首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   84篇
  免费   2篇
  国内免费   5篇
大气科学   2篇
地球物理   19篇
地质学   53篇
海洋学   16篇
自然地理   1篇
  2023年   1篇
  2018年   1篇
  2017年   2篇
  2016年   9篇
  2015年   2篇
  2014年   3篇
  2013年   1篇
  2012年   1篇
  2011年   2篇
  2010年   3篇
  2009年   6篇
  2008年   10篇
  2007年   15篇
  2006年   13篇
  2005年   5篇
  2004年   3篇
  2002年   2篇
  2001年   1篇
  2000年   1篇
  1998年   1篇
  1997年   1篇
  1996年   1篇
  1995年   1篇
  1993年   2篇
  1992年   2篇
  1988年   1篇
  1981年   1篇
排序方式: 共有91条查询结果,搜索用时 15 毫秒
1.
Unknowingly lumping cryptic species in biological monitoring studies hinders progress in understanding their functioning in a wide range of research fields including population dynamics, ecophysiology and community ecology. The common polychaete worm Scoloplos cf. armiger is a cryptic species complex comprising entirely different developmental modes: holobenthic and pelago-benthic development. In the northeast Atlantic, three putative species have been described on the basis of molecular data and a breeding study. We report on the development of a fast genotyping assay and on the occurrence and distribution of different molecular types in the western Wadden Sea. The genotyping assay consists of PCR-RFLP analysis by two enzymes of a mitochondrial (cox3-trnQ-nad6) DNA segment. A new, fourth type was observed and this was the only type whose geographical distribution was not uniform but instead skewed toward the eastern part of the study area. All three hitherto known types were also observed and these three displayed a significant difference in depth distribution within the study area. This is the first ecological difference reported for the ‘Subtidal Clade’ (SC) versus the ‘Type Locality Clade’ (TLC). The new type (‘Intertidal Clade 2’) had a similar distribution to what is known as the Intertidal Clade (IC). However, the most striking observation is the large degree of overlap between the depth distributions in the western Wadden Sea, where the clade names ‘Intertidal’ and ‘Subtidal’ do not necessarily reflect local species composition. The necessity to genotype Scoloplos cf. armiger in monitoring programs is emphasized.  相似文献   
2.
3.
Metal speciation study in combination with major element chemistry of deep sea sediments provided possible metal enrichment pathways in sediments collected from environmentally different locations of Central Indian Ocean Basin (CIB). Metal speciation study suggests that Fe–Mn oxyhydroxide phase was the major binding phase for Ni, Cu and Pb in the sediments. The second highest concentrations of all these metals were present within the structure of the sediments. Easily reducible oxide phase (within the Fe–Mn oxyhydroxide binding phases) was the major host for all the three metals in the studied sediments. Major element chemistry of these sediments revealed that there was an increased tendency of Cu and Ni to get incorporated into the deep sea sediment via the non-terrigenous Mn-oxyhydroxide fraction, whereas, Pb gets incorporated mostly via amorphous Fe-hydroxides into the sediment from the CIB. This is the first attempt to provide an insight into the mechanism of metal enrichment in sediment that host vast manganese nodule.  相似文献   
4.
In this study, headspace single drop microextraction (HS‐SDME) method in combination with electrothermal atomic absorption spectrometry (ETAAS) method was developed and validated for the speciation and determination of inorganic mercury (iHg) and methylmercury (MeHg). MeHg and iHg species were reduced to volatile methylmercury hydride (CH3HgH) and elemental mercury, respectively, in the presence of NaBH4 and trapped onto a drop of acceptor phase in the tip of a microsyringe. Thiourea and ammonium pyrrolydinedithiocarbamate (APDC) were tested as the acceptor phase. The experimental parameters of the method such as microextraction time, temperature, NaBH4 concentration, acceptor phase concentration, and pH of the medium were investigated to obtain distinctive conditions for mercury species. Possible interference effects have also been investigated. In order to validation of the method, analytical figures of merits such as accuracy, precision, limit of detection (LOD), limit of quantitation (LOQ), and linear working range have been evaluated. Accuracy of the method has been verified by analyzing certified reference materials (BCR 453 Tuna fish) and spiked samples. The proposed method was applied for the speciation and determination of mercury species in water and fish samples. Mercury species (MeHg and iHg) have been determined in the real samples with a relative error less than 10%.  相似文献   
5.
