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:      
  示例: 沙坡头地区,人工植被区,变化  检索词用空格隔开表示必须包含全部检索词,用“,”隔开表示只需满足任一检索词即可!
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   3514篇
  免费   47篇
  国内免费   9篇
测绘学   50篇
大气科学   304篇
地球物理   783篇
地质学   1218篇
海洋学   329篇
天文学   488篇
综合类   10篇
自然地理   388篇
  2021年   46篇
  2020年   45篇
  2019年   51篇
  2018年   60篇
  2017年   63篇
  2016年   68篇
  2015年   74篇
  2014年   69篇
  2013年   179篇
  2012年   92篇
  2011年   139篇
  2010年   146篇
  2009年   166篇
  2008年   129篇
  2007年   114篇
  2006年   119篇
  2005年   106篇
  2004年   90篇
  2003年   82篇
  2002年   82篇
  2001年   72篇
  2000年   43篇
  1999年   52篇
  1998年   53篇
  1997年   43篇
  1996年   66篇
  1995年   43篇
  1994年   48篇
  1993年   44篇
  1992年   52篇
  1991年   55篇
  1990年   49篇
  1989年   46篇
  1988年   50篇
  1987年   50篇
  1986年   37篇
  1985年   65篇
  1984年   83篇
  1983年   81篇
  1982年   66篇
  1981年   46篇
  1980年   52篇
  1979年   50篇
  1978年   49篇
  1977年   44篇
  1976年   38篇
  1975年   30篇
  1974年   28篇
  1973年   33篇
  1972年   24篇
排序方式: 共有3570条查询结果,搜索用时 223 毫秒
111.
112.
The δ34S values of dissolved sulfide and the sulfur isotope fractionations between dissolved sulfide and sulfate species in Floridan ground water generally correlate with dissolved sulfate concentrations which are related to flow patterns and residence time within the aquifer. The dissolved sulfide derives from the slow in situ biogenic reduction of sulfate dissolved from sedimentary gypsum in the aquifer. In areas where the water is oldest, the dissolved sulfide has apparently attained isotopic equilibrium with the dissolved sulfate (Δ34S = 65 per mil) at the temperature (28°C) of the system. This approach to equilibrium reflects an extremely slow reduction rate of the dissolved sulfate by bacteria; this slow rate probably results from very low concentrations of organic matter in the aquifer.In the reducing part of the Edwards aquifer, Texas, there is a general down-gradient increase in both dissolved sulfide and sulfate concentrations, but neither the δ34S values of sulfide nor the sulfide-sulfate isotope fractionation correlates with the ground-water flow pattern. The dissolved sulfide species appear to be derived primarily from biogenic reduction of sulfate ions whose source is gypsum dissolution although upgradient diffusion of H2S gas from deeper oil field brines may be important in places. The sulfur isotope fractionation for sulfide-sulfate (about 38 per mil) is similar to that observed for modern oceanic sediments and probably reflects moderate sulfate reduction in the reducing part of the aquifer owing to the higher temperature and significant amount of organic matter present; contributions of isotopically heavy H2S from oil field brines are also possible.  相似文献   
113.
    
