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   1604篇
  免费   63篇
  国内免费   17篇
测绘学   35篇
大气科学   117篇
地球物理   406篇
地质学   574篇
海洋学   154篇
天文学   196篇
综合类   3篇
自然地理   199篇
  2023年   6篇
  2021年   18篇
  2020年   33篇
  2019年   27篇
  2018年   37篇
  2017年   49篇
  2016年   49篇
  2015年   40篇
  2014年   49篇
  2013年   79篇
  2012年   54篇
  2011年   99篇
  2010年   81篇
  2009年   92篇
  2008年   76篇
  2007年   50篇
  2006年   83篇
  2005年   71篇
  2004年   54篇
  2003年   57篇
  2002年   46篇
  2001年   28篇
  2000年   30篇
  1999年   22篇
  1998年   32篇
  1997年   21篇
  1996年   29篇
  1995年   22篇
  1994年   26篇
  1993年   20篇
  1992年   17篇
  1991年   12篇
  1990年   18篇
  1989年   17篇
  1988年   13篇
  1987年   11篇
  1986年   16篇
  1985年   20篇
  1984年   21篇
  1983年   14篇
  1982年   29篇
  1981年   17篇
  1980年   7篇
  1979年   13篇
  1978年   13篇
  1977年   10篇
  1976年   10篇
  1975年   5篇
  1974年   5篇
  1973年   13篇
排序方式: 共有1684条查询结果,搜索用时 15 毫秒
141.
A rank series of lignites and coals of low to moderate maturation levels (vitrinite reflectance (R0): 0.27–0.8%) from the New Zealand Coal Band were investigated using alkaline ester cleavage experiments to reveal compositional changes of ester bound components (fatty acids and alcohols) during increasing maturation. Ester bound alcohols are found to be present in highest amounts in the very immature lignite samples (R0: 0.27–0.29%), but show a rapid decrease during early diagenesis. Ester bound fatty acids also show an initial exponential decrease during diagenesis, but reveal an intermittent increase during early catagenesis before decreasing again during main catagenesis. This intermittent increase was related to the short chain fatty acids. To obtain a maturity related signal and to eliminate facies related scattering in the amounts of fatty acids in the coal samples, the carbon preference index of fatty acids (CPIFA) parameter is introduced here. For the long chain fatty acids (C20–C32) originating from terrigenous plant debris, the CPIFA decreases with increasing maturity, showing a strong maturation related signal. During diagenesis, the same trend can be observed for the short chain fatty acids, but the intermittent increase in the amounts of short chain fatty acids is also accompanied by high CPIFA values. This indicates less altered organic biomass at this advanced maturation level and is in contrast to the mature CPIFA signal of the long chain fatty acids of the same samples. One possible reason for this discrepancy could be extremely different amounts of short and long chain fatty acids in the original source organic matter of these samples. However, another intriguing explanation could be the incorporation of immature bacterial biomass from deep microbial communities containing C16 and C18 fatty acids as main cell membrane components. Deep microbial life might be stimulated at this interval by the increasing release of thermally generated potential substrates from the organic matrix during early catagenesis. In contrast to the fatty acids, the high amounts of alcohols in the immature lignite samples are also visible in the alkene distribution from the open system pyrolysis experiments of the organic matrix before and after saponification.  相似文献   
142.
Spectacular growth has marked the industry initiated by Malcolm McLean with the sailing of the Ideal-X in 1956. While the growth of container shipping has been typically seen in terms of technological advances, increasing vessel capacity, traffic growth, financial performance and competitiveness, it has been shaped also by organisational transformations. This paper provides an overview of the major companies that make up the container shipping industry, tracing the rapid adoption of containerisation by American carriers to its diffusion to Europe and then Asia. While several carriers belong to business conglomerates, the most dynamic in recent years have been those that are those that possess a family structure. About 12 of the present top 20 carriers are largely family controlled, including 4 out of the top 5. Unlike other capital intensive industries, where the power has shifted towards corporate governance, the container shipping industry retains a strong individualistic entrepreneurial character. At a time when North American ownership in container shipping is no more, the spirit of innovation began 50 years ago by an American visionary is still evident in the entrepreneurial dynamism of many of the industry leaders.  相似文献   
143.
There is substantial literature on the importance of bridging across disciplinary and science–management boundaries. One of the ways commonly suggested to cross boundaries is for participants from both sides of the boundary to jointly produce information (i.e., knowledge co-production). But simply providing tools or bringing people together in the same room is not sufficient. Here we present a case study documenting the mechanisms by which managers and scientists collaborated to incorporate climate change projections into Colorado’s State Wildlife Action Plan. A critical component of the project was the use of a collaborative modeling and visualization workspace: the U.S. Geological Survey’s Resource for Advanced Modeling (RAM). Using video analysis and pre/post surveys from this case study, we examine how the RAM facilitated cognitive and social processes that co-produced a more salient and credible end product. This case provides practical suggestions to scientists and practitioners who want to implement actionable science.  相似文献   
144.