This study determines the pollution, fractionation, and ecological risks of sediment-bound heavy metals from coastal ecosystems off the Equatorial Atlantic Ocean. Contamination Factor(CF), pollution load index(PLI), and geoaccumulation index(Igeo) were used to assess the extent of the heavy metal pollution, while the potential ecological risk was evaluated using the risks assessment code(RAC) and Hkanson potential ecological risk. The analysis revealed concentrations(mg/g, dw) of the cadmium(Cd),chromium(Cr), copper(Cu), nickel(Ni), and lead(Pb) in sediments for wet and dry seasons vary from 4.40-5.08, 14.80-21.09. 35.03-44.8, 2.14-2.28, and 172.24-196.39, respectively. The results also showed that the metal fractionation percentages in the residual, oxidizable, and reducible fractions are the most significant, while the exchangeable and carbonate bound trace metals are relatively low. The RAC values indicate no risk for Cd and Ni and low risk for other metals at all the studied sites during both seasons.Potential ecological risk analysis of the heavy metal concentrations indicates that Cd had high individual potential ecological risk, while the other metals have low risk at all investigated sites. The multi-elemental potential ecological risk indices(R_1) indicate high ecological risk in all the ecosystems.  相似文献   
6.
The accumulation of selenium in evaporation basins (or ponds) in the San Joaquin Valley, California is of a great concern due to its potential hazards to environments. In this study, the accumulation, speciation and concentrations of Se were examined in waters as well as sediments in a system of the evaporation ponds. A significant decrease in the total dissolved Se concentration in Cell 1 in which drainage water with higher Se concentration was pumped from Inlet Channels indicated that the immobilization of Se was active in the Cell 1 and resulted in the higher Se concentration in sediments compared to the terminal cell such as Cell 9. The percentage of reduced Se species such as selenite [Se(IV)] and org-Se of total Se in drainage waters was also found increased in Cell 1 compared to Inlet Channels. The total dissolved Se concentrations in water along flow paths from Cell 1 were relatively constant except for terminal cells such as Cells 9 and 10, which showed higher total dissolved Se concentrations due to evapoconcentration. The percentage of reduced Se forms of total Se was inversely proportional to the percentage of Se(VI) depending on the redox condition of evaporation ponds along the flow paths. Sequential extractions of Se species in sediments indicated that organic associated Se and elemental Se were prevalent forms in sediments in the ponds system. The higher concentrations of elemental Se and organic associated Se in sediments in Cell 1 indicated that the immobilization of Se was active in the sediments compared to Cell 9, while the percentage of both fractions of total Se in sediments in Cells 1 and 9 was relatively constant. The organic materials from algae might provide carbon sources for Se reduction and Se sink in sediments in its elemental and organic associated forms.  相似文献   
7.
采用盆栽试验,初步研究了干旱区绿洲土壤—胡萝卜系统中镉、锌、镍3种重金属的形态变化特征及其生物有效性问题。结果表明:供试绿洲土壤原状土中,Cd、Zn、Ni均以稳定的残渣态形式存在,而处理土壤中重金属被钝化的量有限,Cd的存在形式主要以碳酸盐态为主,Zn、Ni则主要以铁锰氧化态为主;3种元素的活性大小依次为Cd>Ni>Zn。根据回归分析,元素Zn对胡萝卜块茎和茎叶吸收Zn量贡献最大的分别是Zn的碳酸盐结合态和铁锰氧化态;元素Ni对胡萝卜各部位吸收贡献最大的均为Ni的铁锰氧化态。  相似文献   
8.
