Institutional Repository of South China Sea Institute of Oceanology, CAS
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Enhanced Chlorophyll Concentrations Induced by Kuroshio Intrusion Fronts in the Northern South China Sea
No full textNew evidences were provided that Kuroshio intrusion in winter is able to increase phytoplankton growth in the open ocean of the northern South China Sea (SCS) based on multiple data sources. Strong fronts due to Kuroshio intrusion and interactions with the SCS water are associated with intense upwelling, supplying high nutrients from the subsurface SCS water and increasing phytoplankton productivity in the frontal region. High chlorophyll is more dynamically related to these fronts than to the alongshore wind, wind stress curl, and eddy kinetic energy on interannual time scale. Further examinations suggest that fronts associated with Kuroshio intrusion into the SCS are linked with large-scale climate variability. During El Nino years, stronger Kuroshio intrusion results in stronger fronts that generate intensified local upwelling and enhanced Luzon winter blooms
Spatiotemporal temperature variations in the East China Sea shelf during the Holocene in response to surface circulation evolution
No full textThe Holocene environment evolution in the East China Sea (ECS) is characterized by the gradual establishment and strengthening of its shelf circulation system, but knowledge about temperature responses in temporal and spatial scales is limited due to the lack of continuous high-resolution records. Here, we compare U-37(K') and TEX86 temperature records for three cores from the ECS shelf, which provide the temporal and spatial patterns of Holocene temperature structure variations. These temperature records revealed broadly consistent temporal trends with three intervals characterized by two distinct shifts. During the early Holocene (10.0-6.0 ka), the modern-type circulation system was not established, which resulted in strong water column stratification; and the higher sea surface temperature (SST) might be associated with the Holocene Thermal Maximum (HTM). The interval of 6.0 to 1.0/2.0 ka displayed a weaker stratification caused by the intrusion of the Yellow Sea Warm Current (YSWC) and the initiation of the circulation system. A decreasing SST trend was related to the formation of the cold eddy generated by the circulation system in the ECS. During 1.0/2.0 to 0 ka, temperatures were characterized by much weaker stratification and an abrupt decrease of SST caused by the enhanced circulation system and stronger cold eddy, respectively. Thus, the temperature structure in the shelf of ECS was closely related with circulation system changes during the mid-late Holocene, which was most likely driven by the intrusion of Kuroshio Current (KC). The significant asynchrony of temperature decreases in the three locations during the late Holocene was likely caused by the gradual expansion of the ECS cold eddy area
Influence of mid-crustal rheology on the deformation behavior of continental crust in the continental subduction zone
No full textAlthough the presence of low-viscosity middle crustal layer in the continental crust has been detected by both geophysical and geochemical studies, its influence on the deformation behavior of continental crust during subduction remains poorly investigated. To illustrate the crustal deformation associated with layered crust during continental subduction, we conducted a suite of 2-D thermo-mechanical numerical studies with visco-brittle/plastic rheology based on finite-differences and marker-in-cell techniques. In the experiments, we established a three-layer