闽江不同季节入海悬浮沉积物的物源分析：来自微量元素和Sr-Nd同位素的约束

Study on the composition and source of sediments discharged by the Zhejiang and Fujian rivers is crucial to have a better understanding of the sediment source-to-sink process in the East Asia continental margin. The river basins in Zhejiang and Fujian have different landform and climate conditions with the big river basins in the East Asia. The sediment transport and the discharged sediment compositional variations, which are probably controlled by both natural and anthropogenic influences on different temporal scales, remain poorly known. As there are distinct parent-rocks in the upper and lower Minjiang river basin and rainfall shows spatial variations in different seasons, Minjiang river is selected as the study area. This project will mainly focus on seasonal suspended sediments from a fixed location near the river mouth in the lower Minjiang river. Geochemical data of sediments from major tributaries and the middle-lower mainstream will be collected and trace elements and Sr-Nd isotopes will be employed in the sediment provenance analysis. The objectives of this project are to figure out the major source of the discharged sediments from Minjiang river and its seasonal variations and to conclude the major controls on the discharged sediment geochemical compositions. The results of this project will not only provide additional significant data for some hot research fields such as provenance study of sediments in the East China Sea and the Taiwan Strait, but will also be beneficial for better understanding the source-to-sink system and reconstruction of the tectonic-sedimentary history of the East Asia continental margin.

浙闽河流入海沉积物组成及其来源的研究对于深入理解东亚大陆边缘沉积物的从源到汇过程具有重要意义。浙闽河流流域的地貌和气候背景与东亚大陆上的大河流域具有一定的差别，在自然环境变化和人类活动双重因素不同时间尺度的影响下，其沉积物输运过程及入海沉积物组分的变化规律，目前并不清楚。考虑到闽江流域上、下游主要母岩类型的不同及降水的季节差异，本项目拟选取闽江下游近河口段不同季节的悬浮沉积物为主要研究对象，运用微量元素和Sr-Nd同位素地球化学手段，进行悬浮沉积物的物源示踪，与主要支流、干流沉积物地球化学分析数据进行对比，明确闽江入海沉积物的源区及其季节时间尺度的演化规律，揭示影响入海沉积物组分的主控因素。上述研究将为东海陆架和台湾海峡沉积物的源区分析等热点领域提供重要的参考数据，对东亚大陆边缘源汇系统的深入认识和构造-沉积演化历史的重建也具有指导意义。

浙闽地区山溪性河流具有与东亚大河不同的地貌和气候背景，其入海沉积物输运过程及组分变化规律的研究程度较低。闽江上、下游分布了不同类型的基岩，降水具有明显的季节地域差异，是开展季节尺度河流入海沉积物物源研究的理想场所。本项目采集了闽江下游近河口段一个固定站位不同季节（覆盖一年水文周期）的悬浮沉积物，利用微量元素和Sr-Nd同位素地球化学等手段进行沉积物源示踪，并与流域不同区域的悬浮物和河床沉积物数据进行了对比分析。结果显示，1）闽江入海悬浮沉积物的Sr-Nd同位素和元素组成均表现出明显的季节性变化，其中Sr-Nd同位素组成差异所指示的物源演变与降水的季节时空变化相一致，即上游流域降水量大时，闽江入海沉积物具有更多上游源区的信号，反之亦然；2）反映母岩岩石类型的微量元素指标（如La/Yb、Th/Sc等）则体现出与流域总径流量的相关性，枯水期入海沉积物具有更明显的河床端元信号，指示暴露河床沉积物的再旋回搬运过程，丰水期入海沉积物则更多来自上游及各支流携带的悬浮颗粒物；3）部分重金属元素（如Pb、V、Co）具有与上述元素相似的季节变化规律，而Cr、Mn元素在枯水期表现出了脉冲式的高浓度，指示了偶然的污染物排放事件，因此，过去的环境评价研究可能低估了闽江流域颗粒重金属的污染程度。上述结果说明闽江沉积物供给-输运过程能快速响应于水文气候变化，具有季节的敏感性，浙闽山溪性中小型河流可能代表了一种被低估的、东亚入海河流输运体系的新类型。研究发现了闽江入海沉积物多时空尺度的地球化学不均一性，强调了水文气候因子（降水和径流）在亚热带构造稳定区山脉侵蚀及沉积物输运过程中的关键角色，为东亚大陆边缘沉积源-汇示踪及环境响应研究提供了重要数据。

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