Knowledge Resource Center for Ecological Environment in Arid Area
| 江苏沿海潮滩和钻孔沉积物磁性矿物特征及其环境意义 | |
| 其他题名 | Magnetic characteristics of the sediments from tidal flat and the drilling along the Jiangsu coast, Eastern China |
| 王龙升 | |
| 出版年 | 2016 |
| 学位类型 | 博士 |
| 导师 | 胡守云 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 江苏沿海潮滩及岸外南黄海辐射沙脊群位于我国东部沿海，是连接青藏高原、中亚干旱区和西太平洋边缘海的重要枢纽。第四纪以来该区接收了巨厚沉积物，其中蕴含着丰富的气候和环境信息，是探讨海陆相互作用、三角洲发育史、海平面变化、以及亚洲宏观环境变迁的重要靶区之一。此外，作为青藏高原侵蚀物质的汇聚地之一，该地区在探索长江迁移、流域化学风化方面的作用也已引起广泛重视。近年来，已有学者对该地区的碎屑矿物分布与组合特征、沉积层结构、形成与演变规律、沉积环境演化进行了研究和分析，然而该地区沉积物的磁性特征研究尚未进行深入探讨。本文选取江苏沿海潮滩沉积物以及南黄海辐射沙脊群的Y2和YZ07孔沉积物进行研究。通过结合古地磁、光释光、AMS14C等测年数据以及粒度、烧失量、有孔虫、硅藻等环境替代指标分析该地区沉积物中磁性矿物特征，并探讨磁性矿物的古环境意义。（1）首先进行江苏海岸现代海安沉积和磁学样品调查和测试，以期获得对现代过程的认识和用来对地质时期的解释。江苏海岸北段为基岩海岸，云台山深入黄海形成了多个砂纸海湾和岛屿。东西连岛中指示磁性矿物含量的磁性参数（c、ARM和SIRM）最低，表明磁性矿物含量较低。除此之外，c、ARM和SIRM的值从北到南显示出明显的升高趋势。c和ARM、SIRM两者相关性较好，表明顺磁性和超顺磁性矿物都对样品磁性贡献不大，亚铁磁性矿物才是数据样品的主要磁贡献者。同时这种线性关系表明磁性矿物的粒径变化不大，因而磁性的变化主要反应了磁性矿物的含量变化。依据磁性矿物和粒度参数变化，结果表明：东西连岛主要来源于近岸岩石风化。在江苏海岸中部历史时期（南宋~清朝晚）形成了黄河入海三角洲沉积，1855年黄河改道后，仍保留了废黄河口。同时在现代长江口采集潮滩沉积物，研究表明：大丰以北潮滩沉积物主要来源于黄河沉积物。如东以南潮滩沉积物主要来源于长江，两者之间受两条大河的共同影响。岩石磁学表明，江苏沿海潮滩沉积物的磁性矿物主要以磁铁矿为主导，含有少量的赤铁矿。粒度与磁性参数相关性分析表明：磁铁矿主要富集在细粉砂中，物源及风化作用的差异是造成江苏沿海潮滩沉积物磁性差异的主要因素。（2）Y2孔位于南黄海辐射沙脊群北翼东沙滩面，现代沉积动力主要受到中低纬度南黄海不规律半日潮汐作用，然而全新世该区经历过长江和黄河古三角洲沉积过程。Y2孔主要包含四个主要的沉积相，自下而上分别是：河床相（51.5-60m）、潮滩相（31.5-51.5）、古土壤（21.5-31.5m）和潮流沙脊相（0-21.5m）。在潮滩相中包含两个古土壤层。河床相主要发育于45-25 ka B.P.，古土壤层发育于25-12 ka B.P.，潮流沙脊相主要发育于7 ka B.P.至今。由于海侵作用，Y2孔并未包含早全新世发育的沉积物。在不同的沉积相中，磁性矿物不同。潮流沙脊相、潮滩相和河床相主要以软磁性矿物（磁铁矿）为主导，含有少量的硬磁性矿物（赤铁矿）。古土壤层和潮滩中包含的薄古土壤层主要以硬磁性矿物（赤铁矿）为主导。粒度指标显示：潮流沙脊相主要是砂为主，古土壤层主要以粘土成分为主，河床相和潮滩相主要以粉砂为主。磁性参数和粒度指标相关性研究显示，磁铁矿主要富集在砂组分中。（3）YZ07取自南黄海辐射沙脊群南翼，现代长江口北翼的南潮滩区，历史时期海平面升降变化，长江河道变迁对采样点的沉积过程产生重要影响，提供了多次海侵海退的沉积记录。YZ07孔的地层层序记录了过去130ka以来的古气候、古环境变化的信息。根据生物指标以及沉积相分析，MIS1时期主要发育潮滩-滨海相，MIS3时期主要发育浅海相，MIS4时期主要发育河口-河漫滩相，MIS5时期主要发育滨海-河口三角洲相，MIS6时期主要发育河流-湖沼相。由于海侵作用及沉积物较薄，YZ07孔中并未保存MIS2时期的沉积物，岩石磁学研究表明：在不同的沉积相中，磁性矿物不同。海相和河流相主要以软磁性矿物（磁铁矿）为主导，含有少量的硬磁性矿物（赤铁矿）。湖沼相主要以硬磁性矿物（赤铁矿）为主导。（4）磁学指标是认识海岸沉积过程和特征的一个重要手段。通过对Y2和YZ07孔的磁学研究，并结合江苏沿海现代潮滩沉积物的磁学分析，发现几点关联和新认识：第一，关于沉积物源：黄河和长江是江苏沿岸沉积物的主要来源，结合黄河和长江现在沉积物的磁学分析，黄河和长江现代沉积物均以低矫顽力的软磁性矿物（磁铁矿）为主，因此，笔者认为沉积物来源的改变并不是影响Y2和YZ07孔磁性矿物的主要因素。第二，结合区域以及全球变化曲线，笔者发现沉积物中磁性矿物的含量受控于区域和全球海平面变化的影响。在高海面时期，主要发育海相沉积，磁性矿物含量较高，主要以磁铁矿为主导。在低海面时期，主要发育河流相和湖沼相，其中河流相磁性矿物含量与海相层相当，以磁铁矿为主导；湖沼相磁性矿物含量明显低于海相和河流相，主要以赤铁矿为主导，这与低海面磁性矿物的来源减少有关。第三，关于还原成岩作用的影响：根据Y2和YZ07孔烧失量和磁学参数分析研究，在岩芯中河流相和海相受到的还原成岩过程的影响较弱，而在湖沼相和古土壤层中受到的还原成岩作用较强，而且在湖沼相和古土壤层形成时期，沉积物中磁性矿物来源明显减少。第四，研究区硬粘土层（包括古土壤层和湖沼相）的判定一直是研究的重点和难点，磁学研究为研究区硬黏土层的磁学解释和判别提供了重要的磁学依据。由于硬黏土层形成时期为低海面时期，此时磁性矿物的供给明显减少；同时，由于硬黏土层中有机质含量明显高于其他层位，沉积环境为还原环境，还原成岩作用明显高于其他层位，细颗粒的磁铁矿大量溶解，而粗颗粒的赤铁矿得以保存。以上两原因造成了硬黏土层中磁性矿物的含量明显低于其他层位，磁性矿物以赤铁矿为主导，这为以后从磁学角度鉴别硬黏土层提供了理论基础。 |
| 英文摘要 | The tidal flat and the radial sand ridge field in the South Yellow Sea along the Jiangsu coast is located on the east coast of China, closely linking the Tibetan Pleteau, the arid central Asia and the west Pacific marginal sea. It accepted the very thick clastic sediment during the Quaternary age, and thus provides great information to better understand and solve the stratigraphy of coastal zones, marine transgression events on the coastal plain and palaeoenvironmental problems involving the evolution of East Asian monsoon and the sediment-to-source process during the Quaternary. Recently, many scholars have studied the detrital mineral distribution and composition composition characteristics, sedimentary structure, formation and evolution, sedimentary environmental evolution in this area. However, there lacks the research about the magnetic characteristic of the sediments. In this study, we select the sediments from the Jiangsu coast and the core Y2 and YZ07 of the radial sand ridge field. Combined the