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941.
942.
Dan Lei Jia Liu Junwei Zhang Andreas Lorke Shangbin Xiao Yuchun Wang Wei Wang Ye Li 《洁净——土壤、空气、水》2019,47(9)
Four field campaigns are carried out to quantify the methane (CH4) oxidation rate in Xiangxi Bay (XXB) of the Three Gorges Reservoir (TGR), China. The water depth of the sampling site varied from 13 to 30 m resulting from the water level fluctuation of the TGR. The CH4 oxidation rates are measured in situ as the decline of dissolved CH4 concentration versus time in incubated, and those rates. The CH4 oxidation rates range from 1.18 × 10?3 to 3.69 × 10?3 µmol L?1 h?1, with higher values and stronger variation during summer. A static floating chamber method is used to measure CH4 emitted to the atmosphere resulting in an annual mean flux of 4.79 µmol m?2 h?1. The CH4 emission rate is significantly negatively correlated with the water level. The results show that a large fraction of CH4 is consumed in the water column with a range of 28.97–55.90 µmol m?2 h?1, accounting for ≈69–98% of the total CH4 input into the water column, and more than 90% is consumed outside the summer, when the water level is lowest. Water depth, which is dominated by water level of the TGR, is a potentially important driver for CH4 oxidation and atmospheric emission in the tributary bay. 相似文献
943.
Wan Bo Wu Fuyuan Chen Ling Zhao Liang Liang Xiaofeng Xiao Wenjiao Zhu Rixiang 《中国科学:地球科学(英文版)》2019,62(12):2005-2016
Numerous continents have rifted and drifted away from Gondwana to repeatedly open ocean basins over the past-500 millionyears.These Gondwana-derived continents drifted towards and collided with components of the Eurasian continent to successively close the preexisting oceans between the two.Plate tectonics satisfactorily describes the continental drift from Gondwana to Eurasia but does not define the geodynamic mechanism of continuously rifting to collisions of continents in the Tethy an Realm.After reappraisal of geological records of the rift,collision and subduction initiation from the surface and various geophysical observations from depth,we propose that Eurasia-directed subducting oceanic slabs would have driven Tethyan system in the Phanerozoic.The Eurasia-directed subduction would have dragged the passive Gondwana margin to rift and drift northwards,giving birth to new oceans since the Paleozoic.The closure of preexisting oceans between the Gondwana-derived continents and Eurasia led to continental collisions,which would have induced the initiation of oceanic subduction in the Tethyan Realm.Multiple episodic switches between collision-subduction-rift repeatedly led to the separation of continental fragments from Gondwana and dragged them to drift towards Eurasia.The final disappearance of Neo-Tethy s would have induced collision of the Gondwana-derived continents with the Eurasian continent,giving rise to the Cenozoic Alpine-Zagros-Himalayan collisional system.Therefore,the Eurasia-directed oceanic subduction would have acted as a 'one-way train' that successively transferred the ruptured Gondwana continental fragments in the south,into the terminal in the north.In this regard,the engine of this "Tethyan one-way train" is the negative buoyancy of subducting oceanic slabs. 相似文献
944.
渤黄海海域悬浮体季节性分布及主要运移路径 总被引:1,自引:0,他引:1
依据国家自然科学基金委公共航次2010年和2012年4个航次在渤黄海海域获得的现场悬浮体浓度数据及CTD海洋水文观测数据,结合MODIS L1B数据第4波段反射率值,建立了悬浮体浓度(SSC)与反射率强度的反演模型,反演了渤黄海海域表层悬浮体的月均分布。综合现场实测数据和遥感反演结果,研究渤黄海海域悬浮体的季节性变化特征。利用HYCOM数值模拟得到的各个层位流速数据,分析渤黄海海域主要断面的悬浮体扩散通量。结果表明,渤黄海海域表层悬浮体浓度高值区主要分布在黄河口莱州湾及渤海湾附近沿岸海域、山东半岛沿岸、苏北浅滩至长江口一带、罗洲群岛附近海域以及沿岸其他小河流入海口。渤黄海海域水体结构具有季节性变化特征。夏季,水体上下温差大、层化较强、水动力条件较弱,海域悬浮体浓度相对较低。而在冬季,海域气温较低,水体上下温差小,同时,渤黄海海域强劲的东北风盛行,水体混合强烈,海域悬浮体浓度较高。同时,黄海暖流和山东半岛沿岸流等流系加强,携带悬浮体的能力和效率均大大提高,冬季成为海域悬浮体输运的主要季节。在冬季,黄河三角洲沿岸、山东半岛沿岸以及苏北海岸等区域在强动力作用下的再悬浮成为海域的悬浮体主要来源。黄河三角洲沿岸再悬浮的沉积物通过渤海海峡的南端输入北黄海,在沿岸流的作用下悬浮体输送通量沿程增大,经过成山头海域后转向南输送,输送通量沿程减少,沉积物在黄海中部泥质沉积区汇聚。沉积动力的分析结果显示,冬春季节在山东半岛区域形成的混合锋面对悬浮体的输运路径有重要的影响。 相似文献
945.
