基于单矿物思路的南海西北大陆架细粒碎屑沉积物物源研究

Recent studies show that the world’s rivers annually discharge about 19 billion tons of fine-grained sediments to the ocean. Studies on the source-to-sink processes of fine-grained sediments are very important to understand the global circulation of materials on the earth. The provenance study is one of important ways to acquire the information about the source-to-sink processes of fine-grained sediments. Until the present, most studies on fine-grained sediment sources have been based on bulk geochemistry and clay mineralogy. However, these methods are always heavily influenced by chemical weathering, grain size and mineral sorting. In the past a few years, substantial progress has been achieved in the provenance study of fine-grained sediments in the South China Sea, promoting the sedimentologic and oceanographic studies in this region. Previous provenance studies based on clay mineralogy and bulk geochemistry identified main sources of fine-grained sediments in the basin scale. However, previous provenance studies haven’t successfully discriminated or identified different sediment sources in the northern South China Sea. In this project the exceptional hydrodynamic property of mica with its plate shape would be utilized to build a bridge between the single-mineral chemistry and the provenance of fine-grained sediments. Single-mineral chemical compositions of biotite and muscovite concentrated in fine-grained sediments will be used to trace fine-grained sediment sources. In this project, the novel approach using single-mineral methods will be used to study the source of the muddy belt on the northwestern shelf of South China Sea, including the modern and Holocene sediments. Moreover, Nd and Hf isotopes of fine-grained fractions will be measured to mutually verify the source results by single-mineral chemistry of biotite and muscovite. This provenance study would greatly minimize the effects of chemical weathering and hydrodynamic sorting and would supply much clearer knowledge about the surficial sediment sources of the muddy belt in the northwestern South China Sea. Integrated provenance and grain-size results of two sedimentary cores will be used to rebuild the depositional history of the muddy belt. This study will deepen our knowledge about the influence mechanism of river-sea interaction history on the shelf sedimentation in the northern South China Sea. Moreover, this study would offer a new provenance approach for fine-grained sediments and would promote the source-to-sink studies on fine-grained sediments in the ocean.

细粒沉积物是河流入海泥沙的主要部分，研究细粒沉积物“源-汇”过程对了解全球物质循环具有重要意义。物源研究是了解沉积物“源-汇”过程的重要途径。目前细粒物源研究，其手段以全岩地球化学和粘土矿物为主，普遍受到化学风化、粒度效应、重矿物分选等影响。在南海，细粒物源研究推动了南海沉积学的发展。全岩地球化学和粘土矿物虽能在大尺度上识别南海细粒沉积物物源，但是在小尺度上还未能有效识别北部各源区。本研究利用云母水动力特性，将应用在砂质沉积物上的单矿物方法引入到细粒物源研究之中，通过研究富集于细粒沉积物中的云母颗粒的矿物地球化学示踪沉积物。本研究拟选择南海西北大陆架，通过云母单矿物地球化学以及细粒组分Nd、Hf同位素，准确判定出大陆架泥质带表层及全新统地层中细粒物源构成。精细的细粒沉积物物源和粒度研究，将查明南海西北大陆架泥质带的形成历史，为深入研究河海交互历史对南海北部大陆架沉积的影响机制提供条件。

南海是一个西太平洋典型封闭的边缘海盆，河流沉积物的输入是海盆沉积物最主要的来源，追溯沉积物的来源是分析南海的区域构造演化、重建古地理以及研究海盆不同时期海洋环流系统重要的途径之一。项目选择南海西北陆架为研究区，开展了细粒沉积物现代的源-汇研究，对影响沉积物矿物、地球化学组成的三个方面展开了深入研究，包括化学风化、沉积物分选以及流域的基岩特征。通过2um粒级粘土组分的元素地球化学组成，确定了华南地区河流沉积物的当前风化趋势线，只有粘土粒级沉积物化学风化强度（CIA）受控于流域现在温度和降水条件。筛选出三个区分华南水系沉积物的元素地球化学和同位素地球化学指标，即Ti/Nd、δEu和εNd。在南海北部华南主要水系小于31um细粒沉积物的Ti/Nd、δEu主控于流域基性岩和酸性岩比例关系，与化学风化强度没有显著相关性，分选效应不会掩盖流域背景地球化学差异。河流沉积物的εNd与流域岩性以及结晶岩年龄相关。大部分华南河流沉积物小于31um细粒组分的εNd的粒度分异都在1以内，与全球大河一致。通过Ti/Nd、δEu、εNd地球化学指标定量计算的珠江细粒沉积物的物源构成与河流泥沙监测数据吻合。华南沿海的粘土矿物主要受控于风化强度，高岭石/伊利石比例与粘土组分的CIA指数一致，珠江的高岭石含量要低于其他入海河流。在华南一些河流入海口显著提高的蒙脱石含量可能非物源因素影响。基于单矿物地球化学方法查清了粤东、闽南河流沉积物在南海东北陆架的分布大势。建立了南海西北陆架两个新获取岩芯的沉积地层和年代框架，构建了陆架泥质带全新世的演变历史，发现泥质带在3000 BP以后存在加速扩展和细化的趋势，提供了泥质带演化与人类活动关系的地球化学证据。通过本项目的研究，深入分析了化学风化、沉积物分选效应对入海河流细粒沉积物的矿物、地球化学组成的影响程度，明确了后续南海北部古风化研究以及细粒沉积物物源判定研究时需要考虑的指标限制性以及粒级组分。