突发性天气事件与流域过程对陆架泥质沉积记录形成与保存的影响

Muddy depositional system on the continental shelf is one of the key research objects for Land-Ocean Interaction Projects, and sedimentary records in the system are important to reveal past climate changes, environment evolutions, sediment sources, and anthropogenic activities. However, the correlation between sediment dynamic processes and sedimentary records in the muddy area is still unclear, which is vital for the interpretation of the record. This project tries to investigate the relation mentioned above, and proposes the muddy depositional system in the northern Taiwan Strait as the study area. By carrying out high-frequency and high-resolution observations of near-bed sediment dynamic processes under normal weather conditions in winter and summer as well as under extreme weather events with accompanying catchment processes, we want to understand the impact of extreme weather events and the accompanying catchment processes on the delivery of mud, formation of muddy belts and their preservation in sedimentary records. In addition, suspended sediment sampling and sediment trap mooring will be conducted at typical stations, and sediment exchange processes at the sediment-water interface and in the benthic layer will be studied by considering suspended particles, settling particles, surface sediment and core sediment as a vertical dynamic system. Based upon the proposed observations and analysis mentioned above, we will attempt to reveal the intensity, processes and associated mechanisms of disturbance contained the sedimentary records by the events, and discuss the sediment deposition and preservation processes influenced by the events. This study could provide scientific support for the sedimentary record interpretation; evaluate the function of the extreme weather events and their accompanying catchment processes on the evolution of the muddy depositional system.

陆架泥质沉沉积体系是陆海相互作用计划重点关注的对象之一，其中的沉积记录对揭示环境变化具有重要的意义，然而其沉积动力过程与沉积记录之间的对应关系目前还不清楚。本项研究拟以台湾海峡北部陆架泥质沉积体系为研究对象，通过对典型位置不同季节正常天气及突发性天气事件与流域过程影响下的海底沉积动力过程观测，结合悬浮体的空间分布调查、典型站位沉积物捕获器锚系同步观测，认识突发性天气事件与流域过程对台湾海峡北部泥质区悬浮颗粒物来源与分布的影响，将水体悬浮颗粒物-沉降颗粒物-表层沉积物-柱状沉积物作为一个垂向整体系统，分析沉积物与底层水体之间的沉积物交换过程，揭示突发性天气事件与流域过程在从源到汇的过程中对陆架原有沉积记录的扰动强度、过程及其扰动机制，探讨突发性天气事件与流域过程引起的沉积物沉降与埋藏过程及其控制机制，为正确解译沉积记录提供科学依据，并尝试探讨事件性天气与流域过程在泥质沉积体系演化过程的作用。

泥质沉积区是大陆架上广泛分布的一种沉积体系，是在单条或多条河流作为物质供应、在物理、化学和生物过程等多时空尺度综合作用下的一种现代活动沉积体系，其中的沉积记录对揭示气候变化、环境演变、沉积物来源和输送及其对人类活动的响应具有重要的意义。本项目以台湾海峡北部-闽江河口为研究区，通过断面重复调查，海底三脚架观测和沉积物捕获器观测，将水体悬浮颗粒物-沉降颗粒物-表层沉积物-柱状沉积物作为一个垂向整体，系统研究了台湾海峡北部泥质沉积体系演化与闽江入海泥沙“源-汇”过程的关系，揭示了闽江入海泥沙输运与极端天气事件在台湾海峡北部泥质沉积体系形成与演化过程中的作用。研究结果表明，夏季台湾海峡北部泥质区海底冲淤变化幅度较大，单个潮周期内的平均冲淤幅度可达到14.8 mm，最大值可达到17.9 mm；再悬浮起来的沉积物进入水体后，一般可影响到海底以上20 m水层。近底层沉降通量为0.86~10.68 g/m2•d，沉降颗粒物中有机碳以海源为主，其贡献率平均为83%；沉降颗粒物主要来源于海底沉积物再悬浮、南部上升流区的平流输送及北部闽江入海物质的输入，其中再悬浮贡献率平均为76%，底部温盐跃层的上移及跃层以下水体混合强度的增大了底部沉降颗粒物中再悬浮的贡献率。断面观测结果显示，闽江冲淡水在夏季可以到达平潭岛东北部陆架泥质区，但大量泥沙主要沉积在河口区，只有少量泥沙逃逸出水下三角洲前缘泥质沉积区而进入台湾海峡北部泥质区。遥感分析结果表明，秋、冬季节，在强烈东北季风的影响，闽江入海物质在沉积物再悬浮、陆架环流、潮汐运动等综合作用下向南输运，并沉积到台湾海峡中、北部海域，为泥质沉积区的形成提供了大量的物源。台风对闽江河口外陆架及台湾海峡北部陆架沉降颗粒物的影响主要体现在台风过境期间波浪作用导致的强烈再悬浮与再沉降，另一个是台风带来的强降雨导致大量陆源物质快速入海并被输运至闽江口外陆架及台湾海峡北部泥质沉积区，从而促进了台湾海峡北部泥质沉积体系的形成与演化。