海洋地质与环境重点实验室知识库

     本文通过对东海陆架表层沉积物及岩芯沉积物的粒度、碎屑矿物、单矿物和环境磁学分析，研究了东海陆架近代沉积的源汇过程，揭示了人类活动及洪水、台风等突发事件对陆架沉积的影响以及近代气候变化和东亚冬季风的强度变化在东海内陆架沉积中的记录。
     研究结果表明，东海陆架表层沉积物粒度分布存在明显的季节变化，且泥质沉积区粒度的季节变化较砂质沉积区更明显。沿岸河流洪、枯季入海输沙量的差异，季风驱动下的长江冲淡水、沿岸流及台湾暖流方向、强度的变化是表层沉积物粒度发生季节性变化的主控因素。
     碎屑矿物鉴定结果显示，东海陆架表层沉积物中的重矿物以角闪石、绿帘石、片状矿物和白云石为主，轻矿物则以长石和石英为主。按碎屑矿物组成可将东海陆架划分为三个矿物区，其中内陆架矿物区碎屑矿物组合与现代长江物质一致；外陆架矿物区沉积物中比重大的矿物含量相对较高，且矿物多蚀变，主要为古长江物质；而虎皮礁矿物区则受到长江、黄河物质以及火山物质的共同影响。

     磁学分析表明，东海陆架表层沉积物中的磁性矿物主要由磁铁矿、磁赤铁矿和赤铁矿组成，其中磁铁矿主导了研究区沉积物的磁学性质。研究区西部内陆架及东北部济州岛西南泥质区沉积物中磁性矿物颗粒细，以单畴、准单畴为主，同时含有较多的超顺磁颗粒。而在研究区中外陆架，磁性矿物颗粒较粗，以准单畴和多畴为主，很少出现超顺磁颗粒。

     表层沉积物的碎屑矿物特征及环境磁学特征均为判断沉积物来源提供了依据。将陆架沉积物碎屑矿物组合特征，单矿物石榴子石的化学成分特征以及沉积物磁学参数S_-100/SIRM与长江、黄河沉积物进行对比发现东海陆架沉积物主要来源于长江，北部受到黄河物质的影响。黄河沉积物在31°N以北的东海陆架广泛分布，向南黄河物质对东海陆架的影响不明显。此外，闽浙沿岸部分站位沉积物受到人类活动及瓯江物质的影响。

     东海陆架表层沉积物的粒度分布在浙江北部沿岸（29°N附近）出现一明显的跨陆架细粒沉积物高值区。该细粒沉积物高值区平均粒径较小>4.4 Ф，向东扩散的最大距离超过200 km。同时，在29°N附近，重矿物百分含量存在一跨陆架低值区，而轻矿物中的片状矿物含量、重矿物中白云母含量则有一明显的跨陆架舌状高值区。环境磁学参数χ_fd%、χ_ARM平面等值线图在相同的位置也出现跨陆架舌状高值区。这表明，东海内陆架的现代陆源物质不仅沿岸输送也存在跨陆架扩散。从细粒沉积物含量分布看细粒级沉积物（尤其是粒级小于7 Ф组分）能被输运至124°40′E的外陆架甚至更远的区域。跨陆架细粒沉积物高值区的位置及其季节性变化的特征与水文现象跨陆架锋相一致。

      东海内陆架沉积物中记录了自二十世纪五十年代以来由人类活动（尤其是兴建水坝）导致的长江输沙量的骤减。由于长江河道和水下三角洲的侵蚀，东海内陆架沉积物自1987年开始变粗，2003年三峡大坝投入使用后沉积物粒度进一步加粗。同时，东亚季风也通过控制沿岸流的强弱影响了东海内陆架的沉积过程。岩芯沉积物中细敏感粒级平均粒径的变化与东亚冬季风的强度变化有很好的一致性，部分站位沉积物记录了自1987年东亚冬季风的减弱。沉积物粒度短时间尺度的突然变化通常是由洪水和台风导致的，但洪水和台风事件在东海内陆架的影响范围有限。功率谱分析结果显示岩芯沉积物平均粒径表现出3–8 a，10–11 a和20–22 a的变化周期，分别与ENSO周期和太阳活动的周期（Schwabe周期和Hale周期）相对应。
     东海内陆架岩芯SS4为约1.5 ka以来的沉积，沉积物中普遍存在的风化云母及高含量的白云石表明SS4沉积物主要来源于长江。但含量相对较高的绿帘石表明闽浙沿岸小河流物质对SS4孔也有一定贡献。SS4孔沉积物碎屑矿物含量和粒度变化很好的记录了隋唐暖期和小冰期的古气候变化及东亚冬季风的强度波动。沉积物敏感粒级的平均粒径记录了小冰期中的后两个冷期（1620–1710 a AD冷期和1800–1860 a AD冷期）。在1250–1550 a AD低温时期敏感粒级并没有表现出粒径的大幅度增加，而几种主要重矿物含量对此却有记录，在1250–1550 a AD低温时期闪石类矿物及钛铁矿磁铁矿含量较高，而云母类矿物含量很低，反映了活跃的水动力环境和较强的东亚冬季风。此外，以约800 a BP (1215 a AD) 为界限，绿帘石/角闪石比值的显著变化表明了东亚冬季风强度的变化。

    Based on the grain size, detrital minerals and magnetic characteristics analyses of the surface sediments and sediment cores collected in the East China Sea continental shelf, the source to sink processes were studied. In this paper, the deposition process in the modern East China Sea continental shelf, the effects of human activities and climate disasters such as floods and typhoon on the sediment deposition and the sediment records of the climate and the East Asian Winter Monsoon changes during the past 1.5 ka have been discussed.