crustal model with a quartz-rich middle crustal layer embedded between the upper and lower continental crust. Results show that the middle crustal layer determines the amount of the accreted upper crust, maximum subduction depth, and exhumation path of the subducted upper crust. By varying the initial effective viscosity and thickness of the middle crustal layer, the further effects can be summarized as: (1) a rheologically weaker and/or thicker middle crustal layer results in a larger percentage of the upper crust detaching from the underlying slab and accreting at the trench zone, thereby leading to more serious crustal deformation. The rest of the upper crust only subducts into the depths of high pressure (HP) conditions, causing the absence of ultra-high pressure (UHP) metamorphic rocks; (2) a rheologically stronger and/or thinner middle crustal layer favors the stable subduction of the continental crust, dragging the upper crust to a maximum depth of similar to 100 km and forming UHP rocks; (3) the middle crustal layer flows in a ductile way and acts as an exhumation channel for the HP-UHP rocks in both situations. In addition, the higher convergence velocity decreases the amount of subducted upper crust. A detailed comparison of our modeling results with the Himalayan collisional belt are conducted. Our work suggests that the presence of low-viscosity middle crustal layer may be another possible mechanism for absence of UHP rocks in the southern Tibet
Structures within the oceanic crust of the central South China Sea basin and their implications for oceanic accretionary processes
No full textInternal structures in mature oceanic crust can elucidate understanding of the processes and mechanism of crustal accretion. In this study, we present two multi-channel seismic (MCS) transects across the northern flank of the South China Sea basin to reveal the internal structures related to Cenozoic tectono-magmatic processes during seafloor spreading. Bright reflectors within the oceanic crust, including the Moho, upper crustal reflectors, and lower crustal reflectors, are clearly imaged in these two transects. The Moho reflection displays varied character in continuity, shape and amplitude from the continental slope area to the abyssal basin, and becomes absent in the central part of the basin where abundant seamounts and seamount chains formed after the cessation of seafloor spreading. Dipping reflectors are distinct in most parts of the MCS data but generally confined to the lower crust above the Moho reflection. These lower crustal reflectors merge downward into the Moho without offsetting it, probably arising from shear zones between the crust and mantle characterized by interstitial melt, although we cannot exclude other possibilities such as brittle faulting or magmatic layering in the local area. A notable feature of these lower crustal reflector events is their opposite inclinations. We suggest the two groups of conjugate lower crustal reflector events observed between magnetic anomalies C11 and C8 were associated with two unusual accretionary processes arising from plate reorganizations with southward ridge jumps. (C) 2018 The Author(s). Published by Elsevier B.V
A Locally Generated High-Mode Nonlinear Internal Wave Detected on the Shelf of the Northern South China Sea From Marine Seismic Observations