This note critically examines the relative importance of several effects which influence the diurnal variation of atomic hydrogen abundance near the critical level.It is pointed out that the neglect of exospheric hydrogen in a recent theoretical treatment causes an overestimation of the diurnal variation at high exospheric temperatures, and an underestimation at low exospheric temperatures. The fluxes due to lateral flow are large compared to other fluxes only to the extent that the actual diurnal variation is very different from the diurnal variation corresponding to zero net lateral flow, which does not seem to be the case in the real atmosphere. Two effects which are probably important are charge exchange reactions with thermal oxygen ions, resulting in a diurnal exchange with the plasmasphere; and charge exchange reactions with high velocity protons, resulting in enhanced escape and diurnal variation.  相似文献   
145.
146.
Recent research on the role of public capital in the economy focuses primarily on assessing its economic, and sometimes spatial economic, impacts. Access to more detailed and disaggregate data than typically used in these analyses allows us to take a fresh perspective on the often conflicting goals of interregional equity and aggregate efficiency. Using the state of Ohio as a case study, and classic definitions of equity and efficiency, we assess the correspondence between distributions of infrastructure investment and the social/economic distress they are intended to alleviate. Traditional map and statistical analysis combined with a graphical device we call the variegated distribution plot reveals that, in both rates and levels, investment is highest in areas of greatest distress. Both patterns are consistent with equity-driven investment distributions.  相似文献   
147.
This article describes the design, operation, and field testing of the depthimeter. The depthimeter merges heave and acoustically derived vessel depth to form estimates of instantaneous vessel depth and instantaneous sea surface height, both relative to mean sea level. Results from sea trials held in December 1997 demonstrate successful operation of the system.  相似文献   
148.
149.
The SW England Rhenohercynian passive margin initiated with rift-related non-marine sedimentation and bimodal magmatism (Late Lockhovian). Continued lithospheric extension resulted in the exhumation of mantle peridotites and limited seafloor spreading (Emsian-Eifelian). Variscan convergence commenced during the Late Eifelian and was coeval with rifting further north. Collision was marked by the Early Carboniferous emergence of deep marine sedimentary/volcanic rocks from the distal continental margin, oceanic lithosphere, pre-rift basement and upper plate gneisses (correlated with the Mid-German Crystalline High of the Saxothuringian Zone). Progressive inversion of the passive margin was strongly influenced by rift basin geometry. Convergence ceased in the Late Carboniferous and was replaced by an extensional regime that reactivated basin controlling/thrust faults and reorientated earlier fabrics (Start-Perranporth Zone). The resultant exhumation of the lower plate was accompanied by emplacement of the Early Permian SW England granites and was contemporaneous with upper plate sedimentary basin formation above the reactivated Rhenohercynian suture. The Rhenohercynian passive margin probably developed in a marginal basin north of the Rheic Ocean or, possibly, a successor basin following its closure. The Lizard ophiolite is unlikely to represent Rheic Ocean floor or associated forearc (SSZ) crust. The Rheic and Rhenohercynian sutures may be coincident or the Rheic suture may be located further south in the Léon Domain.  相似文献   
150.
In this study, we combined grain size and geochemical analyses with radioisotope analysis of lead-210 (210Pb), caesium-137 (137Cs) and radiocarbon (14C) ages to reconstruct the sedimentation history of two urbanised coastal lagoons in south-east Australia. Towradgi and Fairy Lagoons were both found to exhibit slow initial sedimentation of less than 1 mm year−1 prior to anthropogenic influences. Land clearing in the catchments increased runoff and erosion in the creeks feeding into the estuaries, and has resulted in progradation of fluvial material into the estuarine systems with a marked increase in sedimentation to between 2 and 7 mm year−1. The upper 20–50 cm of the sediment column in both lagoons contained elevated concentrations of heavy metals such as Pb, Cu, Ni and Zn. This pollution trend was found to be consistent with the history of industrialisation and urbanisation in the region, which expanded rapidly post World War 2. The total metal concentrations were consistent with other urbanised/industrialised estuaries around the world. Despite the fairly disturbed nature of these coastal lagoons, the use of 210Pb, 137Cs and 14C dating in combination with bulk geochemical analyses allowed detailed reconstruction of sedimentation history.  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