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   37714篇
  免费   749篇
  国内免费   592篇
测绘学   949篇
大气科学   3141篇
地球物理   7587篇
地质学   12496篇
海洋学   3453篇
天文学   8831篇
综合类   122篇
自然地理   2476篇
  2021年   203篇
  2020年   268篇
  2019年   273篇
  2018年   632篇
  2017年   578篇
  2016年   898篇
  2015年   637篇
  2014年   876篇
  2013年   1932篇
  2012年   1066篇
  2011年   1483篇
  2010年   1218篇
  2009年   1811篇
  2008年   1608篇
  2007年   1518篇
  2006年   1434篇
  2005年   1312篇
  2004年   1226篇
  2003年   1192篇
  2002年   1106篇
  2001年   997篇
  2000年   1008篇
  1999年   932篇
  1998年   850篇
  1997年   850篇
  1996年   726篇
  1995年   654篇
  1994年   571篇
  1993年   525篇
  1992年   512篇
  1991年   492篇
  1990年   483篇
  1989年   431篇
  1988年   405篇
  1987年   458篇
  1986年   438篇
  1985年   536篇
  1984年   594篇
  1983年   571篇
  1982年   545篇
  1981年   483篇
  1980年   449篇
  1979年   414篇
  1978年   428篇
  1977年   377篇
  1976年   341篇
  1975年   346篇
  1974年   350篇
  1973年   352篇
  1972年   207篇
排序方式: 共有10000条查询结果,搜索用时 500 毫秒
971.
We present a comprehensive paleomagnetic study on Paleoproterozoic (2173–2060 Ma) plutonic and metamorphic rocks from French Guiana, representative of the full range of the main Transamazonian tectonothermal steps. Twenty-seven groups of directions and poles were obtained from combination of 102 sites (613 samples) based on age constraint, similar lithology and/or geographical proximity. Paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases. This is also reinforced by positive field tests (baked contact and reversal tests). Recent U/Pb and Pb/Pb on zircon and complementary 40Ar/39Ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages. Estimated magnetic ages, based on amphibole dating as a proxy, enable us to construct a Guiana Shield apparent polar wander path for the 2155–1970 Ma period. It is also possible to present paleolatidudinal evolution and continental drift rates related to specific Transamazonian tectonic regimes.French Guiana and probably the Guiana Shield were located at the Equator from ca. 2155 to 2130 Ma during the Meso-Rhyacian D1 magmatic accretion phase, related to subduction of Eorhyacian oceanic crust. After closure of the Eorhyacian Ocean and collision of West African and Amazonian plates, the Guiana Shield moved. The first evolution towards 60° latitude, occurs after 2080 Ma, during the Neorhyacian D2a post collisional sinistral transcurrent phase. During the Late Rhyacian D2b phase, up to 2050 Ma, the Guiana Shield reaches the pole and starts to move to lower latitudes on an opposite meridian. By the Orosirian D2c phase, from ca. 2050 to 1970 Ma, the Guiana Shield reaches the Equator.Based on the amphibole 40Ar/39Ar dates, we estimate the continental drift between 12 and 16 cm/y for the Meso to Late Rhyacian period followed by a lower rate between 9 and 14 cm/y up to Orosirian time. This study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100–200 Ma time period. Our results confirm the possibility of APWP construction on Paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi-method radiometric dating.  相似文献   
972.
Ingestion of enhanced zinc can cause memory impairments and copper deficiencies. This study examined the effect of zinc supplementation, with and without copper, on two types of memory. Rats raised pre- and post-natally on 10 mg/kg ZnCO3 or ZnSO4 in the drinking water were tested in a fear-conditioning experiment at 11 months of age. Both zinc groups showed a maladaptive retention of fearful memories compared to controls raised on tap water. Rats raised on 10 mg/kg ZnCO3, 10 mg/kg ZnCO3 + 0.25 mg/kg CuCl2, or tap water, were tested for spatial memory ability at 3 months of age. Significant improvements in performance were found in the ZnCO3 + CuCl2 group compared to the ZnCO3 group, suggesting that some of the cognitive deficits associated with zinc supplementation may be remediated by addition of copper.  相似文献   
973.
