lead enrichment records show that, the characteristics of Pb unnatural enrichment in three cores are mostly identical. They all began in the early 1980s and declined in the 2000. Based on the high-resolution age of deposition rate and environmental events, which was closely related with the lead pollution history such as the beginning of the lead pollution and 2000 which the year of leaded petrol has been banned in China, it is thought that the result is caused by human activity impacts. By comparing and contrasting three lead enrichment records of three sample cores, the similar trend of three curves were found. It is indicated that oxbow lakes' river/lake sedimentary facies had no effects on the unnatural enrichment of Pb element. Based on the above study, it reveals that the possibility of using the unnatural enrichment events of Pb element as a time marker in the study area.