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   562篇
  免费   32篇
  国内免费   6篇
测绘学   13篇
大气科学   36篇
地球物理   125篇
地质学   227篇
海洋学   60篇
天文学   76篇
综合类   1篇
自然地理   62篇
  2024年   1篇
  2023年   1篇
  2022年   2篇
  2021年   5篇
  2020年   14篇
  2019年   14篇
  2018年   13篇
  2017年   19篇
  2016年   25篇
  2015年   26篇
  2014年   19篇
  2013年   24篇
  2012年   28篇
  2011年   38篇
  2010年   43篇
  2009年   39篇
  2008年   25篇
  2007年   33篇
  2006年   24篇
  2005年   26篇
  2004年   29篇
  2003年   20篇
  2002年   13篇
  2001年   8篇
  2000年   13篇
  1999年   6篇
  1998年   15篇
  1997年   8篇
  1996年   4篇
  1995年   4篇
  1994年   6篇
  1993年   8篇
  1992年   2篇
  1991年   5篇
  1990年   4篇
  1989年   4篇
  1988年   2篇
  1987年   6篇
  1985年   4篇
  1984年   4篇
  1983年   2篇
  1981年   2篇
  1980年   2篇
  1979年   2篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
  1973年   1篇
  1972年   1篇
排序方式: 共有600条查询结果,搜索用时 31 毫秒
591.
The occurrence of planktonic prosobranch larvae identified during the summers of 1977, 1979 and 1981 off Beaufort, North Carolina is given. Forty-two species were recorded; 29 of these were found each summer. Observations and identifications were made on 12 newly described larvae: Lacuna sp, Epitonium sp. 1 (apiculatum), Epitonium sp. 2, Strombus alatus, Erato maugeriae, Simnia acicularis, Natica sp. 1, Natica sp, 2, Natica sp. 3, Natica sp. 4, Olivella adelae, Mangelia sp. and Sp.? Turridae. A key for the identification of planktonic prosobranch veligers occuring in summer months is included.  相似文献   
592.
Chemical and physical datas for stilpnomelanes are given. A study of correlations between physical datas and chemical composition gives the following results: 1. There is a linear correlation between chemical composition and indices of refraction and birefringence (a mathematical expression of this correlation is given). 2. There is no linear correlation between chemical composition and specific gravity. Detailed datas (bibliography, chemical analysis, X-ray diffraction and so on) will be published later.  相似文献   
593.
594.
New evidences based on a combination of field and laboratory investigations reinforce the hypotheses that the circulation of warm fluids has remarkably contributed to the origin and development of the Devonian Kess Kess mounds of the Hamar Laghdad Ridge (eastern Anti‐Atlas, Morocco). The limestones of the Hamar Laghdad Ridge were deposited above a structural high generated by calc‐alkaline volcanic activity that has probably fuelled the circulation of warm fluids throughout the overlying carbonate units. The geological and palaeontological attributes described throughout the succession of the Hamar Laghdad Ridge (from the Lochkovian to Frasnian intervals) are interpreted as the result of hydrothermal processes related to a volcanic system. In particular, these attributes seem consistent with a chemo‐physical environment fuelled by the circulation of warm and late magmatic fluids. These attributes include a very low oxygen stable isotope signature (δ18O ~ −10‰) for carbonates. Evidences for a late magmatic fluid circulation consist of volcanic glass and pyroclasts replacement with hydrothermal minerals such as quartz, anatase and clinochlore. Fluids circulating through veins and pores into sediments, and venting to the seafloor, probably induced the formation of cavities where monospecific trilobite communities were detected. The partially silicified trilobite remains are associated with traces of goethite. This iron‐bearing oxide mineral is also present in the upper part of the Hamar Laghdad Ridge. All these attributes are here interpreted as possible evidences for a low‐temperature hydrothermal venting system active during the Lochkovian–Frasnian time span. This study combines an updated revision with new petrographic, geological and geochemical results aimed at providing an overall framework on the origin and early diagenesis of the Devonian succession of Hamar Laghdad. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
595.
Economics of climate policy and collective decision making   总被引:1,自引:0,他引:1  
This paper explores the reasons why economic instruments of climate change are reluctantly applied and stresses the need for interdisciplinary research linking economic theory and empirical testing to deliberative political procedures. It is divided in three parts. The first one recalls the main issues in implementing Cost-Benefit Analysis such as information problems, uncertainties, discounting the future and irreversibilities. The second part shows how these issues can be treated in integrated assessment and techno-economic models and presents a case study, which shows that
  • The chosen scenario tends to stabilize atmospheric CO2 concentration at around 550 ppm in the long run.
  • Exclusion of possibility to trade CO2 emission permits under a cap regime would increase the cost of emission abatement for OECD countries.
  • Combining different flexibility instruments might lead to significant gains in the overall cost of climate policy.
