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引用本文:李安春,张凯棣.东海内陆架泥质沉积体研究进展.海洋与湖沼,2020,51(4):705-727.
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东海内陆架泥质沉积体研究进展
李安春, 张凯棣
中国科学院海洋研究所 海洋地质与环境重点实验室 青岛 266071
摘要:
陆架海现代泥质沉积研究具有重要的地质学、环境科学和气候学意义,数十年来一直受到海洋科学家的关注,尤其是近年来在我国兴起了一股研究陆架泥质沉积体的热潮,取得一大批重要成果。本文对我国最大的东海内陆架泥质沉积体的研究进展进行回顾,作为典型区域呈献给读者。东海内陆架泥质沉积区从长江口水下三角洲向南,沿闽浙近岸浅海一直延伸到台湾海峡中部,全长800km,宽约100km,面积约80000km2,相当于两个台湾岛的大小,实属我国乃至亚洲浅海规模最大(体积排第二)的楔式泥质沉积体和现代沉积区。该区中晚全新世沉积地层厚度较大,局部厚达40—80m,总体上呈近岸厚、向海方向逐渐变薄,一般在50—60m等深线、局部可达75m甚至90m等深线附近尖灭。该区泥质沉积物粒度较细,主要由黏土和粉砂组成(大于90%),砂含量很低(小于10%),黏土和粉砂含量在泥质沉积区外缘急剧降低,而砂含量突然增加。沉积物类型为粉砂质黏土和黏土质粉砂,外侧与黏土-粉砂-砂(混合沉积)或泥质砂为界。粒度分布南北有一定差异,北段(长江口外至瓯江口外)近岸较粗外侧较细,近岸为黏土质粉砂,外侧为粉砂质黏土;而南段(自瓯江口至台湾海峡北部)则相反;深入台湾海峡中部的远端泥质沉积也较粗,为黏土质粉砂。现代沉积速率从长江口水下三角洲至闽浙沿岸近海以及从近岸向外陆架方向逐渐降低,与地层厚度分布相一致。悬浮体浓度空间分布,尤其是冬季悬浮体的分布与沉积速率的分布基本一致,表明沉积物是从长江口沿闽浙近海向南和从近岸向海输运的。矿物、化学和环境磁学指标等均显示沉积物主要来自长江,老黄河对该区北部,台湾物质对南部有一定影响,闽浙沿岸河流在局部也有少量贡献。该泥质沉积体的形成与全新世中期约7.3ka BP以来持续高海面及相应的总体沉积动力过程密切相关,主要包括闽浙沿岸流将长江物质源源不断地向南输送和沉积过程、台湾暖流、上升流在其外侧的阻挡作用,以及下降流和穿刺锋的横向输运等动力控制因素。沉积物输运主要发生在冬季,冬季风导致海洋动力加强的作用功不可没,热带气旋-台风风暴对泥质体起到了一定助长与破坏的双重作用。由于该泥质沉积体的形成与季风和沿岸流的密切关系,在形成过程中打上了气候环境的烙印,是冬季风和夏季风演化记录的良好载体,近年来的研究成果很好地揭示了中晚全新世以来千年、百年、十年尺度甚至更高分辨率的气候演化历史及气候突变事件。该泥质沉积区对人类活动的响应也较敏感,自3.0ka BP以来对长江流域燃火变化以及历史上中国人口的几次大迁徙均显现在沉积记录中,特别是对近几十年,尤其是三峡水库蓄水以来,长江来沙的变化也有明显的响应。
未来的研究需要进一步澄清泥质体形成发育过程不同阶段中长江、黄河、台湾、闽浙河流物质以及残留区物质的定量贡献及时空差异;深入了解人类活动的响应及环境记录研究;加强现代沉积动力过程的观测和精细的数值模拟研究,揭示泥质积区动力背景的空间差异性,这不仅是深化泥质沉积形成机制的需要,也可为古环境恢复提供科学支撑。
关键词:  东海内陆架  泥质沉积  形成机制  物质来源  气候记录  环境响应
DOI:10.11693/hyhz20200500145
分类号:
基金项目:国家自然科学基金,41430965号;国家自然科学基金,41806056号。
RESEARCH PROGRESS OF MUD WEDGE IN THE INNER CONTINENTAL SHELF OF THE EAST CHINA SEA
LI An-Chun, ZHANG Kai-Di
Key Laboratory of Marine Geology and Environment, Institute of Oceanology, CAS, Qingdao 266071, China
Abstract:
The study of modern mud sediments in the continental shelf sea has important geological, environmental, and climatological significance. Therefore, marine scientists have been paying close attention to the issues for several decades. Especially in the recent years, there has been a great upsurge in the study of continental shelf mud sediments in China, and a large number of important achievements have been made. The research progresses of the largest mud wedge in the East China Sea were reviewed.
