长江口沉积物~(210)Pb分布及沉积环境解释

在长江河口潮滩、分流河道和水下三角洲共获得18个柱样,进行沉积学分析和210Pb测定,并对其中6根柱样进行137Cs测定。经研究发现,长江口外在水深25～30m,122°30′N,31°00′E附近存在一个泥质沉积中心,沉积速率达2.0～6.3cm/yr。另外,在潮滩和涨潮槽也获得较高沉积速率,其中南汇和横沙岛潮滩沉积速率(1.03～1.94cm/yr)高于崇明东滩(0.51～0.76cm/yr),涨潮槽沉积速率也达0.86cm/yr。此外,在石洞口、南汇、九段沙潮滩和三角洲前缘有部分柱样未获沉积速率,推测为沉积环境不稳定或沉积速率过快所致。

~(210)Pb Distribution of the Changjiang Estuarine Sediment and the Implications to Sedimentary Environment

Eighteen sediment vibrocores, 0.2~6 m long, were collected from the Yangtze estuary in different sedimentary facies, including tidal flat, flood tidal channel, delta front, prodelta and delta-shelf transition zone, to examine the modern depositional characteristics in the study area. Measurement of 210Pb was performed for all sediment vibrocores and 137Cs applied for 6 of them. CIC (constant initial concentration) dating model was used to calculate the sedimentation rate using 210Pb data. The results obtained show the silty clay and clayey silt in the tidal flat and tidal channel, fine sand and silt in the delta front facies. Fine-grained sediment, mostly the clayey silt, consists of the pro-delta facies, and clay-silt-sand constitutes the delta-shelf transition zone. On the basis of relative continuous sediment record, our radiometric measurement demonstrates that the highest sedimentation rates (~2.0 to 6.0 cm/yr) were found in the prodelta facies, reflecting processes of the interface of estuarine mixed water and the seawater. Lower sedimentation rate of 0.81 cm/yr can be recognized form the prodelta margin, but the rate is hardly recorded in the delta-transition zone due to terminating modern sedimentation seaward. The higher sedimentation rates (1.03~1.94 cm/yr) occur in the tidal flat of Hengsha Island and Nanhui, and lower ones (0.51~0.76 cm/yr) in the Chongming Island, indicating the southeastward transport of the suspended sediment in the Changjiang estuary. The sedimentation rate in the flood tidal channel is about 0.69~0.86 cm/yr, showing the rapid deposition due to the discrete flood and ebb currents. The sedimentation rates proposed here is of significance to evaluate the sediment budget from the upstream in the past centennial time scale and in particular is to provide an analogue for after Damming study at Three-Gorges.