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   2922篇
  免费   99篇
  国内免费   75篇
测绘学   80篇
大气科学   255篇
地球物理   649篇
地质学   943篇
海洋学   260篇
天文学   692篇
综合类   28篇
自然地理   189篇
  2022年   16篇
  2021年   37篇
  2020年   50篇
  2019年   42篇
  2018年   70篇
  2017年   74篇
  2016年   92篇
  2015年   71篇
  2014年   73篇
  2013年   152篇
  2012年   96篇
  2011年   122篇
  2010年   129篇
  2009年   136篇
  2008年   137篇
  2007年   117篇
  2006年   106篇
  2005年   108篇
  2004年   120篇
  2003年   118篇
  2002年   155篇
  2001年   146篇
  2000年   97篇
  1999年   66篇
  1998年   51篇
  1997年   48篇
  1996年   41篇
  1995年   45篇
  1994年   45篇
  1993年   29篇
  1992年   21篇
  1991年   30篇
  1990年   30篇
  1989年   19篇
  1988年   13篇
  1987年   15篇
  1986年   20篇
  1985年   28篇
  1984年   34篇
  1983年   24篇
  1982年   19篇
  1981年   22篇
  1980年   14篇
  1978年   20篇
  1976年   19篇
  1975年   17篇
  1974年   26篇
  1972年   21篇
  1971年   16篇
  1970年   13篇
排序方式: 共有3096条查询结果,搜索用时 15 毫秒
141.
De Gao 《湿地科学》2013,11(1):21-26
The application of arbuscular mycorrhizal (AM) inocula in severely degraded wetlands could ensure success in restoration.Mycorrhizal fungi play an important role in plant individual’s survival and development in a low nutrient condition.Based on the importance that mycorrhizal fungi have to their host plants,mycorrhizal inocula have been produced and applied in terrestrial ecosystems in order to let the plants become mycorrhizal.However,mycorrhizal inocula have not been used in wetland restorations,despite increasing evidence that mycorrhizal fungi are commonly found in various wetland systems and have the ability to survive under anoxic conditions.Evidence also shows that mycorrhizal fungal inocula in the soil could have been destroyed in the degraded wetland or could be destroyed during traditional wetland restoration process.Therefore,AM inocula production is strongly recommended for wetland restoration.In this paper,I will argue that AM inocula production is required when introduced recovery is necessary,and aeroponic culture technique is a preferable method to produce AM inocula.Last,a renewed wetland restoration flow chart is summarized.  相似文献   
142.
We present the application of novel diagnostics to the spectroscopic observation of a Coronal Mass Ejection (CME) on disk by the Extreme Ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft. We apply a recently developed line profile asymmetry analysis to the spectroscopic observation of NOAA AR 10930 on 14?–?15 December 2006 to three raster observations before and during the eruption of a 1000 km?s?1 halo CME. We see the impact that the observer’s line-of-sight and magnetic field geometry have on the diagnostics used. Further, and more importantly, we identify the on-disk signature of a high-speed outflow behind the CME in the dimming region arising as a result of the eruption. Supported by recent coronal observations of the STEREO spacecraft, we speculate about the momentum flux resulting from this outflow as a secondary momentum source to the CME. The results presented highlight the importance of spectroscopic measurements in relation to CME kinematics, and the need for full-disk synoptic spectroscopic observations of the coronal and chromospheric plasmas to capture the signature of such explosive energy release as a way of providing better constraints of CME propagation times to L1, or any other point of interest in the heliosphere.  相似文献   
143.
In light of the many improvements within 3D urban modeling and Location‐Based Services, this article provides a timely review of the state‐of‐the‐art on integrating indoor and outdoor spaces in pedestrian navigation guidance aids. With people moving seamlessly between buildings and surrounding areas, navigation guidance tools should extend from merely outdoor or indoor guidance, to provide support in the combined indoor‐outdoor context. This article first examines the challenges and complexities of integrating indoor and outdoor spaces into a single navigation system. Next, by using objective selection criteria, 36 relevant studies were withheld and further reviewed on their specific developments in data model requirements, and algorithmic and context support for integrated IO navigation systems. This review shows that the challenges of dealing with both indoor and outdoor space structures, while taking into account pedestrian's freer use of space, currently complicate the proposition of a unified IO space concept for navigation. However, there are some ongoing developments (e.g. context definitions, algorithmic extensions, increased data availability, growing awareness of pedestrians’ perception during wayfinding) that will help to bring outdoor and indoor spaces closer together in the realm of combined geospatial analysis.  相似文献   
144.