paleomagnetism, OSL, AMS14C, grain-size, Loss-on-ignition(LOI), foraminifer and diatom, we discussed the palaeoenvironmental significance of magnetic minerals. The major research results are as follows:(1) The changes of magnetization of the tidal flat sediments mainly reflect the changes of concentration rather than those in grain sizes of magnetic particles. The sediments of Liandao Island have the lowest values of magnetic susceptibility (c), anhysteretic remanent magnetization (ARM) and saturation isothermal remanence magnetization (SIRM). In addition to this, the values of c, ARM and SIRM display a generally increasing trend from the north to the south along the Jiangsu coast. The results indicated that the tidal flat sediments of Liandao Island were mainly from the nearshore rock weathering. The Yellow River is the dominant supplier for the north of Dafeng, while the Yangtze River is the dominant supplier for the south of Rudong. The coast between Dafeng and Rudong is the transition zone influenced by both rivers. Rock magnetic measurements show that the main magnetic phases is magnetite, with a small amount of hematite. The strong relationships between c and the 2-16 μm fraction suggest that magnetite is enriched in the finer silt fraction. The sediment sources are the main factors that influence the magnetic properties of the tidal flat sediments from the Jiangsu coast. This study provides a baseline for future assessment of possible variations in sediment composition caused by various sources.(2) Sediments of radial sand ridge field are commonly represented by a set of flood plain, paleosol, and tidal sand facies. In this paper, measurements of magnetic properties and particle size were obtained from Core Y2 from the radial sand ridge field in the South Yellow Sea. The results show that each facies has specific magnetic minerals and particle size distributions. In the flood plain, sand and coarse silt are the main grain sizes, magnetite and subordinate hematite comprise the magnetic minerals. The clay and fine silt are the main grain-size distributions of paleosol, the magnetic minerals are dominated by hematite, and the soft-magnetic mineral content is very low. In the tidal sand ridge, sand is the predominant grain-size distribution, and magnetite dominates the bulk magnetic properties, with small amounts of hematite. The strong relationships between c and the >63-μm fraction suggest that magnetite is enriched in the coarse sand fraction. Based on the combined mineral-magnetic, particle size and loss-on-ignition data, the low c value of paleosol mainly resulted from the decrease in the magnetic mineral inputs and post-depositional dissolution of the magnetic minerals. The results show that the magnetic minerals were controlled by changes in the climate and hydrodynamic environment. This study not only provides a basis for a radial sand ridge field paleoenvironmental reconstruction by integrating detailed logs of magnetic properties and grain-size parameters, but it also provides constraints for exploring the land-sea interactions recorded in the sediment cores as a part of the earth system, and thus demonstrates the effectiveness of this approach .(3)The core YZ07 have recorded the palaeoclimatical and palaeoenvironmental information since 130ka. Based on the analysis of sedimentary facies and biological indicator, the main sedimentary facies during MIS1 are tidal flat and coastal facies. The main sedimentary facies during MIS3 are neritic facies. The main sedimentary facies during MIS4 are estuarine and alluvial flat facies. The main sedimentary facies during MIS5 are coastal and river delta facies. The main sedimentary facies during MIS6 are fluvial and lacustrine facies. Owing to the transgression, core YZ07 can’t contain the sediments formed during MIS2. Rock magnetization suggested that different facies have different magnetic mineral. The main magnetic mineral in marine and fluvial facies are magnetite, with small amount of hematite. The main magnetic mineral in lacustrine facies are hematite.