The mud sedimentary zone in the inner shelf of the East China Sea extends southward from the underwater delta of the Changjiang (Yangtze) River estuary to the middle of the Taiwan Strait along the shallow coastal waters off Fujian and Zhejiang Provinces. The total length is about 800km, the width is about 100km, and the area is about 80000km2, which is about two Taiwan Islands in area. It is the largest wedge-type mud sedimentary body and modern sedimentary area in the China shelf sea and even in the coastal waters of Asia (the second in volume). The middle and late Holocene sedimentary strata in this area are relatively thick, with a local thickness of 40-80m. Generally, they are thicker near-shore, gradually thin out oceanward, and vanish near the 50-60m isobath or even deeper at 75-90m isobath. Mud sediments in this area have a relatively fine grain size and are mainly composed of clay and silt (more than 90%); however, sand content is very low (less than 10%). The clay and silt contents decrease sharply in the outer edge of the mud zone, while sand contents increase suddenly. The type of the sediment is silty clay and clayey silt, which is bounded laterally towards the sea by clay-silt-sand (mixed deposit) or muddy sand. There are some differences in the grain size distribution between the north and the south. In the northern section (from the Changjiang River estuary to the outside of Oujiang River mouth), the grain size is coarser (clayey silt) nearshore, and finer (silty clay) laterally; while in the southern section (from the outside Oujiang River mouth to the north of the Taiwan Strait), the trends are opposite. The distal mud sediment to the middle Taiwan Strait is coarser (clayey silt). The modern deposition rates are higher in the subaqueous delta of the Changjiang River estuary and near the coastal waters off Fujian-Zhejiang Provinces, but they gradually decrease alongshore to the outer continental shelf, which are consistent with the thickness distribution of the Holocene strata. The spatial distribution of the suspended matter concentration, especially in winter, was consistent with the distribution of the deposition rate, which indicates that the sediment was transported from the Changjiang River estuary to the south along coast off Fujian and Zhejiang Provinces and from the nearshore to the sea. The mineral, chemical elements, isotopes, and environmental magnetism indexes further indicate that the mud deposits are derived mainly from the Changjiang River, followed by the old Huanghe River contributes in the north and from Taiwan Island in the south, and the coastal rivers of Fujian and Zhejiang Provinces locally. The formation of this mud sediment wedge was closely related to the continuous high sea level about 7.3ka BP ago since the middle Holocene and the corresponding sedimentation, which includes the continuous southward transport of Changjiang River materials by the Fujian-Zhejiang coastal current, the blocking of the warm current and the upwelling current as well as the lateral transport of the downwelling and the front puncture currents, etc. Transport of the sediment occurs mainly in winter, and enhanced by winter monsoon. Tropical cyclone and typhoon storm have played a dual role in building and destroying the mud wedge. Because of the close relationship between the formation of the mud sediment and the monsoon-driven coastal current, the mud wedge recorded the imprint of the climate environment and became an ideal object of study for the evolution of winter and summer monsoons. In recent years, abundant research results of this region have well revealed the climatic evolution history and events on the scales of millennium, centennial, decadal, and even higher resolution since the middle and late Holocene. The muddy deposit is also very sensitive to human activities, and it documented major anthropological events such as variation in fire about 3000 years ago in the Changjiang River reaches, and several great migrations of Chinese population in history, etc. Especially in the recent decades, particularly since the impoundment of the Three Gorges reservoir, there has also been an obvious response to the changes in the sediment source from the Changjiang River.
In the future, it is needed to further quantify the contribution and spatial and temporal differences of the substances from the Changjiang River, Huanghe River, and rivers of Taiwan Island and Fujian-Zhejiang lands, and the residual areas at different stages of the formation and development of mud bodies; deepen the understanding the impacts of human activities and environmental change on the mud wedge; strengthen the observation of the modern sedimentary dynamic process with fine numerical simulation; and reveal the spatial difference of the dynamic background in the mud sediment zone. These works will help understand the formation mechanism of the mud sediments, and provide scientific clues for the reconstruction of the paleo environment.
Key words:  East China Sea inner shelf  mud deposits  formation mechanism  material source  climate records  environmental response
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