    Surficial sediment grain size distribution in the East China Sea continental shelf presents apparent spatial characteristics and seasonal variations. Mud area presented more variations in grain size than sand one. This phenomenon indicates that sediment supply is the main controlling factor of the surficial sediment grain size distribution on the East China Sea continental shelf.

    A total of 49 species of heavy minerals were distributed in the research region, dominated by hornblende and followed by epidote, schistose mineral, dolomite,garnet, magnetite and so on. Light minerals mainly consist of feldspar followed by quartz; schistose minerals and calcite are respectively in the third and fourth row. According to the characteristics of detrital mineral assemblages, the East China Sea continental shelf can be divided into three districts: Inner-shelf area (Unit I), Outer-shelf area (Unit II) and Tiger Reef area (Unit III). Sediments in Unit I are mainly from the Yangtze River and the Fujian and Zhejiang coastal rivers. In Unit II, sediments are mainly composed of the ancient Yangtze River material. Unit III has relatively varied provenances and complicated hydrodynamic environment. The materials from the Yellow River, the Yangtze River and submarine volcanos both influence there.
    Magnetic mineralogy in the East China Sea continental shelf is dominated by magnetite with a small amount of hematite and maghematite, which is consistent with the results of detrital minerals identification under the microscope. Magnetic parameters (frequency-dependent magnetic susceptibility (χ_fd%), χ_ARM, χ_ARM/χ and χ_ARM/SIRM) suggested that the magnetic mineral particles are finer in the west of the study area. In the southwest Cheju Island sediments are dominated by single-domain and pseudo-single-domain particle with relatively high concentration of super-paramagnetic particles. While in the middle-outer continental shelf the magnetic mineral particles are dominated by pseudo-single-domain and multi-domain particles.

   The detrital mineral and magnetic characteristics of surface sediments provide the basis for judging the provenance of sediments in the continental shelf. Compared the detrital mineral assemblages, the garnet component and the magnetic parameters of the sediments in the continental shelf with the sediments of Yangtze River、Yellow River, we found that the sediments in the East China Sea continental shelf are mainly derived from the Yangtze River and the northern part is affected by the Yellow River. The sediments of the Yellow River are widely distributed in the East China Sea shelf north of 31°N, mixing with the Yangtze river sediments. In Zhejiang Fujian coastal, parts of the sediments are affected by human activities and Oujiang material.
   Surficial sediment grain size distributions show significant cross-shelf transport trend in the East China Sea. Fine-grained sediment can be spread to the outer continental shelf beyond the east of 124°40'E or even farther near 29°N. This phenomenon is also reflected in the distribution of detrital minerals and the magnetic properties of sediments. In the area near 29°N, heavy mineral content appears a cross shelf low value area while schistose minerals show a high value front. This is the result of the cross transport of the schistose minerals which have relatively small density and easily to be suspended. Magnetic parameters such as χ_fd%、χ_ARM also show a high value cross shelf front in the same area. It indicates that the modern terrigenous materials on the East China Sea inner shelf not only transported along the coast but also diffused crossing the shelf. Locations and directions of the cross-shelf fine-grained sediment fronts have seasonal variations, with northeastward trend in summer and eastward one in autumn. These trends were closely related to the hydrological phenomena of Cross-shelf Penetrating Fronts.
   Sediments in the East China Sea inner shelf have recorded the sudden drop in sediment load of the Yangtze River since 1950s, caused by human activities (dam constructions, especially the Three Gorges Dam closed). Sediment grain size became finer after 1969 and turned to coarsen since 1987 and then further coarsen since 2003, which accord well to the three steps of sediment load drop in the Yangtze River. Simultaneously, the East Asian Winter Monsoon has influenced the deposition process in the East China Sea inner shelf by changing the intensity of coastal current. Mean grain size variations of the fine population (divided by grain size vs. standard deviation method) are coincided with that of the East Asian Winter Monsoon strength and recorded its weakness in 1987. Abrupt changes of sediment grain size in a short time scale are generally caused by floods and typhoons. However, the effects of river floods and typhoons are limited in scope. Spectral analyses of the sediment cores show statistically periodicities centering on 10–11 and 20–22 years, corresponding to the periodicity of solar activity (Schwabe cycle and Hale cycle). Mean grain size time series also displays a 3–8 years periodicity corresponding to El Niño Southern Oscillation (ENSO) periodic change.
   The widespread weathering mica and high content of dolomite in core SS4 located in the East China Sea inner shelf indicate that the sediments are mainly from the Yangtze River. However, the relatively high content of epidote may represent the impact of the local material. The variation of the detrital mineral content and grain size of sediments in core SS4 have well recorded the climate changes in the Sui-Tang periods and in Little Ice Age (LIA) and the intensity fluctuation of the East Asian Winter Monsoon. Mean grain size variations of the sensitive components have reflected the last two cold periods in the LIA (1620–1710 a AD and 1800–1860 a AD), but showed no significant increase during the cold period in 1250–1550 a AD which has reflected both in the oxygen isotope curve and the temperature curve. However, several major detrital mineral contents have recorded this cold event. During 1250–1550 a AD, the content of amphibole, ilmenite and magnetite was high, but the content of mica was very low indicating an active hydrodynamic environment and strong East Asian Winter Monsoon. Therefore, the combination of the analysis results of detrital minerals and sensitive grain size can more accurately reflect the changes of paleoclimate and make up for the deficiency of reconstructing paleoclimate by selecting different sensitive grain components. The obvious change of the ratio of epidote / amphibole also indicates the variation of the East Asian Winter Monsoon.