(4) Magnetism is an important proxy for understanding the sedimentary process and characteristics. By the magnetic research of the core Y2 and YZ07, and combined with magnetism analysis of the modern tidal flat sediments along the Jiangsu coast, some new understanding was found: Firstly, about the sedimentary source. The magnetic analysis show the main minerals of the sediments along the Jiangsu coast was soft magnetic minerals. Therefore, we think the change of sedimentary source is not the main factor influencing the magnetic minerals of core Y2 and YZ07. Secondly, Combined with the regional and global curve, we found that the magnetic mineral of the sediments was influenced by the changes of regional and global sea level. The main sedimentary facies during MIS1 are tidal flat and coastal facies. The main sedimentary facies during MIS3 are neritic facies. The main sedimentary facies during MIS4 are estuarine and alluvial flat facies. The main sedimentary facies during MIS5 are coastal and river delta facies. The main sedimentary facies during MIS6 are fluvial and lacustrine facies. Owing to the transgression, core YZ07 can’t contain the sediments formed during MIS2. Rock magnetization suggested that different facies have different magnetic mineral. The main magnetic mineral in marine and fluvial facies are magnetite, with small amount of hematite. The main magnetic mineral in lacustrine facies are hematite.Thirdly, based on the analysis of LOI and magnetic parameters of core Y2 and YZ07, The negative correlation between χ and LOI indicate that there are more detrital input in the marine and fluvial facies, therefore low LOI values lead to weak reductive dissolution, and magnetite was preserved in the marine and fluvial facies. On the contrary, there are less detrital input in lacustrine facies, high LOI values lead to strong reductive dissolution, and little magnetite left. As a whole, detrital input was low in lacustrine facies, and generally a strong reductive dissolution because of high LOI values.Fourthly, it was always difficult to distinguish terrestrial stiff mud including the palesols and lacustrine facies. Magnetic research provides an important magnetic basis for identify terrestrial stiff mud. Because the terrestrial stiff mud was formed in the low water period, at this time, the supply of magnetic minerals decreased significantly. At the same time, because the organic matter content in the terrestrial stiff mud was obviously higher than that of other facies. Sedimentary environment was the reductive environment. Reductive diagenesis is significantly more serious than the other facies. fine particles of magnetite was dissolved, and coarse grained hematite is preserved. Above two reasons, the magnetic content of the terrestrial stiff mud is lower than that of the other facies. The main magnetic mineral was hematite. This provides a theoretical basis for indentify the terrestrial stiff mud from the perspective of magnetism. |
| 中文关键词 | 江苏沿海 ; 环境磁学 ; 还原成岩 ; 海平面变化 |
| 英文关键词 | Jiangsu coast Environmental magnetism Post-depositional diagenesis sea level change |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 自然地理学 |
| 来源机构 | 中国科学院南京地理与湖泊研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/287734 |
| 推荐引用方式 GB/T 7714 | 王龙升. 江苏沿海潮滩和钻孔沉积物磁性矿物特征及其环境意义[D]. 中国科学院大学,2016. |
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