This work is devoted to the physico-chemical study of cadmium and lead interaction with diatom–water interfaces for two marine planktonic (Thalassiosira weissflogii = TW, Skeletonema costatum = SC) and two freshwater periphytic species (Achnanthidium minutissimum = AMIN, Navicula minima = NMIN) by combining adsorption measurements with surface complexation modeling. Reversible adsorption experiments were performed at 20 °C after 3 h of exposure as a function of pH, metal concentration in solution, and ionic strength. While the shape of pH-dependent adsorption edge is similar among all four diatom species, the constant-pH adsorption isotherm and maximal binding capacities differ. These observations allowed us to construct a surface complexation model for cadmium and lead binding by diatom surfaces that postulates the constant capacitance of the electric double layer and considers Cd and Pb complexation with mainly carboxylic and, partially, silanol groups. Parameters of this model are in agreement with previous acid–base titration results and allow quantitative reproduction of all adsorption experiments.  相似文献   
974.
Bacterially mediated methanogenesis in municipal solid waste landfills has been shown to cause an enrichment of carbon stable isotope ratios of dissolved inorganic carbon and hydrogen stable isotope ratios of water in landfill leachate. In the present study, we investigate the universality of this enrichment in leachate obtained from four diverse landfill sites in New Zealand. At each site, surface water samples upstream and downstream of landfills were analysed to examine the applicability of stable isotope ratios as a tool for monitoring leachate contamination in landfill-associated streams. The design of leachate collection systems, operational history, and landfill location appeared to strongly influence leachate isotopic values and the effectiveness of isotope ratios as an environmental monitoring tool for surface water.  相似文献   
975.
The Capim River kaolin, located in the eastern Brazilian Amazon, constitutes one of the most important kaolin deposits in the world. Known for its high whiteness, its noble application is in the paper industry. Studies were carried out on samples from the six facies of the deposit (sand kaolin, soft kaolin, lower transition facies, ferruginous crust, upper transition facies and flint kaolin) in order to trace its geochemical evolution. The kaolin developed at the expense of Cretaceous sandy–clayey sediments of the Ipixuna Formation. Intense lateritic processes characterized by ferruginization and deferruginization mechanisms led to the distinction of the different facies.  相似文献   
976.
A multiproxy approach (textural characteristics, heavy metal concentrations and benthic foraminifera data) was adopted to study the OMEX core KSGX 40, collected at the Galicia Mud Deposit (NW Iberian outer continental shelf) and recording the last ca. 4.8 ka cal BP. Geochemical profiles of Fe, Mn, Zn, Co, Cr, Cu, Ni, Pb, Al and Ca show significant temporal variations. Benthic foraminifera productivity proxy suggests that the recorded muddy events were contemporaneous of a higher Corg flux to the marine benthic environment. The higher flux of organic matter, following a long-term intensification of the upwelling and coinciding with a finer sedimentation, led to depressed levels of oxygen beneath the water-sediments interface and to early diagenetic processes of several redox-sensitive elements.  相似文献   
977.
Eluvial concentration of platinum-group minerals (PGM) has developed in the south of New Caledonia, from the weathering of a mafic and ultramafic cumulate. The platinum/palladium (Pt/Pd) ratio evolution from the bottom to the top of the weathering profile indicates a chemical mobilization of Pd in supergene environment. The examination by scanning electron microscopy of the residual PGM collected in the weathering profile and their chemical characterization by electronic microprobe show a preservation of the structure and chemical signature (in the core of the grains) of the fresh PGM. This clearly demonstrates that the PGM studied here are residual and are affected by dissolution process.  相似文献   
978.
The characterisation of waste is entering a new phase with the latest developments of characterisation leaching tests and associated modelling capabilities. The currently applied too simple testing approaches lead to poor choices in waste management. With the increased insight in release controlling processes chemical speciation aspects can be addressed even in the most complex and heterogeneous waste materials. From a composition perspective materials may be highly variable, but often materials are far more consistent in their leaching behaviour. This aspect should be exploited more extensively as it holds the key to treat waste materials in such a way that long term solutions are achieved rather than reaching temporary gains.  相似文献   
979.
Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.  相似文献   
980.
Airborne gamma-ray spectrometry data (uranium, potassium and thorium contents) reveal geochemical heterogeneities within the monolithological Hyrôme watershed (ca. 150 km2) in the Armorican massif (western France). Our observations and computations provide important constraints on the spatial distribution and the associated magnitudes of chemical erosion processes at the scale of a small watershed. Two distinct, partially preserved, weathering profiles exhibit a strong correlation between regolith evolution and airborne-derived K/Th ratios, suggesting that the variability is linked to supergene processes. Using both airborne data and laboratory measurements on rock samples, the total net export of potassium has been estimated at 422 ± 50 kg/m2 and the chemical weathering rate of potassium at 17 ± 2 kg/km2/a.  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