946.
地震预报意见以大量资料为基础,依靠专家经验和多学科知识并通过会商讨论形成。显著地震发生后,快速、准确、有效地研判震情可为后期应急指挥和地震现场工作奠定基础。本文以Datist软件为平台,基于四川及邻区的基础资料,设计研发符合四川震情的震后会商资料快速产出系统,当四川及邻区发生中强地震时,通过该系统可在收到地震短信5分钟内以请求触发的方式快速产出应急会商资料,并以微信、PowerPoint和Word的形式将基础资料发送给分析预报人员,为后期趋势分析奠定基础。 相似文献
947.
Seismic force reduction factor(SFRF) spectra of shear-type multi-degree-of-freedom(MDOF) structures are investigated. The modified Clough model, capable of considering the strength-degradation/hardening and stiffnessdegradation, is adopted. The SFRF mean spectra using 102 earthquake records on a typical site soil type(type C) are constructed with the period abscissa being divided into three period ranges to maintain the peak features at the two sitespecific characteristic periods. Based on a large number of results, it is found that the peak value of SFRF spectra may also exist for MDOF, induced by large high-mode contributions to elastic base shear, besides the mentioned two peak values. The variations of the stiffness ratio λk and the strength ratio λF of the top to bottom story are both considered. It is found that the SFRFs for λF≤λk are smaller than those for λF λk. A SFRF modification factor for MDOF systems is proposed with respect to SDOF. It is found that this factor is significantly affected by the story number and ductility. With a specific λF(= λk0.75), SFRF mean spectra are constructed and simple solutions are presented for MDOF systems. For frames satisfying the strong column/weak beam requirement, an approximate treatment in the MDOF shear-beam model is to assign a post-limit stiffness 15%-35% of the initial stiffness to the hysteretic curve. SFRF spectra for MDOF systems with 0.2 and 0.3 times the post-limit stiffness are remarkably larger than those without post-limit stiffness. Thus, the findings that frames with beam hinges have smaller ductility demand are explained through the large post-limit stiffness. 相似文献
948.
Liang Chen Zhao Yan Qin Feng Zheng Zhuo Xiao Xiayun Ma Chunmei Li Huan Zhao Wenwei 《中国科学:地球科学(英文版)》2020,63(8):1144-1160
Quantitative climate reconstruction on long timescales can provide important insights for understanding the climate variability and providing valuable data for simulations. Unfortunately, the credibility of some attempts was hampered by incomplete reconstruction procedures. We here establish a comprehensive framework resting on high-quality Chinese modern pollen database, including modern pollen data screening, calibration set selection, major climate factor analysis, appropriate model selection, strict statistical assessment of results and ecological interpretation. The application of this framework to three high-resolution pollen records from the eastern Tibetan Plateau allows accurate quantitative inferences of Holocene temperature changes, which is the major control of regional vegetation. The results show that the mean warmest month temperature(MTwa)during the early Holocene was ca. 10.4℃ and reached the highest value at 8.5–6 ka BP(ca. 11℃). The early and mid-Holocene(11–5 ka BP) warmth was followed by 1.2℃ temperature decrease, culminating in the coolest temperatures of the Holocene during the Neoglacial cooling. Superimposing on the general cooling trend, MTwareveals a significant 500-yr periodicity with varying intensities through time, showing that warm(cold) intervals are in phase with solar maxima(minima) periods. This spectral similarity indicates a possible connection of multi-century scale climate fluctuations with solar forcing. 相似文献
949.
950.
为提高多波束数据处理的准确性和简便性,研究了多波束水深点云处理的策略和关键算法,构建了kd-tree(k-dimensional tree)结构加速点云查找,针对大尺度噪点设计了半径滤波和统计滤波算法,针对小尺度噪点提出了双边滤波算法,并对以上算法的适用性进行了探讨分析。实验结果表明,当设置适合的滤波阈值,半径滤波和统计滤波能高效去除大尺度噪点,且保留地形特征,总误差分别为2.25%和2.76%。在大尺度噪点去除的基础上,改进的双边滤波可以实现地形平滑的同时,保持水深点云的有效数据量。研究成果为多波束点云数据的自动化处理提供了解决方案,对多波束测量工作的效果和效率提升做了有益尝试。 相似文献