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   192篇
  免费   5篇
  国内免费   2篇
测绘学   2篇
大气科学   8篇
地球物理   51篇
地质学   83篇
海洋学   17篇
天文学   21篇
自然地理   17篇
  2024年   1篇
  2022年   1篇
  2021年   7篇
  2020年   3篇
  2019年   5篇
  2018年   3篇
  2017年   8篇
  2016年   8篇
  2015年   5篇
  2014年   11篇
  2013年   15篇
  2012年   14篇
  2011年   18篇
  2010年   10篇
  2009年   13篇
  2008年   10篇
  2007年   7篇
  2006年   9篇
  2005年   3篇
  2004年   5篇
  2003年   11篇
  2002年   4篇
  2001年   1篇
  2000年   4篇
  1999年   7篇
  1998年   3篇
  1996年   4篇
  1995年   1篇
  1993年   2篇
  1984年   1篇
  1983年   1篇
  1982年   1篇
  1981年   2篇
  1978年   1篇
排序方式: 共有199条查询结果,搜索用时 31 毫秒
61.
<正>One new genus and five new species of the family Evaniidae are described from the Early Cretaceous (Albian)Spanish amber of Penacerrada-Ⅰ(Province of Burgos),San Just and Arroyo de la Pascueta(both in the Province of Teruel):Cretevania alonsoi sp.nov.,C.montoyai sp.nov.,C.alcalai sp.nov.,C.rubusensis sp.nov.,and Iberoevania roblesi gen.and sp.nov.Taxonomic changes include Cretevania pristina(Zhang and Zhang,2000) comb.nov.,C.exquisita(Zhang,Rasmtsyn,Wang and Zhang,2007)comb.nov.,C.vesca(Zhang,Rasnitsyn,Wang and Zhang,2007)comb.nov.,and C.cyrtocerca(Deans,2004)comb.nov.,as a result of the reinterpretation of the genera Procretevania and Eovernevania.The new well preserved specimens of the genus Cretevania,together with the characters shown by the type specimens of the synonymized genera,give new information about their anatomical characters of taxonomical importance,and the genus Cretevania Rasnitsyn,1975 is re-diagnosed.The holotypes of the Russian species in amber have been revised.A cladistic analysis of fossil and extant groups of the superfamily Evanioidea is included.Cretevania had a wide palaeogeographic distribution,with the highest diversity known from Spain.The 13 known Cretevania species show a high interspecific variation mainly in wing characteristics,and a wide range of body and wing size.  相似文献   
62.
Ground water/surface water interaction in a fractured rock aquifer   总被引:1,自引:0,他引:1  
In a recent field study of ground water/surface water interaction between a bedrock stream and an underlying fractured rock aquifer, it was determined that the majority of ground water discharge occurred through sparsely located vertical fractures. In this paper, the dominant mechanisms governing ground water/surface water exchange in such an environment are investigated using a numerical model. The study was conducted using several conceptual models based on the field study results. Although the field results provided the motivation for the modeling study, it was not intended to match modeling and field results directly. In addition, the extent of capture zones for discharging or recharging fractures was explored. The results of this study are intended to provide a better understanding of contaminant migration in the vicinity of bedrock streams. Based on the numerical results, the rate of ground water discharge (or recharge) was found to depend on the aperture size of the discharging feature, and on the distribution of hydraulic head with depth within the fracture network. It was determined that the extent of both the capture zone and reverse capture zone for an individual fracture can be extremely large, and will be determined by the height of the stream stage, the fracture apertures of the network, and the hydraulic-head distribution within the network. Because both the stream stage and the hydraulic-head distribution are transient, the size of the capture zone and/or the reverse capture zone for an individual fracture may change significantly over time. As a result, the migration path for contaminants within the fracture network and between the surface and subsurface will also vary significantly with time.  相似文献   
63.
The biological processes have been proposed as climate variability contributors. Dimethylsulfide (DMS) is the main biogenic sulfur compound in the atmosphere; it is mainly produced by the marine biosphere and plays an important role in the atmospheric sulfur cycle. Currently it is accepted that terrestrial biota not only adapts to environmental conditions but also influences them through regulations of the chemical composition of the atmosphere. In the present study we used a wavelet method to investigate the relationship between DMS, Low cloud cover (LCC), Ultraviolet Radiation A (UVA), Total Solar Irradiance (TSI) and Sea Surface Temperature (SST) in the so called pristine zone of the Southern Hemisphere. We found that the series analyzed have different periodicities which can be associated with large scale climatic phenomena such as El Niño (ENSO) or the Quasi-Biennial Oscillation (QBO), and/or to solar activity. Our results show an intermittent but sustained DMS-SST correlation and a DMSUVA anti correlation; but DMS-TSI and DMS-LCC show nonlinear relationships. The time-span of the series allow us to study only periodicities shorter than 11 years, then we limit our analysis to the possibility that solar radiation influences the Earth climate in periods shorter than the 11-year solar cycle. Our results also suggest a positive feedback interaction between DMS and solar radiation.  相似文献   
64.