用场发射扫描电子显微镜及能谱仪和X射线衍射仪,对淄博市中心城区大气降尘的矿物组成、微形貌和微区成分进行了分析.结果表明,淄博市大气降尘的矿物组成有石英、长石、石膏、方解石、赤铁矿、白云母、伊利石和非晶质等.矿物微形貌特点和矿物组成揭示了降尘组分主要来源于3种途径:与高温过程有关的工业活动排放的产物,自然成因和大气化学反应的产物.球形赤铁矿、板状方解石和絮状石膏是本研究中观察到的3种典型矿物微形貌.  相似文献   
145.
Lack of high-spatial-resolution soil and sediment arsenic data for Hawai‘i has generated substantial disagreement between researchers and regulators regarding the magnitude of natural levels of arsenic in Hawai‘i and rendered difficult the defining of areas of anthropogenically elevated arsenic. Our earlier research into the occurrence of arsenic in terrestrial and marine environments revealed widely disparate concentrations of arsenic with no apparent spatial pattern. To better understand the distribution and abundance of arsenic in soils and sediments of O‘ahu, we collected an additional 64 samples at locations chosen to represent different environments with varying degrees of human impact. We found surface arsenic values that ranged from 0.28 to 740 ppm with a median concentration of 8.1 ppm, which is above the global median of 5 ppm and US soil median of 5.2 ppm. Higher concentrations of arsenic (up to 913 ppm) were encountered at depth in soil cores. The median arsenic in streambed sediments from one of our earlier studies of 6.1 ppm was comparable to the conterminous US median of 6.3 ppm; however, we encountered arsenic concentrations as high as 43.9 ppm (median = 8.60 ppm, n = 75) in marine sediments in recent work off the leeward coast of O‘ahu. Overall, arsenic in the soils and sediments of O‘ahu is elevated relative to world and national values, but there still is no readily discernible pattern in the distribution of arsenic to explain these elevated values.  相似文献   
146.
The Mancha Oriental System (MOS) has a surface area of 7,260 km2, making it one of the largest carbonate aquifer systems in Spain. The system sustains about 1,000 km2 of irrigated crops and supplies groundwater to 275,000 inhabitants. The economic transformation brought about by the development of extensive irrigated cropland has led to a water-balance disequilibrium of about 75 million m3/year. This input–output deficit has negative consequences in the quantity and quality of the available resources, in the river–aquifer relationship, and in the associated ecosystems as well. To understand the hydrogeological behaviour of the system, it is necessary to design a conceptual model. Further, the conceptualisation of a groundwater flow system is a requirement of the European Water Framework Directive for the characterisation of groundwater bodies. The robustness of the conceptual model depends heavily on the user capability of representing the real system. In this work, a multidisciplinary approach has been used to represent the three-dimensional geological framework and the groundwater flow conceptualisation of the MOS. Data management and three-dimensional visualisation have been carried out by means of geographical information system (GIS) tools and software for contouring and three-dimensional surface mapping.  相似文献   
147.
Summary A metadolerite of the ophiolites of the Northern Apennines (Vignola, Panaro Valley, near Modena, Italy) behaves as ocean-floor tholeiites when Ti, P, Zr, Cr, Y and Nb are considered. However, it deviates from typical oceanic tholeiites in that(1) it contains primary alkali amphiboles, (2) its clinopyroxenes plot in the diagram TiO2–MnO–Na2O not in the fields of the ocean-floor basalts, and (3) the calculated uranium content of the liquid in equilibrium with its magmatic minerals is similar to the one reported for alkali basalts.These results put constraints on the generalized significance attributed to the ophiolites of the Northern Apennines, i. e. not all of them were generated at spreading ridges similar to the normal ridge segments of the present-day ocean-floors, as has been generally accepted by now.The metamorphic parageneses indicate that the rock underwent low-pressure transformations, probably in oceanic environment, followed by recrystallizations at higher pressure, reasonably in concomitance with an orogenesis.