No full textIn this work, a secondary nonlinear internal wave (NIW) on the continental shelf of the northern South China Sea is investigated using high-resolution seismic imaging and joint inversion of water structure properties combined with in situ hydrographic observations. It is an extraordinary wave combination with two mode-2 NIWs and one elevated NIW occurring within a short distance of 2 km. The most energetic part of the NIW could be regarded as a mode-2 NIW in the upper layer between 40 and 120 m depth. The vertical particle velocity of approximate to 41 cm/s may exceed the critical value of wave breaking and thus collapse the strong stratification followed by a series of processes including internal wave breaking, overturning, Kelvin-Helmholtz instability, stratification splitting, and eventual restratification. Among these processes, the shear-induced Kelvin-Helmholtz instability is directly imaged using the seismic method for the first time. The stratification splitting and restratification show that the unstable stage lasts only for a few hours and spans several kilometers. It is a new observation that the elevated NIW could be generated in a deepwater region (as deep as approximate to 370 m). Different from the periodical NIWs originating from the Luzon Strait, this secondary NIW is most likely generated locally, at the continental shelf break during ebb tide
Frictional properties of the South China Sea oceanic basalt and implications for strength of the Manila subduction seismogenic zone
No full textWe conduct friction experiments on a basalt sample (No. U1431E41R6W) from International Oceanic Discovery Program (IODP) Expedition 349 in the temperature range of 100-600 degrees C and effective normal stress of 50 MPa with 100 MPa pore fluid pressure. Loading rates are stepped up and down in the range of 0.04-1.0 mu m/s to obtain the velocity dependence of friction. Based on the rate and state friction law, we numerically fit the experimental curves to acquire constitutive parameters that cannot be obtained by direct measurement. Experimental results show that basalt friction coefficient varies from 0.70 to 0.74, similar to the range of gabbro documented in a previous study (He et al., 2007). Stable velocity-strengthening behavior is observed at temperatures of 100 degrees C and 200 degrees C, and unstable velocity-weakening at temperatures above 300 degrees C with occurrences of periodic stick slips. Both a and b values inferred by numerical fittings show positive temperature dependence in the temperature range of 100-600 degrees C. The stick-slip motions above 300 degrees C seem to be due to a dramatic decrease in the characteristic slip distance d(c), from 11 to 15 pm to 0.4-2.5 mu m. Based on our results and plastic flow laws of mafic rocks from a previous study, fault strength profile and slip stability of Manila subduction zone are discussed with calculations in the context of a unified friction-flow model. The inferred lower boundary of seismogenic zone of Manila subduction fault is around 40 km, which is in a reasonable range compared with seismicity data
Application of large capacity air gun in three-dimensional crustal structure exploration of the Pearl River estuary area
No full textThe air gun source has been widely used as an artificial source in seismic exploration of the sea area because of its simple operation, low risk, automatic continuous operation and almost no influence on the observation environment. In 2015, we carried out the 3-D crustal structural exploration project in the Pearl River estuary, 13 seismic sounding explorations were set up in the sea area, and activated air gun source for more than 12,000 times. Through records of ocean bottom seismograph (OBS) and fixed seismic stations, we found that seismic records of OBS can be clearly identified Pg, PmP, and other phases, the records of the fixed seismic stations can be clearly identified the air gun shot events. The results show that the energy of the air gun shot in this field exploration is quite good, and data can support the study of the three-dimensional structure of the crust in the sea by air gun shot records of seismometers
