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   1712篇
  免费   480篇
  国内免费   762篇
测绘学   20篇
大气科学   147篇
地球物理   267篇
地质学   2081篇
海洋学   145篇
天文学   8篇
综合类   60篇
自然地理   226篇
  2024年   16篇
  2023年   28篇
  2022年   59篇
  2021年   84篇
  2020年   67篇
  2019年   121篇
  2018年   111篇
  2017年   127篇
  2016年   103篇
  2015年   133篇
  2014年   107篇
  2013年   164篇
  2012年   142篇
  2011年   110篇
  2010年   108篇
  2009年   137篇
  2008年   156篇
  2007年   133篇
  2006年   146篇
  2005年   111篇
  2004年   102篇
  2003年   115篇
  2002年   84篇
  2001年   74篇
  2000年   95篇
  1999年   61篇
  1998年   44篇
  1997年   34篇
  1996年   33篇
  1995年   39篇
  1994年   18篇
  1993年   17篇
  1992年   19篇
  1991年   17篇
  1990年   15篇
  1989年   3篇
  1988年   4篇
  1987年   4篇
  1986年   3篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
  1982年   1篇
  1979年   1篇
  1978年   2篇
  1973年   1篇
排序方式: 共有2954条查询结果,搜索用时 15 毫秒
31.
Ozone has been observed in elevated concentrations by satellites over areas previously believed to be background. There is meteorological evidence, that these ozone plumes found over the Atlantic Ocean originate from vegetation fires on the African continent.In a previous study (DECAFE-88), we have investigated ozone and assumed precursor compounds over African tropical forest regions. Our measurements revealed large photosmog layers at altitudes from 1.5 to 4 km. Both chemical and meteorological evidence point to savanna fires up to several thousand km upwind as sources.Here we describe ozone mixing ratios observed over western Africa and compare ozone production ratios from different field measurement campaigns related to vegetation burning. We find that air masses containing photosmog ingredients require several days to develop their oxidation potential, similar to what is known from air polluted by emissions from fossil fuel burning. Finally, we estimate the global ozone production due to vegetation fires and conclude that this source is comparable in strength to the stratospheric input.  相似文献   
32.
Ophiomorpha irregulaire is a large and extremely rare trace fossil. The type section is in the Book Cliffs, Utah, USA. A second on-shore locality is reported here, in the Nuussuaq Basin of West Greenland. Both occurrences date from the Late Cretaceous. Other reports of O. irregulaire are restricted to two-dimensional sections in well cores and are mostly Jurassic in age. The West Greenland occurrence is in the Atane Formation in shallow marine clean sandstone. The two on-shore localities lie in the same biogeographical region, in connection with the Cretaceous Western Interior Seaway of North America. O. irregulaire is characterized by a sinuous, branched meander maze lying mostly within a single horizontal plane, probably not far beneath the seafloor. The burrow lining has remarkably elongate pellets that probably were emplaced by the burrower. In some cases, diagenetic compression may have deformed the lining pellets to produce their elongated shape. The muddy sediment comprising the lining is almost unrepresented in the clean sands of the substrate, and it was probably trapped from suspension during suspension-feeding activity.  相似文献   
33.
The precipitation climatology and the underlying climate mechanisms of the eastern Mediterranean, West Asia, and the Indian subcontinent are reviewed, with emphasis on upper and middle tropospheric flow in the subtropics and its steering of precipitation. Holocene climate change of the region is summarized from proxy records. The Indian monsoon weakened during the Holocene over its northernmost region, the Ganges and Indus catchments and the western Arabian Sea. Southern regions, the Indian Peninsula, do not show a reduction, but an increase of summer monsoon rain across the Holocene. The long-term trend towards drier conditions in the eastern Mediterranean can be linked to a regionally complex monsoon evolution. Abrupt climate change events, such as the widespread droughts around 8200, 5200 and 4200 cal yr BP, are suggested to be the result of altered subtropical upper-level flow over the eastern Mediterranean and Asia.The abrupt climate change events of the Holocene radically altered precipitation, fundamental for cereal agriculture, across the expanse of late prehistoric-early historic cultures known from the archaeological record in these regions. Social adaptations to reduced agro-production, in both dry-farming and irrigation agriculture regions, are visible in the archaeological record during each abrupt climate change event in West Asia. Chronological refinement, in both the paleoclimate and archaeological records, and transfer functions for both precipitation and agro-production are needed to understand precisely the evident causal linkages.  相似文献   
34.
Erratum to: GeoJournal 62: 51–58 DOI 10.1007/s10708-005-8562-2 The erroneous version of this article was originally published in GeoJournal 62: 51–58. This amended version corrects the formatting errors which are evident in the original version.Abstract It is universally accepted that the need for defining precise lines of separation and points of contacts between states is the byproduct of the emergence of nation-states and ‘world economy’ in the nineteenth century Europe. Nevertheless, it is hard to overlook the fact that these modern notions are rooted in periods prior to the emergence in Europe of nation-states. There are indications that ancient civilizations were familiar with the notion of ‘state’ in connection with the concepts of territory and boundary. Ancient texts reveal that this basic principle existed in ancient Persian literature in respect of matters of state, territory, and boundary. Similarly, the likelihood exists that these Persian notions could have influenced Roman civilization. It is widely believed that a combination of ancient Greco-Roman and Persian civilizations is a major contributor to what culturally constitutes ‘West’. Later in the Sassanid period the inter-linked notions of state, territory, and boundary developed substantially, coming quite close to their contemporary forms. On the other hand, considering that ‘justice’ was the corner stone of ancient Persian Political philosophy, the idea that ancient␣Iranian spatial arrangement might have contributed to the evolution of the concept of democracy in the West may not be too difficult to contemplate.The online version of the original article can be found at  相似文献   
35.