Klinopyroxene und Amphibole in einem Metadolerit aus dem Nördlichen Apennin. Folgerungen für die paläogeographische Stellung der Ophiolithe
Zusammenfassung Ein Metadolerit aus den Ophiolithen des Nördlichen Apennins (Vignola im Panaro-Tal bei Modena, Italien) entspricht in den Gehalten an Ti, P, Zr, Cr, Y und Nb den Tholeiiten der Ozeanböden. Er weicht hingegen von typischen ozeanischen Tholeiiten dadurch ab, daß er 1. primäre Alkaliamphibole enthält, daß 2. seine Klinopyroxene im Diagramm TiO2–MnO–Na2O nicht in die Felder der Tholeiite der Ozeanböden fallen und daß 3, der berechnete U-Gehalt für die mit seinen magmatischen Mineralen im Gleichgewicht stehenden liquiden Phase ähnlich jener ist, die für Alkalibasalte angegeben wird.Diese Ergebnisse schränken die generelle Bedeutung, die den Ophiolithen des Nördlichen Apennins bisher allgemein zugeschrieben wurde, ein: Nicht alle entstanden an sich ausdehnenden Rücken ähnlich jenen der heutigen Ozeanböden.Die metamorphen Parageneses zeigen, daß das Gestein bei niedrigem Druck, wahrscheinlich in ozeanischer Umgebung, Umwandlungen erlitt; dann folgten Rekristallisationen unter höherem Druck, die wohl zeitlich mit einer Orogenese zusammenfielen.


With 4 Figures  相似文献   
148.
The absence of human occupation sites in southeastern Brazil during the mid Holocene has been referred to as the 'Archaic Gap' (8970–1940 cal. a BP) and is predicted to have resulted from increased aridity. A ca. 14 000 cal. a pollen history from two well‐dated lake sediment cores located within the archeological district of Lagoa Santa, in the State of Minas Gerais, southeastern Brazil, was used to test this hypothesis. Our analyses indicated that the present cerrado and tropical semi‐deciduous forest mosaic persisted throughout the mid Holocene, until ca. 5500 cal. a BP, and the Lagoa Santa region did not experience especially dry conditions during the Holocene period. The early Holocene pollen spectra contained an assemblage of cold‐adapted taxa such as Podocarpus, Myrsine and Araucaria, co‐occurring with taxa from cerrado, e.g. Caryocar. A replacement of cold taxa by the modern cerrado–semi‐deciduous forest vegetation took place progressively, but appears to have been completed by the mid Holocene. No evidence of sustained drought was found in sedimentation or forest composition, nor any prolonged dry event in the study region. Holocene dryness as an explanation for the abandonment of Lagoa Santa region is not supported by the palynological analyses conducted in this study. Rather it is suggested that unpredictable climate may have underlain that abandonment. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
149.
In this work, we investigated a 3 ha sulphide-bearing waste-rock dump (Libiola Mine, Italy) using mineralogical, geochemical, and geostatistical analyses. The dumped materials were highly heterogeneous in grain size and lithology and varied both laterally and vertically. Other than the host rock of the ore, basalts and serpentinites, the dumped materials contained high amounts of low-grade chalcopyrite- and pyrite-rich mineralisations. Due to these characteristics and to the absence of minerals able to neutralise acidity, this waste-rock dump can be classified as an acid mine drainage (AMD) producer. The study confirms that AMD is still active and, in the best scenario, can persist for up to 6.17 × 103 years. The consequences of this process are of serious environmental concern as it involves strong acidification of the circulating waters, the release of potentially toxic metals into the soil, streams and rivers and the precipitation of huge quantities of secondary Fe-oxides and Fe-oxyhydroxides.  相似文献   
150.
Chemical analyses are given for actinolitic hornblendes of tonalitic rocks from the Hercynian belt of Northern Portugal. The distribution of elements between amphibole and co-existing biotite is studied. The composition of the amphiboles is analysed in the light of experimental data on amphiboles and the physical conditions of crystallization inferred from the study of the biotite and rock series. The data on the biotites lead to the definition of a temperature of 800°C for the crystallization of actinolitic hornblendes with Mg/(Mg + Fe) ratios of 0·72-0·61 at pressures of about 3 Kb and fO2 defined by FMQ.  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