|关键词||北黄海 冷水团 悬浮体 沉积环境 絮凝作用|
北黄海冬、夏季温、盐分布显示，冬季水体垂向混合均匀，而夏季水体层化强烈。夏季鲁北沿岸为冬季残留的相对高盐水，鲁北沿岸流消失，在其外侧有自渤海经海峡中部流向北黄海中西部的低盐水，研究区东部存在由南黄海向西北进入北黄海的高盐水，二者与辽南沿岸流在表层形成向北黄海中部汇聚的气旋式环流。中部海域下层大部分区域被冷水团占据，冷中心（5.97 ℃）位于122 °E，38.5 °N附近。夏、冬季温、盐数据的欧氏距离计算结果证实了前人对冷水团垂向环流的模拟结果，即冷水团边缘锋区和冷中心为上升流，两者中间区域为微弱的下降流。夏季蓬莱、成山头近岸的强潮流区存在显著的潮汐锋。
北黄海夏季悬浮体以大颗粒为主，按粒径特征可分为小型（<32 μm），中型（32-256 μm）以及大型颗粒（>256 μm）三类。大型颗粒以生源物质为主，是表、中层悬浮体的主要部分，包括胶状物质粘连小颗粒形成的颗粒聚集体以及浮游生物单体；中型颗粒体积浓度在底层最高，由再悬浮的细颗粒物质在水体中絮凝而成。沿岸水体以及冷水团内不同区域的悬浮体性质有显著差异，这主要与冷水团导致的动力环境差异和生物活动有关：其内部强潮流区，悬浮体有机质含量较低、剪切力较强，致使絮团粒径较小（<128 μm），结构较紧实；而冷中心附近底层悬浮体中有机质含量较高、动力环境较弱，促进了粒径较大的絮团形成（>128 μm），其结构较松散；鲁北沿岸底部水体环境虽然也有较强的剪切力，但其再悬浮的细颗粒较多，有机质含量较高，大絮团在强动力下破碎的同时也有新的大絮团形成，使悬浮体中粒径较大和较小的絮团共存。另外，烟台-威海中段近岸的冷水团边缘，由于潮流较弱，又有较强上升流减缓细颗粒物质沉速，促进大絮团的形成，从而使该区域底层悬浮体中絮团粒径显著大于其他区域。
北黄海冷水团的环流、动力环境对陆源物质的输运和沉积有重要作用。浊度的断面分布显示，沿岸水体受陆架潮汐锋阻碍难以直接向北黄海中部扩散。但是，潮汐锋区浊度的剖面分布在跃层附近出现小峰值，指示出锋区受潮混合影响而再悬浮的沉积物在上升流和跃层附近离岸流的共同作用下向冷水团内部扩散的过程。底层无机颗粒浓度、粒度分析结果显示，夏季水体运移的陆源物质主要为粒径< 64 μm的细颗粒。无机颗粒体积浓度在底层的分布由成山头西北近岸的强潮流区向北黄海中部降低，在位于弱潮流区的冷中心附近最低，显示出沿岸物质在潮流作用下向冷水团中部输运的过程。其最大粒径与体积浓度变化一致，显示出沿岸物质向冷水团中部输运过程中粗、细颗粒先后沉降的分选过程。现场悬浮体沉降速率的计算结果显示，细颗粒主要通过絮凝过程沉积。在冷水团内部，悬浮体的静水沉降速率在成山头西北的强潮流区较低、在冷中心和烟台-威海中段近岸的锋区较高。加之成山头西北区域的潮流较强，湍流扰动使悬浮体实际沉降速率更低，因此，在冷水团环流结构影响下，细颗粒向冷中心汇聚，并在冷水团提供的弱动力环境中通过絮凝加速沉积。北黄海冷水团对细颗粒输运、沉积过程的影响显示，其对泥质沉积区的形成有重要作用
In order to analyze the sedimentary environment of the cold water mass and its effect on the composition, structure characteristics, transport, and deposition processes of the suspended particulate matters (SPM), the cruises were conducted onboard R/V “DONGFANGHONG 2” in January 2016 (winter) and July (summer). During the cruises, water temperature, Salinity turbidity and Fluorescent chlorophyll were profiled by using a SBE9plus11 CTD, simultaneously, volume concentration and size of SPM were measured using a type C LISST-100X, in 34,30 work stations during summer and winter cruise, respectively. Water samples were taken simultaneously from standard layers in two typical sections during summer cruise, including a surface layer, middle layer (near the thermocline) and bottom layer. Dispersed particle size distributions (DPSDs) and inorganic particle size distributions (IPSDs) were obtained through laboratory analysis. Additionally, the SPM collected on the filters was inspected using a microscope to primarily identify the characteristics and general types of coarse particles. Based on the above data, the issues such as the dynamic environment of the cold water mass, the distribution characteristics of SPM in the north Yellow Sea and the influencing factors, the influence mechanism of the cold water mass (CWM) on the characteristics, transportation, deposition of SPM were discussed.
