三峡大坝建坝前后长江水下三角洲及邻近陆架沉积速率变化

Changes of Sedimentation Rates in the Yangtze River Subaqueous Delta and Its Adjacent Inner Continental Shelf Before and After the Construction of the Three Gorges Dam

沉积速率是沉积物供给和沉积作用的综合结果,也是表征沉积环境状态的重要指标。本文基于长江水下三角洲及邻近海域收集及补充的254个放射性^(210)Pb数据,对数据进行同化处理并使用统一的方法重新估算了沉积速率,分析三峡大坝建成前后研究区沉积速率的变化,探讨了长江入海泥沙减少及性质的变异对河口三角洲冲淤的影响。结果显示,2006年之后长江水下三角洲及邻近陆架沉积速率明显降低,其中水下三角洲沉积速率由最高5.4 cm/a降低到整体不足2 cm/a,而内陆架降幅较小由1~2 cm/a降低到不足1 cm/a,水下三角洲还出现了局部无沉积的站位。2006年之后沉积体表层混合作用增强,混合层厚度从4~10 cm增加到5~30 cm。沉积速率改变主要受制于长江入海沉积物的通量的巨减以及沉积物颗粒的粗化,而沉积速率的改变促进了底栖生物发育从而导致混合作用增强。

Sedimentation rate is the combined result of sediment supply and deposition processes,and is also an important indicator to characterize the state of the depositional environment.Based on 254 radioactive ~(210)Pb data collected and supplemented from the Yangtze River subaqueous delta and its adjacent areas,this paper assimilated the data and recalculation the sedimentation rate using a unified method to analyze the changes of sedimentation rate in the study area before and after the construction of the Three Gorges Dam,and to explore the influence of the reduction of sediment discharge and grain size change of the Yangtze River subaqueous delta and its adjacent areas.The results show that the sedimentation rate of the Yangtze River subaqueous delta and the adjacent continental shelf decreased significantly after 2006,in which the sedimentation rate of the subaqueous delta decreased from a maximum of 5.4 cm/a to less than 2 cm/a overall,while that of the inner continental shelf decreased less from 1~2 cm/a to less than 1 cm/a,and the subaqueous delta also showed some non-sedimentation station.The surface mixing layer of the sediment increased after 2006,and the thickness of the surface mixed layer increased from 4~10 cm to 5~30 cm.The change of sedimentation rate is mainly governed by the huge decrease of sediment flux of the Yangtze River into the sea and the coarsening of sediment particles,while the change of sedimentation rate promotes the development of benthic organisms which leads to the increase of mixing processes.