The impacts of 2008 snowstorm in China on the ecological environments in the Northern South China Sea
No full textAt the beginning of 2008, disastrous weather with snowstorms truck South China. Analysis of satellite and in situ data revealed obvious changes in ecological environments in the northern South China Sea after snowstorms. The sea surface temperature (SST) data from NOAA and Moderate Resolution Imaging Spectroradiometer (MODIS) from 2000 to 2013, and the chlorophyll-a (Chla) concentration from MODIS are analysed. In the study area, the multiple-day-averaged SST dropped by 20.01% and Chla concentration increased by 22.42% during and after snowstorm. SST decreased more significantly and Chla increased in the coastal waters, while suspended sediment concentration (SSC) increased more remarkably offshore. The biggest drop of SST reached 6 degrees C and striking elevation in Chla with 52.55% in the Taiwan Strait, and SSC changed significantly near to the Taiwan bank after the snowstorm. Further analysis indicated that the remarkable reductions in SST were caused by large cold water input from Min-Zhe coastal water and the Pearl River Estuary, as a result of strong sea-land-air interaction in the coastal waters during the snowstorm. The increase in Chla may be related to the abundant nutrients from the plenty of cold water, a strong front and vertical mixing in the coastal waters
底栖有孔虫群落对南黄海中部泥质区全新世海洋环境演化的响应
No full text南黄海是一个地理位置独特的陆架浅海,在独特的环流体系下形成了巨厚泥质沉积体,作为高沉积速率的稳定沉积体,它蕴含着丰富的古环境变化信息,是研究过去海洋环境和气候变化的绝佳档案库。深入研究南黄海中部泥质区对理解区域环境变化机制有重要意义。本文通过对南黄海中部泥质区不同位置7个沉积物柱样的底栖有孔虫群落分析,通过综合对比系统探讨了南黄海中部泥质区全新世期间古环境阶段性演化特征,首次重建了全新世期间黄海中部冷水团的演化史,并初步探讨了影响黄海中部冷水团强弱变化的驱动因素。所得主要认识和结论如下:南黄海中部泥质区14 ka以来经历了3个主要的演化阶段:14~10 ka期间,南黄海中部泥质区北缘整体呈滨岸环境,水深较浅,底栖有孔虫丰度相对较高,南黄海中部泥质区中心区域离岸较远,水深较深,底栖有孔虫丰度很低;在10~8 ka期间,海平面呈现快速上升时期,整体陆架环境较动荡,水动力环境较强,南黄海中部泥质区几乎无底栖有孔虫出现;8 ka以来,南黄海中部泥质区中心区域进入泥质沉积,黄海中部冷水团核心区域形成,底栖有孔虫丰度逐渐增加并达到与现在相当的水平,南黄海中部泥质区北缘在8 ka左右虽已进入泥质沉积,但7 ka左右才开始受到黄海中部冷水团的影响。通过多柱样的粗组分数据和年代数据综合对比,我们认为南黄海中部泥质区的形成年代在10~7.5ka期间,不同位置泥质沉积的形成年代有显著区域差异。总体上,南黄海泥质区西北缘、西缘泥质沉积较早形成;南黄海中部泥质区中心和北缘的泥质沉积形成略晚一些。结合各柱样中底栖有孔虫丰度数据进行分析,我们认为南黄海中部泥质区泥质沉积的形成时间可能早于黄海中部冷水团的形成时间,且南黄海中部泥质区不同位置受到黄海冷水团影响的起始时间也存在区域差异。根据南黄海中部泥质区7个沉积物柱样中底栖有孔虫群落结构特征以及Q型因子分析结果,首次重建了近7 ka来南黄海中部冷水团的阶段性演化序列:7~5 ka期间,南黄海泥质区各柱样中底栖有孔虫以Ammonia ketienziensis为主,指示黄海中部冷水团为全新世最强盛期,黄海沿岸流对南黄海中部泥质区底层水体影响较小;5.0~3.0 ka期间,各柱样中Ammonia ketienziensis含量显著减少,而Hanzawaia nipponica显著增加,表明南黄海中部冷水团较前一阶段有所减弱,,其范围有减小的趋势,而黄海沿岸流对南黄海中部泥质区西北缘和西缘的影响略有增加但影响依然较小,且无法影响到泥质区中心位置;3.0 ka以来,指示黄海沿岸流的低温、低盐属种Protelphidium tuberculatum和Buccella属分子开始在南黄海中部泥质区西北缘和西缘较多出现,反映了黄海沿岸流对黄海中部泥质区外围底层水体的影响显著增强,而黄海中部冷水团则为7 ka来最弱阶段。整体上,全新世期间黄海中部冷水团和黄海沿岸流对南黄海中部泥质区西北缘、北缘底层水体的影响呈现此消彼长的关系。全新世期间南黄海环流体系的波动演化可能是区域气候因素(ENSO、东亚冬季风及北半球太阳辐射)在不同时期综合影响黄海环流体系进而作用于南黄海区域的结果
封闭式虾池水体环境因子变化及浮游微藻优势种群演替规律
No full text为分析封闭式虾池水质环境因子季节变化对浮游微藻群落结构的影响,对虾池水体的温度、p H、总氮(TN)、总磷(TP)和叶绿素a含量等环境因子进行测试,分析同期水体中浮游微藻群落结构及优势种群变换,解释在没有外来水源干扰的情况下微藻群落对环境因子变化的响应。结果表明,养殖池塘水温和总磷(TP)含量从4月至9月底逐渐上升,10月起开始下降,至次年1月达到最低;p H先下降后升高;总氮(TN)和叶绿素a含量先升高后下降。养殖初期(4~5月)为小环藻(Cyclotella sp.)、舟形藻(Navicula sp.)、波吉卵囊藻(Oocystis borgei)和四尾栅藻(S.quadricauda);中期(6~7月)为衣藻(Chlamydomonas sp.)、小球藻(Chlorella sp.)、四尾栅藻、多芒藻(Golenkinia radiata)和假鱼腥藻(Pseudanabeana);中后期(8~9月)为微囊藻(Microcystis)、假鱼腥藻(Pseudanabeana)、小球藻和四尾栅藻;后期(10~12月)为微囊藻、假鱼腥藻、颤藻(Oscillatoria sp.)和小球藻;末期(12月后)为小球藻、多芒藻和四尾栅藻。优势微藻种群演替与环境因子变化密切相关,在低温、氮磷营养较低且N/P较高和有一定盐度的早期水体中以硅藻和绿藻为主;随着温度回升、水体完全淡化和氮磷营养的积累,以绿藻为优势种群;高温和富营养化状态下的中后期蓝藻成为强优势种群,其中也有喜高温和耐污染的绿藻;后期以蓝藻为绝对优势,水温下降后变成以绿藻为优势种,池中浮游微藻生物量减少