Fifty-seven surface sediment samples from 35 Florida lakes were collected to study testate amoebae. Seven genera, 17 species, and 28 strains were identified in the 46 sediment samples from 31 lakes that contained testate rhizopods. Seven species accounted for ≥90% of the individuals in all samples. Sediment total phosphorus (TPsed), organic matter (OM), and total carbon:total nitrogen ratio (TC:TN) were measured to assess the effect of these variables on thecamoebian assemblages. OM content was the only sediment variable that influenced presence/absence of thecamoebians. Samples with <5% OM contained no thecamoebians. Lakes with multiple surface sediment samples showed high Morisita–Horn similarity values (0.74–0.99), indicating that all sites at which samples were collected in a lake provided representative thecamoebian assemblages. No relationship was observed between thecamoebian diversity indices and sediment variables. Lake trophic state and pH were examined to explore potential water column influences on thecamoebian communities. Highest thecamoebian diversity indices were found in mesotrophic to eutrophic lakes with pH near 8.0. These results suggest that water column conditions have a greater influence on thecamoebian assemblages than do sediment variables. We used multivariate analysis to evaluate the relations between water quality variables and testate rhizopod assemblages. Canonical correspondence analysis (CCA) showed that alkalinity and pH are the water column variables that most influence the relative abundance of species. Thecamoebians thus hold promise as bioindicators of acidification in Florida lakes. Thecamoebian remains in lake sediment cores should be useful to infer past anthropogenic shifts in lake pH.  相似文献   
65.
The Azores archipelago is located at the North America, Eurasia and Africa triple junction and shows seismicity and volcanism typical of an active plate boundary. Terceira Island has been affected by several damaging earthquakes being the 1980 January 1st event (M=7.2) the strongest in Azores during the last century.  相似文献   
66.
Geomorphic analysis, employing topographic, morphologic, geologic, and bathymetric maps, and field studies show that the morphology of the southern coast of Mexico can be linked to lateral variations in the geometry and tectonism of the subduction zone. A reconnaissance study, based on the regional morphological characteristics and correlation with seismotectonic segments, regional tectonics and major bathymetric features, allows identification of several morphotectonic zones along the coast of southern Mexico: (1) Jalisco zone, (2) Colima zone, (3) Michoacan zone, (4) Guerrero zone, (5) Oaxaca zone, and (6) Tehuantepec zone. A range of geomorphological evidence, including marine terraces, river terraces, uplifted notches, and elevated wave-cut platforms, indicates local and regional uplift along the coast of the Jalisco, Michoacan, Guerrero and Oaxaca zones. Coasts of the Colima and Tehuantepec zones show morphological evidence of subsidence.  相似文献   
67.
Natural Hazards - Recife Metropolitan Region, in the northeast of Brazil, is formed by a coastal plain bounded by hills and by the Atlantic Ocean. Recife Metropolitan Region has about 4 million...  相似文献   
68.
Abstract. Using principal component analysis, the zooplanktonic communities of the divergence zone in the northwestern sector of the Alboran Sea were studied. The study was based on 35 vertical trawls in the 200 metres superficial level. The two first components account for 65 % of the total variance. The first component is a contrast between a warm water coastal community (characterized by low specific diversity and the abundance of Cladocera and Copepoda such as Acartia clausi, Centropages chierchiae and Temora stylifera) and a community of cold subsuperficial water distinguished by high specific diversity and a low number of individuals, one of its most characteristic elements being the eggs and larval stages of the Conostomasidae Maurolicus muelleri. The second component appears to be associated with an ecotone between the communities previously cited, and with a specific grouping (Rhincalanus nasutus, Eucalanus monachiis, Pseudocalanus elongatus, Temora longicornis , etc.) resulting from the peripheral displacement of subsuperficial elements brought to the surface by the cyclonic circulation of water masses between the Atlantic current and the Spanish coast.  相似文献   
69.
70.
Modern geostatistical techniques allow the generation of high-resolution heterogeneous models of hydraulic conductivity containing millions to billions of cells. Selective upscaling is a numerical approach for the change of scale of fine-scale hydraulic conductivity models into coarser scale models that are suitable for numerical simulations of groundwater flow and mass transport. Selective upscaling uses an elastic gridding technique to selectively determine the geometry of the coarse grid by an iterative procedure. The geometry of the coarse grid is built so that the variances of flow velocities within the coarse blocks are minimum. Selective upscaling is able to handle complex geological formations and flow patterns, and provides full hydraulic conductivity tensor for each block. Selective upscaling is applied to a cross-bedded formation in which the fine-scale hydraulic conductivities are full tensors with principal directions not parallel to the statistical anisotropy of their spatial distribution. Mass transport results from three coarse-scale models constructed by different upscaling techniques are compared to the fine-scale results for different flow conditions. Selective upscaling provides coarse grids in which mass transport simulation is in good agreement with the fine-scale simulations, and consistently superior to simulations on traditional regular (equal-sized) grids or elastic grids built without accounting for flow velocities.  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