The third part presents results of a survey conducted among the main economic and environmental associations in Switzerland. The survey reveals conflicting views on economic instruments. It shows how the social acceptability of these instruments can be improved in taking explicitly into account these opposing views of special interest groups. Therefore, policy scenarios should be selected in combining techno-economic models with empirical studies about their political and normative context.  相似文献   
596.
597.
Due to the severity of arsenic contamination of soil and water resources around the world, finding new adsorbents for arsenic removal from the water is of high importance. The present study investigates the possible use and effectiveness of starch-stabilized Fe/Cu nanoparticles for adsorption of arsenic from aqueous solutions. First, Fe/Cu nanoparticles at various starch concentrations of 0, 0.02, 0.04 and 0.06 wt% were synthesized and characterized by X-ray diffraction, transmission electron microscopy and zeta potential/particle size analyzer. Then 0.04 wt% stabilized Fe/Cu nanoparticles were tested for the sorption of As(III) and As(V) from synthetic arsenic-contaminated water. To have an understanding about the arsenic adsorption mechanism of nanoparticles, X-ray photoelectron spectroscopy (XPS) was performed before and after adsorption. The results showed that starch provides nanoparticles with a neutral surface and stabilization of nanoparticles is possible with 0.04 wt% or higher concentrations of starch. For 0.04 wt% starch-stabilized Fe/Cu nanoparticles, the adsorption isotherms fit well within the Langmuir equation, with maximum sorption capacities of 90.1 mg/g for As(III) and 126.58 mg/g for As(V) at a pH of 7.0 from the aqueous arsenic solutions. Examining the XPS spectra of nanoparticles before and after adsorption showed that arsenic adsorption by this nanoparticle can be due to the formation of inner-sphere arsenic complexes on the particle surface, and the surface oxygen-containing functional groups involved in adsorption. The high sorption capacity suggests the potential for applying starch-stabilized Fe/Cu nanoparticles to the contaminated waters for removal of arsenic.  相似文献   
598.
A new stegosaur species, Jiangjunosaurus junggarensis, gen. et sp. nov., is erected based on a specimen collected from the Upper Jurassic upper section of the Shishugou Formation in the Junggar Basin, Xinjiang, China. It represents the first stegosaur from the Jurassic of Xinjiang and increases the diversity of the dinosaur fauna in the Shishugou Formation. The new genus is characterized by symmetrical and proportionally wide tooth crowns, a sub-rectangular axial neural spine seen in lateral view, and large openings on the lateral surfaces of the cervical centra. A preliminary character analysis suggests that this new taxon is more derived than the Middle Jurassic stegosaur Huayangosaurus but more primitive than most other known stegosaur species.  相似文献   
599.
The hydrogen content of nominally anhydrous minerals is of great interest, because it can influence many physical and mechanical properties of mantle rocks. Moreover, the hydrogen diffusion profiles can be used to constrain timescales related to magma eruptions. Here, we report models of ionic diffusion for trace elements in anisotropic crystals and apply them to hydrogen diffusing out of mantle-derived olivine. We first compare and discuss the characteristics of 1D and 3D models and show that only 3D anisotropic diffusion models can lead to diffusion profiles exhibiting non-equilibrium plateau at the center of the solid along the slowest axis, as measured in natural samples. In a second part, we discuss the differences between hydration and dehydration of olivine for diffusion that is linked to two different atomic sites involved in hydrogen mobility. Finally, we apply our 3D anisotropic model to previous results on mantle-derived olivine from Pali-aike to better characterize diffusion coefficients and their anisotropy that could be relevant for dehydration of olivine. Our results show that dehydration has to be strongly anisotropic, with a fast [100] axis and a significantly slower [001] axis.  相似文献   
600.
This multidisciplinary study aims to decipher the impact of ancient charcoal production on past and present-day soils in the northern Vosges Mountains. Soil observations in the field and laboratory were complemented by charcoal and phytolith studies on large thin sections, molecular analyses of organic pollutants, and phytolith analysis on bulk samples. The complex technosol platform records an ancient natural soil sequence buried by a human-made platform on which charcoal accumulated. The current upslope soil is an entic Podzol. Palaeoecological data collected in the buried soil are reliable owing to low bioactivity due to soil acidity. Podzolisation predated the platform construction. The presence of ashes induced low soil alkalisation developed in the charcoal hearth remains and appears to have generated the migration of subsequent iron/clay/organic bands throughout the platform sediment and the buried soil. Charcoal studied in thin sections revealed mainly Quercus and Fagus taxa. Phytolith studies suggest that a less dense or degraded forest preceded platform construction, probably due to former woodland coppicing or earlier disorganised wood gathering. The specific distribution of polycyclic aromatic hydrocarbons sorbed on charcoal has persisted in soils throughout centuries, but we have no evidence that charcoal-making activities contributed to diffuse global pollution.  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