Paleontological study of Upper Jurassic and Lower Cretaceous sediments recovered by boreholes in the Agan-Vakh and Nadym-Vengapur interfluves clarified environments of their deposition. As is shown, influx of siliciclastic material to central areas of the West Siberian sea basin varied through time. Taxonomic composition and ecological structure of nektonic and benthic fossil assemblages are analyzed and considered in terms of environmental factors such as hydrodynamics, aeration, temperature, and salinity of seawater.  相似文献   
36.
东准噶尔巴塔玛依内山组是典型陆相火山.沉积体系(盆地),主体由基性及中酸性火山熔岩组成,火山碎屑岩及火山碎屑沉积岩较少,通过对纸房地区晚石炭世巴塔玛依内山组陆相火山岩的岩石化学、地球化学特征进行系统研究,认为它属于造山期后固结初期,新陆壳裂谷向高原火山岩演化的同岩浆源、同沉积盆地、同火山作用的钙碱系列双峰式火山岩.  相似文献   
37.
滇西潞西上芒岗卡林型金矿   总被引:1,自引:0,他引:1  
王学武  杨世瑜 《云南地质》2005,24(4):354-360
上芒岗金矿赋存于二叠系沙子坡组碳酸岩和侏罗系勐戛组下段砂岩、灰岩中,明显受上芒岗断裂控制,与美国模式卡林型金矿极为相似,具有相同的物质来源和成因模式。上芒岗地区具有广阔的深部找矿前景。  相似文献   
38.
对新疆准噶尔地区浅成低温热液型金矿床中富硫型的阔尔真阔腊金矿、贫硫型的石英滩金矿进行了流依包裹体的均一温度、爆裂温度、包裹体气液相成分、H、O 同位素、矿体围岩及脉石英包裹体 C 同位素、矿体中黄铁矿等 S、Ph 同位素等系统地进行了研究,综合研究表明,本区该类型金矿成矿流体一般温度低、盐度低,来源主要为循环的大气水、矿石中黄铁矿的 S、Pb 同位素均为深源,暗示金的深部来源:矿体石英包裹体中 CO_2的δ~(13)C 为低于-10‰的有机碳,反映了本区年轻的富含有机质的沉积地层参与了金的成矿。此外,本文首次提出了富硫型阔尔真阔腊金矿床成矿流体中有侵入岩浆热液参与,深部有多金属成矿远景;贫硫型石英滩金矿没有侵入岩浆热液的参与,成矿仅与火山古热液活动有关,其成矿较单一。此外,阔尔真阔腊金矿中低温流体活动较强,金矿化也较强:石英滩金矿低温流体活动相对较弱,金矿化也较弱,也体现了该类型金矿床低温流体活动的越强烈,金矿化越强的规律。  相似文献   
39.
中国西部地区气温与太平洋海温的关系   总被引:5,自引:0,他引:5  
利用1961~1990年我国西部(110 E以西)155个测站的地面月平均气温和同期太平洋地区月平均海温资料和经验正交函数(EOF)展开方法,研究了中国西部地区各个季节的气温距平时空分布规律以及西部地区气温距平与太平洋海温异常之间的相互联系。结果发现:利用EOF展开得到的特征向量和时间系数能够较好地反映西部地区实际气温异常的时空分布特征。El Nino和La Nina年西部气温距平分布(冬季除外)呈现基本相反的状况,即在El Nino年气温偏暖(冷)的区域,在La Nina年则是气温偏冷(暖)的区域。各季节西部地区的气温变化与太平洋海温异常存在着相互影响和作用。  相似文献   
40.
中国西北部盆地岩石热导率和生热率特征   总被引:19,自引:0,他引:19       下载免费PDF全文
邱楠生 《地质科学》2002,37(2):196-206
本文根据大量实测数据,首次系统地报道了中国西北地区塔里木盆地、准噶尔盆地和柴达木盆地内的岩石热导率、岩石放射性生热率数据及其分布特征.对600多个岩石热导率和100多个实测岩石生热率的统计分析表明,沉积盆地中岩石的热物理性质与其岩性、埋藏深度和地层时代密切相关.随深度和地层时代的加大,岩石热导率增大;塔里木盆地的岩石热导率的总体平均值最大,而柴达木盆地的最小.岩石生热率在上地壳的分布是随深度的增加而减小的,但在沉积盆地的深度范围内几乎不变,其分布是均匀的,仅不同岩性的生热率差别较大.估算的岩石放射性生热产生的热量可以占到盆地地表热流的25%~45%.因此,岩石热物理性质的参数不仅与盆地的地温分布和大地热流特征密切相关,还可以为该地区盆地热历史恢复及深部地球物理的研究提供有效的参数和边界条件.  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