Shoreface sandstone deposits within the Early Carnian part of the Snadd Formation of the Norwegian Barents Sea can be traced for hundreds of kilometres in the depositional strike direction and for tens of kilometres in the depositional‐dip direction. This study uses three‐dimensional seismic attribute mapping and two‐dimensional regional seismic profiles to visualize the seismic facies of these shoreface deposits and to map their internal stratigraphic architecture at a regional scale. The shoreface deposits are generally elongate but show variable width from north‐east to south‐west, which corresponds to a sediment source in the northern part of the basin and a southward decrease in longshore sediment transport. The Snadd Formation presents an example of how large‐scale progradational shoreface deposits develop. The linear nature of its shoreface deposits contrasts with more irregular, cuspate wave‐dominated deltaic shorelines that contain river outlets, and instead implies longshore drift as the main sediment source. In map view, discrete sets of linear features bounded by truncation surfaces scale directly to beach ridge sets in modern counterparts. The shoreface deposits studied here are characteristic in terms of scale and basin‐wide continuity, and offer insight into the contrast between shallow marine deposition under stable Triassic Greenhouse and fluctuating Holocene Icehouse climates. Findings presented herein are also important for hydrocarbon exploration in the Barents Sea, because they describe a hitherto poorly understood reservoir play in the Triassic interval, wherein the most prominent reservoir plays have so far been considered to be found in channelized deposits in net‐progradational delta‐plain strata that form the topsets to shelf‐edge clinoforms. The documented presence of widespread wave‐dominated shoreface deposits also has implications for how the relative importance of different sedimentary processes is considered within the basin during this period.  相似文献   
114.
Chromophoric dissolved organic matter (CDOM) is the major light absorber in the Baltic Sea. In this study, excitation emission matrix (EEM) fluorescence spectra and UV–visible absorption spectra of CDOM are reported as a function of salinity. Samples from different locations and over different seasons were collected during four cruises in 2002 and 2003 in the Baltic Sea in both Pomeranian Bay and the Gulf of Gdansk. Absorption by CDOM decreased with increased distance from the riverine source and reached a relatively stable absorption background in the open sea. Regression analysis showed that fluorescence intensity was linearly related to absorption by CDOM at 375 nm and aCDOM(375) absorption coefficients were inversely related to salinity. Analysis of CDOM-EEM spectra indicated that a change in composition of CDOM occurred along the salinity gradient in the Baltic Sea. Analysis of percent contribution of respective fluorophore groups to the total intensity of EEM spectra indicated that the fluorescence peaks associated with terrestrial humic components of the CDOM and total integrated fluorescence decreased with decreasing CDOM absorption. In contrast, the protein-like fraction of CDOM decreased to a lesser degree than the others. Analysis of the percent contribution of fluorescence peak intensities to the total fluorescence along the salinity gradient showed that the contribution of protein-like fluorophores increased from 2.6% to 5.1% in the high-salinity region of the transect. Fluorescence and absorption changes observed in the Baltic Sea were similar to those observed in similar transects that have been sampled elsewhere, e.g. in European estuaries, Gulf of Mexico, Mid-Atlantic Bight and the Cape Fear River plume in the South Atlantic Bight, although the changes in the Baltic Sea occurred over a much smaller salinity gradient.  相似文献   
115.
116.
    
The paper presents the concept, the objectives, the approach used, and the expected performances and accuracies of a radioscience experiment based on a radio link between the Earth and the surface of Mars. This experiment involves radioscience equipment installed on a lander at the surface of Mars. The experiment with the generic name lander radioscience (LaRa) consists of an X-band transponder that has been designed to obtain, over at least one Martian year, two-way Doppler measurements from the radio link between the ExoMars lander and the Earth (ExoMars is an ESA mission to Mars due to launch in 2013). These Doppler measurements will be used to obtain Mars’ orientation in space and rotation (precession and nutations, and length-of-day variations). More specifically, the relative position of the lander on the surface of Mars with respect to the Earth ground stations allows reconstructing Mars’ time varying orientation and rotation in space.Precession will be determined with an accuracy better by a factor of 4 (better than the 0.1% level) with respect to the present-day accuracy after only a few months at the Martian surface. This precession determination will, in turn, improve the determination of the moment of inertia of the whole planet (mantle plus core) and the radius of the core: for a specific interior composition or even for a range of possible compositions, the core radius is expected to be determined with a precision decreasing to a few tens of kilometers.A fairly precise measurement of variations in the orientation of Mars’ spin axis will enable, in addition to the determination of the moment of inertia of the core, an even better determination of the size of the core via the core resonance in the nutation amplitudes. When the core is liquid, the free core nutation (FCN) resonance induces a change in the nutation amplitudes, with respect to their values for a solid planet, at the percent level in the large semi-annual prograde nutation amplitude and even more (a few percent, a few tens of percent or more, depending on the FCN period) for the retrograde ter-annual nutation amplitude. The resonance amplification depends on the size, moment of inertia, and flattening of the core. For a large core, the amplification can be very large, ensuring the detection of the FCN, and determination of the core moment of inertia.The measurement of variations in Mars’ rotation also determines variations of the angular momentum due to seasonal mass transfer between the atmosphere and ice caps. Observations even for a short period of 180 days at the surface of Mars will decrease the uncertainty by a factor of two with respect to the present knowledge of these quantities (at the 10% level).The ultimate objectives of the proposed experiment are to obtain information on Mars’ interior and on the sublimation/condensation of CO2 in Mars’ atmosphere. Improved knowledge of the interior will help us to better understand the formation and evolution of Mars. Improved knowledge of the CO2 sublimation/condensation cycle will enable better understanding of the circulation and dynamics of Mars’ atmosphere.  相似文献   
117.
    