The distribution of temperature and salinity in the North Yellow Sea showed that the water column was well mixed in winter, and the water in summer was stratified. During summer, the water with relatively high salinity is located along the northern coast of the Shandong Peninsula, and the Shandong coast current disappeared. On its outer side, there is low salt water flowing from the middle of the Bahia Strait to the north Yellow Sea. In addition, the water with high salinity entering the north Yellow Sea from the south Yellow Sea in the eastern part of the study area. Together with the Liaonan coast current, the two formed a cyclonic circulation that converged in the middle of the north Yellow Sea. Most of the lower part of the central sea area is occupied by CWM, and the center (5.97°C) is located near 122°E and 38.5°N. The Euclidean distance calculation results between the summer and winter water confirmed the previous simulation results of the vertical circulation of the CWM, that is, upwelling occurs in the regime and center of the CWM, between which there is a weak down flow. There are significant tidal fronts in the strong current areas near Penglai and Chengshantou in summer.
There is a significant difference between the distribution of SPM in winter and summer, which are mainly affected by the factors including wind, waves, tidal fronts, and thermocline. In winter, there is a strong resuspension of the sediments with the strong wind waves and coastal currents, while in summer, the lower water is controlled by the cold water mass and the resuspension is weak, resulting in a significantly higher concentration of SPM in winter than that in summer. Concentration of SPM and average particle size are stratified.
The average particle size is relatively higher upon the thermocline than below the thermocline, and is highest near the thermocline, indicating that the thermocline not only hinders the upward diffusion of the resuspended material, but also reduces the settling velocity of the SPM from the upper layer and accelerates the accumulation and aggregation of them. In addition, the thermocline provides a suitable environment for the growth of phytoplankton and the abundance of phytoplankton near the thermocline promotes the aggregation of fine particles into large aggregates. The diffusion of water with high turbidity along the coast is limited by the tidal front in the upper water and is obstructed by the edge of the CWM at the lower water so that it cannot directly enter the middle of the Yellow Sea. The waters with high concentration of SPM located in coast of Penglai and Chengshantou in both winter and summer, which show a trend of spreading from Chengshantou to the northwest, indicate that the sediments is transported to the middle of the north Yellow Sea by the tide both in winter and summer.
The observations show that coarse particles dominate the total volume concentrations (VCs) of the SPM. The in situ SPM is divided into three types according to size differences after dispersion: (1) small particles (<32 μm), (2) medium particles (32-256 μm) and (3) large particles (>256 μm). Large particles are mainly derived from biogenic materials and are the main part of the SPM in surface and middle layer, existing as mucus aggregates and individual plankton. The medium particles are primarily flocs consisting of the resuspended fine particles. There are significant differences in the properties of SPM in coastal water and in different areas of CWM, which are mainly related to differences in dynamic environment and biological activities caused by CWM. In the area with strong tidal current inside the CWM, the organic matter content of the SPM is low and the shearing force is strong, resulting in a smaller floc size (< 128 μm) and a tighter structure. While in the lower water near the center of CWM, the organic matter content of the SPM is high and the dynamic environment is weak, which promotes the formation of flocs with a larger particle size (>128 μm) and looser structure. Although the dynamic environment on the north shore of the Shandong Peninsula is strong, high concentration of re-suspended fine particles and a higher content of organic matter promote the particle aggregation while large flocs are broken under strong shear forces, resulting in the coexistent of larger flocs and smaller flocs. On the edge of the cold water mass near the Yantai-Weihai coast, in addition to the weaker currents, there is a strong upward flow that reduces the settling velocity of fine-grained matter and promotes the formation of large flocs, thus enabling the flocs size larger than that in other areas.
The circulation and hydrodynamic environment of the CWM play an important role in the transport and deposition of terrestrial materials. Section distribution of turbidity shows that the coastal terrestrial material is difficult to directly spread to the middle of the Yellow Sea due to the tidal front.
However, the profile of turbidity in the tidal front area shows a small peak near the thermocline, indicating that the re-suspended sediments in the front area are diffused into the CWM under the combined effects of upwelling and offshore flow near the thermocline. The results show that the terrigenous materials transported by summer water are mainly fine particles with a size < 64 μm. The volume concentration of inorganic particle in the bottom layer was reduced from the area near Chengshantou with strong tidal current to the central part of the northern Yellow Sea, indicating that the coastal material was transported by the tide to the middle of the CWM. The effective density and settling velocity of the in situ SPM shows that the fine particles were deposited mainly through the flocculation process. The settling velocity is higher in the regime and center of CWM. Combined with the distribution of turbidity, inorganic particle concentration, and particle size, the relationship between the north Yellow Sea mud and the cold water mass was further discussed. It was found that the transport and sedimentary process of the SPM in the CWM was well related with the north Yellow Sea mud.
|学科领域||地球科学 ; 地质学 ; 沉积学|
|张凯南. 北黄海冷水团对悬浮体物质组成和沉积环境的影响机制[D]. 中国科学院大学. 中国科学院大学,2018.|