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   1269篇
  免费   39篇
  国内免费   8篇
测绘学   19篇
大气科学   95篇
地球物理   291篇
地质学   402篇
海洋学   138篇
天文学   206篇
综合类   4篇
自然地理   161篇
  2022年   8篇
  2021年   12篇
  2020年   18篇
  2019年   24篇
  2018年   22篇
  2017年   21篇
  2016年   34篇
  2015年   39篇
  2014年   35篇
  2013年   96篇
  2012年   51篇
  2011年   61篇
  2010年   48篇
  2009年   64篇
  2008年   50篇
  2007年   57篇
  2006年   42篇
  2005年   45篇
  2004年   38篇
  2003年   45篇
  2002年   41篇
  2001年   37篇
  2000年   29篇
  1999年   32篇
  1998年   27篇
  1997年   22篇
  1996年   13篇
  1995年   12篇
  1994年   19篇
  1993年   14篇
  1992年   15篇
  1991年   11篇
  1990年   13篇
  1989年   11篇
  1988年   10篇
  1987年   17篇
  1986年   15篇
  1985年   10篇
  1984年   16篇
  1983年   14篇
  1982年   18篇
  1981年   15篇
  1980年   16篇
  1979年   16篇
  1978年   6篇
  1977年   5篇
  1976年   10篇
  1975年   7篇
  1974年   8篇
  1973年   12篇
排序方式: 共有1316条查询结果,搜索用时 15 毫秒
71.
As the world’s largest importer of marine ornamental species for the aquaria, curio, home décor, and jewelry industries, the United States has an opportunity to leverage its considerable market power to promote more sustainable trade and reduce the effects of ornamental trade stress on coral reefs worldwide. Evidence indicates that collection of some coral reef animals for these trades has caused virtual elimination of local populations, major changes in age structure, and promotion of collection practices that destroy reef habitats. Management and enforcement of collection activities in major source countries such as Indonesia and the Philippines remain weak. Strengthening US trade laws and enforcement capabilities combined with increasing consumer and industry demand for responsible conservation can create strong incentives for improving management in source countries. This is particularly important in light of the March 2010 failure of the parties to the Convention on International Trade in Endangered Species (CITES) to take action on key groups of corals.  相似文献   
72.
73.
Process length variation of cysts of the dinoflagellate Protoceratium reticulatum (Claparède et Lachmann) Bütschli in surface sediments from the North Pacific was investigated. The average process length showed a significant inverse relation to annual seawater density: σt annual = ?0.8674 × average process length + 1029.3 (R2 = 0.84), with a standard error of 0.78 kg m?3. A sediment trap study from Effingham Inlet in British Columbia revealed the same relationship between average process length and local seawater density variations. In the Baltic–Skagerrak region, the average process length variation was related significantly to annual seawater density: σt annual = 3.5457 × average process length ? 993.28 (R2 = 0.86), with a standard error of 3.09 kg m?3. These calibrations cannot be reconciled, which accentuates the regional character of the calibrations. This can be related to variations in molecular data (small subunit, long subunit and internal transcribed spacer sequences), which show the presence of several genotypes and the occurrence of pseudo‐cryptic speciation within this species. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
74.
The coal-bearing sediments and coal seams of the Karoo Basin, Southern Africa are described and discussed. The Karoo Basin is bounded on its southern margin by the Cape Fold Belt, onlaps onto the Kaapvaal Craton in the north and is classified as a foreland basin. Coal seams are present within the Early Permian Vryheid Formation and the Triassic Molteno Formation.The peats of the Vryheid Formation accumulated within swamps in a cool temperate climatic regime. Lower and upper delta plain, back-barrier and fluvial environments were associated with peat formation. Thick, laterally extensive coal seams have preferentially accumulated in fluvial environments. The coals are in general inertinite-rich and high in ash. However, increasing vitrinite and decreasing ash contents within seams occur from west to east across the coalfields. The Triassic Molteno coal seams accumulated with aerially restricted swamps in fluvial environments. These Molteno coals are thin, laterally impersistent, vitrinite-rich and shaly, and formed under a warm temperate climatic regime.Palaeoclimate, depositional systems, differential subsidence and basin tectonics influence to varying degrees, the maceral content, thickness and lateral extent of coal seams. However, the geographic position of peat-forming swamps within a foreland basin, coupled with basin tectonics and differential subsidence are envisaged as the primary controls on coal parameters. The Permian coals are situated in proximal positions on the passive margin of the foreland basin. Here, subsidence was limited which enhanced oxidation of organic matter and hence the formation of inertinitic coals. The coals in this tectonic setting are thick and laterally extensive. The Triassci coals are situated within the tectonically active foreland basin margin. Rapid subsidence and sedimentation rates occurred during peat formation which resulted in the preservation of thin, laterally impersistent, high ash, vitrinite-rich, shaly coals.  相似文献   
75.
76.
77.
Digital techniques for the correction of signal distortions that arise in planar laser-induced fluorescence (PLIF) measurements (by PC-based video digitizing systems) of jets and plumes and the production of flow statistics in both time and space are reviewed. The entire concentration field is repeatedly imaged in s intervals over hundreds of thousands a points in a plane. By the recognition of signal distortion sources and the employment of corrections, a clearer picture of tracer concentrations may be realized.Fluorescence studies are made with a planar sheet of laser light 430 mm tall and 1.5 mm thick. The fluorescence excitation produced from trace concentrations of Rhodamine 6G is used to visualize and measure the propagation of a jet or plume in a density stratified laboratory tank. The emitted light is collected by a CCD camera in a 512 × 480 pixel format over a 940 × 715 mm field of view. The captured images are corrected for transverse laser sheet intensity distribution; laser beam attenuation; refraction; lens vignette; time varying and spatial noise; digitization aspect ratio; camera response. The measurement and methods of correction are discussed in detail. The resulting image data can then be used to collect tracer concentration statistics for jets and plumes. Instantaneous (i.e. over of a second intervals), average, maximum, minimum, standard deviation, and coefficient of variation are given as introductory examples of image statistics realizable for a buoyant jet.  相似文献   
78.
79.
80.
Data collected since 1985 from test drilling, fluid sampling, and geologic and geophysical investigations provide a clearer definition of the hydrothermal system in Long Valley caldera than was previously available. This information confirms the existence of high-temperature (> 200°C) reservoirs within the volcanic fill in parts of the west moat. These reservoirs contain fluids which are chemically similar to thermal fluids encountered in the central and eastern parts of the caldera. The roots of the present-day hydrothermal system (the source reservoir, principal zones of upflow, and the magmatic heat source) most likely occur within metamorphic basement rocks beneath the western part of the caldera. Geothermometer-temperature estimates for the source reservoir range from 214 to 248°C. Zones of upflow of hot water could exist beneath the plateau of moat rhyolite located west of the resurgent dome or beneath Mammoth Mountain. Lateral flow of thermal water away from such upflow zones through reservoirs in the Bishop Tuff and early rhyolite accounts for temperature reversals encountered in most existing wells. Dating of hot-spring deposits from active and inactive thermal areas confirms previous interpretations of the evolution of hydrothermal activity that suggest two periods of extensive hot-spring discharge, one peaking about 300 ka and another extending from about 40 ka to the present. The onset of hydrothermal activity around 40 ka coincides with the initiation of rhyolitic volcanism along the Mono-Inyo Craters volcanic chain that extends beneath the caldera's west moat.  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