The physical geography of the limestone islands of Palau permits permanent water-column stratification in 13 tropical sea-level marine lakes, each with unique water-column physics, chemistry, and biology. Embayments and lagoons amid the coral became isolated as marine lakes after Miocene uplifting. Surface mixing of lake water by wind is reduced by jungle-covered karst ridges. Surface tidal exchange through fissures in fenestrated karst is slow while midwater exchange through submarine tunnels is fast, but both produce damped, delayed tides with modest seawater exchange from the barrier-reef lagoon. Topographic protection from wind, heavy regular rain throughout the year, with precipitation exceeding evaporation, and modest tidal exchange produce stratified water columns with brackish waters above permanently anoxic saline hypolimnia. Permanent lake stratification is documented for 18 years; sediment cores (by others) show stratification for >100 years, and recent constant sea level implies ecosystem stability for thousands of years. Therefore, the marine lakes in Palau are small, closed, simple ecosystems that do not change over time–steady-state chemostats permitting replicate field measurements of biological and physical attributes from day to day, month to month, or decade to decade. [Key words: Palau, meromictic, marine lake, karst, lake stratification, tides, model ecosystem.]  相似文献   
118.
    
Abundances of O, Si, Al and Mn have been determined in Luna 20 fines sample 22001,9 by instrumental neutron activation analysis. The abundances of O, Si and Al are among the highest we have observed in lunar samples and reflect a highlands origin for much of this regolith sample. The Luna 20 abundances reported here most closely resemble those we have determined in four samples of two Apollo 16 fines, rock 14310, and a clast from breccia 15459. The Luna 20 OSi ratio of 1.96 ± 0.05 is similar to that in most other lunar samples, but the AlSi ratio of 0.532 ± 0.024 is exceeded only by our data on the Apollo 16 fines. This AlSi ratio is in agreement with the value of 0.55 ± 0.06 determined by the remote X-ray fluorescence experiment for the highlands between Mare Crisium and Mare Smythii which lie near the Luna 20 site (Adleret al., 1972).  相似文献   
119.
  总被引:1,自引:0,他引:1  
Crustal shortening of the ocean floor in the eastern Mediterranean is recognized by a marked thickening of the sedimentary layer seaward of the Hellenic and Calabrian island arcs. Steep gradients and large negative free-air anomalies in the gravity field along with a highly uniform, low regional heat flow are manifestations of the thickened crust. Bodies of recently deformed sediment in and seaward of the Hellenic Trough reveal the style, polarity, and dynamics of the thickening mechanism.

A linear buried anticlinal structure, inferred from analysis of surface ship gravity profiles, may mark the site of contemporary intrabasinal underthrusting. The distribution of earthquakes beneath the Mediterranean Ridge supports the interpretation that the Anaximander, Ptolomy, and Strabo Mountains are features comparable to large basement nappes. Cyprus is one such structure, offset to the south, where the oceanic crust and part of the upper mantle have been involved in the décollement.  相似文献   

120.
We present a numerical method for solving a class of systems of partial differential equations (PDEs) that arises in modeling environmental processes undergoing advection and biogeochemical reactions. The salient feature of these PDEs is that all partial derivatives appear in linear expressions. As a result, the system can be viewed as a set of ordinary differential equations (ODEs), albeit each one along a different characteristic. The method then consists of alternating between equations and integrating each one step-wise along its own characteristic, thus creating a customized grid on which solutions are computed. Since the solutions of such PDEs are generally smoother along their characteristics, the method offers the potential of using larger time steps while maintaining accuracy and reducing numerical dispersion. The advantages in efficiency and accuracy of the proposed method are demonstrated in two illustrative examples that simulate depth-resolved reactive transport and soil carbon cycling.  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