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   54439篇
  免费   304篇
  国内免费   277篇
测绘学   1469篇
大气科学   3010篇
地球物理   9561篇
地质学   23261篇
海洋学   4340篇
天文学   11296篇
综合类   245篇
自然地理   1838篇
  2022年   487篇
  2021年   707篇
  2020年   758篇
  2019年   827篇
  2018年   3972篇
  2017年   3528篇
  2016年   2836篇
  2015年   727篇
  2014年   1447篇
  2013年   2050篇
  2012年   2376篇
  2011年   3985篇
  2010年   3542篇
  2009年   3884篇
  2008年   3236篇
  2007年   3914篇
  2006年   1868篇
  2005年   1215篇
  2004年   1093篇
  2003年   1143篇
  2002年   1033篇
  2001年   833篇
  2000年   682篇
  1999年   437篇
  1998年   444篇
  1997年   491篇
  1996年   349篇
  1995年   370篇
  1994年   360篇
  1993年   297篇
  1992年   296篇
  1991年   312篇
  1990年   348篇
  1989年   289篇
  1988年   274篇
  1987年   275篇
  1986年   222篇
  1985年   321篇
  1984年   319篇
  1983年   305篇
  1982年   281篇
  1981年   281篇
  1980年   298篇
  1979年   208篇
  1978年   243篇
  1977年   202篇
  1976年   178篇
  1975年   187篇
  1974年   174篇
  1973年   208篇
排序方式: 共有10000条查询结果,搜索用时 639 毫秒
991.
992.
993.
994.
Coastal ecosystems are ecologically and commercially valuable, productive habitats that are experiencing escalating compromises of their structural and functional integrity. The Clean Water Act (USC 1972) requires identification of impaired water bodies and determination of the causes of impairment. Classification simplifies these determinations, because estuaries within a class are more likely to respond similarly to particular stressors. We reviewed existing classification systems for their applicability to grouping coastal marine and Great Lakes water bodies based on their responses to aquatic stressors, including nutrients, toxic substances, suspended sediments, habitat alteration, and combinations of stressors. Classification research historically addressed terrestrial and freshwater habitats rather than coastal habitats. Few efforts focused on stressor response, although many well-researched classification frameworks provide information pertinent to stressor response. Early coastal classifications relied on physical and hydrological properties, including geomorphology, general circulation patterns, and salinity. More recent classifications sort ecosystems into a few broad types and may integrate physical and biological factors. Among current efforts are those designed for conservation of sensitive habitats based on ecological processes that support patterns of biological diversity. Physical factors, including freshwater inflow, residence time, and flushing rates, affect sensitivity to stressors. Biological factors, such as primary production, grazing rates, and mineral cycling, also need to be considered in classification. We evaluate each existing classification system with respect to objectives, defining factors, extent of spatial and temporal applicability, existing sources of data, and relevance to aquatic stressors. We also consider classification methods in a generic sense and discuss their strengths and weaknesses for our purposes. Although few existing classifications are based on responses to stressors, may well-researched paradigms provide important information for improving our capabilities for classification, as an investigative and predictive management tool.  相似文献   
995.
996.
997.
998.
999.
In the low‐pressure, high‐temperature metamorphic rocks of western Maine, USA, staurolite porphyroblasts grew at c. 400 Ma, very late during the regional orogenesis. These porphyroblasts, which preserve straight inclusion trails with small thin‐section‐scale variation in pitch, were subsequently involved in the strain and metamorphic aureole of the c. 370 Ma Mooselookmeguntic pluton. The aureole shows a progressive fabric intensity gradient from effectively zero emplacement‐related deformation at the outer edge of the aureole ~2900 m (map distance) from the pluton margin to the development of a pervasive emplacement‐related foliation adjacent to the pluton. The development of this pervasive foliation spanned all stages of crenulation cleavage development, which are preserved at different distances from the pluton. The spread of inclusion‐trail pitches in the staurolite porphyroblasts, as measured in two‐dimensional (2‐D) thin sections, increases nonlinearly from ~16° to 75° with increasing strain in the aureole. These data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing crenulation cleavage. The data spread is qualitatively modelled for both pure and simple shear, and both solutions match the data reasonably well. The spread of inclusion‐trail orientations (40–75°) in the moderately to highly strained rocks is similar to the spread reported in several previous studies. We consider it likely that the sample‐scale spread in these previous studies is also the result of porphyroblast rotation relative to one another. However, the average inclusion‐trail orientation for a single sample may, in at least some instances, reflect the original orientation of the overgrown foliation.  相似文献   
1000.
Analyzing the tables and probability maps posted by Yan Y. Kagan and David D. Jackson in April 2002–September 2004 at http://scec.ess.ucla.edu/~ykagan/predictions_index.html and the catalog of earthquakes for the same period, the conclusion is drawn that the underlying method could be used for prediction of aftershocks, while it does not outscore random guessing when main shocks are considered.  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