Mining wastes and calcines from two abandoned mining areas (Valle del Azogue and Bayarque in Almería) have been characterized. In the mining wastes, the dominant mercury phases are cinnabar and elemental mercury in the matrix. In the calcines, however, the dominant mercury phase is elemental mercury bounded to the matrix. Water-leaching experiments were conducted on low-grade stockpiles and calcines in order to simulate the mobilization of mercury by runoff under environmental conditions. The laboratory column-leaching experiments show a possible mobilization of mercury from Hg0 dissolution, colloid transport and a possible dissolution of calomel and other soluble phases in the mine wastes from the Valle del Azogue and Bayarque mines. Equilibrium speciation modeling of Hg, conducted using the numerical code MINTEQ, showed that the theoretical dominant mercury species in the calcine and mining wastes samples are Hg(OH)2, HgCl2, HgClOH and Hg0. In some leachates obtained from the Valle del Azogue mining wastes (sample A06), the high Hg concentrations may indicate the possible dissolution of mineral phases such as calomel and other soluble phases, which are subsaturated. The environmental results indicate a great environmental mobility of mercury, especially during wet episodes associated with intense precipitation events, when there are significative amounts of secondary soluble minerals.  相似文献   
9.
The purpose of this study was to establish the distribution, speciation and bioavailability of mercury in mangrove sediments. A systemic survey of surficial sediments from 13 mangrove wetlands of China was carried out. Hg concentrations ranged from 2.3–903.6 ng g−1, with an average value of 189.4 ng g−1. Of the 13 areas surveyed, the Hg content in sediments was similar to background levels in 6 areas but was much higher in the other seven areas. Hg levels were affected by natural and anthropogenic factors, including terrestrial pollutants, geomorphic properties, and indirectly by economic status. Hg levels were positively correlated with organic matter, pH, and silt and clay fractions, but Hg was negatively correlated with sand fraction. In most mangrove wetlands, Hg existed primarily in the form of volatile Hg. Hg is easily bioaccumulated in mangrove wetlands and may be the natural source of Hg emissions to the atmosphere.  相似文献   
10.
Mercury speciation and its distribution in surface and deep waters of the Mediterranean Sea were studied during two oceanographic cruises on board the Italian research vessel URANIA in summer 2003 and spring 2004 as part of the Med Oceaneor and MERCYMS projects. The study included deep water profiles of dissolved gaseous Hg (DGM), reactive Hg (RHg), total Hg (THg), monomethyl Hg (MeHg) and dimethyl Hg (DMeHg) in open ocean waters. Average concentrations of measured Hg species were characterized by seasonal and spatial variations. Overall average THg concentrations ranged between 0.41 and 2.65 pM (1.32 ± 0.48 pM) and were comparable to those obtained in previous studies of the Mediterranean Sea. A significant fraction of Hg was present as “reactive” Hg (average 0.33 ± 0.32 pM). Dissolved gaseous Hg (DGM), which consists mainly of Hg0, represents a considerable proportion of THg (average 20%, 0.23 ± 0.11 pM). The portion of DGM typically increased towards the bottom, especially in areas with strong tectonic activity (Alboran Sea, Strait of Sicily, Tyrrhenian Sea), indicating its geotectonic origin. No dimethyl Hg was found in surface waters down to the depth of 40 m. Below this depth, its average concentration was 2.67 ± 2.9 fM. Dissolved fractions of total Hg and MeHg were measured in filtered water samples and were 0.68 ± 0.43 pM and 0.29 ± 0.17 pM for THg and MeHg respectively. The fraction of Hg as MeHg was in average 43%, which is relatively high compared to other ocean environments. The concentrations reported in this study are among the lowest found in marine environments and the quality of analytical methods are of key importance. Speciation of Hg in sea water is of crucial importance as THg concentrations alone do not give adequate data for understanding Hg sources and cycling in marine environments. For example, photoinduced transformations are important for the presence of reactive and elemental mercury in the surface layers, biologically mediated reactions are important for the production/degradation of MeHg and DGM in the photic zones of the water column, and the data for DGM in deep sea indicate the natural sources of Hg in geotectonicaly active areas of the Mediterranean Sea.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号