全文获取类型
收费全文 | 5801篇 |
免费 | 227篇 |
国内免费 | 89篇 |
专业分类
测绘学 | 168篇 |
大气科学 | 548篇 |
地球物理 | 1460篇 |
地质学 | 1998篇 |
海洋学 | 555篇 |
天文学 | 766篇 |
综合类 | 22篇 |
自然地理 | 600篇 |
出版年
2021年 | 60篇 |
2020年 | 64篇 |
2019年 | 87篇 |
2018年 | 126篇 |
2017年 | 116篇 |
2016年 | 157篇 |
2015年 | 145篇 |
2014年 | 204篇 |
2013年 | 326篇 |
2012年 | 244篇 |
2011年 | 266篇 |
2010年 | 192篇 |
2009年 | 306篇 |
2008年 | 292篇 |
2007年 | 265篇 |
2006年 | 222篇 |
2005年 | 189篇 |
2004年 | 196篇 |
2003年 | 190篇 |
2002年 | 182篇 |
2001年 | 133篇 |
2000年 | 136篇 |
1999年 | 113篇 |
1998年 | 110篇 |
1997年 | 94篇 |
1996年 | 85篇 |
1995年 | 85篇 |
1994年 | 76篇 |
1993年 | 63篇 |
1992年 | 65篇 |
1991年 | 69篇 |
1990年 | 66篇 |
1989年 | 59篇 |
1988年 | 58篇 |
1987年 | 65篇 |
1986年 | 43篇 |
1985年 | 73篇 |
1984年 | 81篇 |
1983年 | 73篇 |
1982年 | 66篇 |
1981年 | 76篇 |
1980年 | 63篇 |
1979年 | 57篇 |
1978年 | 38篇 |
1977年 | 54篇 |
1976年 | 63篇 |
1975年 | 46篇 |
1974年 | 56篇 |
1973年 | 47篇 |
1972年 | 25篇 |
排序方式: 共有6117条查询结果,搜索用时 15 毫秒
171.
A recently exposed section across a ?rst‐order valley buried beneath the regional blanket peat on hillside slopes in the upper Liffey valley, Co. Wicklow, is described. The section shows two alluvia within a shallow valley form underlain by an extensive boulder and stone line over regional till and weathered granite. 14C dates from wood in the alluvia indicate the older alluvium to have formed between 4324 ± 53 BP and 4126 ± 45 BP and the younger between 3217 ± 53 BP and 2975 ± 53 BP . The basal layer of the overlying peat yielded a date of 2208 ± 61 BP . The younger alluvium shows the effects of soil paludi?cation prior to the peat expansion. Dated pollen analyses elsewhere in the upper catchment con?rm the spread of blanket peat over most areas above 350 m after 4000–3600 BP . The buried valley was contributing sediments to the mid‐Holocene ?oodplains in the upper Liffey valley prior to the extension of blanket peat over the catchment after which sediment yields from it and the other catchment slopes declined. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
172.
Jean-Louis Le Mouël Pierre-Noël Mayaud Peter Shebalin 《Comptes Rendus Geoscience》2003,335(13):935-941
Big perturbations of the magnetic field (amplitudes larger than 250 nT) are simply detected by subtracting the values of a model from the measurements of CHAMP satellite. Taking a full year of CHAMP data and organizing them in four subsets of three months length (spring, summer, autumn, winter), it is found that: (a) the two domains where such big perturbations mainly exist are limited, in both hemispheres, by a parallel of high latitude of the corrected geomagnetic coordinates system; (b) a conspicuous seasonal (annual) variation affects the density of the perturbations and is opposite in the two hemispheres. We hold that these perturbations are linked to the midday magnetic activity within the auroral zone, long ago described by one of us (Mayaud, 1956). The source of the perturbations observed at the satellite altitude would be field-aligned currents resulting from the penetration of the solar wind into the magnetospheric cusps. To cite this article: J.-L. Le Mouël et al., C. R. Geoscience 335 (2003). 相似文献
173.
174.
Gábor Dobosi Pamela D. Kempton Hilary Downes Antal Embey-Isztin Matthew Thirlwall Peter Greenwood 《Contributions to Mineralogy and Petrology》2003,144(6):671-683
Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like 143Nd/144Nd but 87Sr/86Sr is elevated relative to most MORBs. Their '18O values cover a wide range from +3.8 to +9.5. A group of LREE-enriched mafic granulites have higher 87Sr/86Sr (0.704-0.708) and lower 143Nd/144Nd (0.5128-0.5124), with higher '18O values on average (+7.8 to +10.6) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in )Hf-)Nd isotope space has a shallower slope than the crust-mantle array, with the metasedimentary rocks forming the low )Hf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher 208Pb/204Pb. The wide range in '18O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of '18O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low 143Nd/144Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny. 相似文献
175.
Recent Precipitation Trends in Hungary in the Context of Larger Scale Climatic Changes 总被引:1,自引:0,他引:1
The time series of monthly precipitation totals from 14 Hungarian observing stations (1901–1998) were analysed to reveal the long term changes in precipitation characteristics occurred in the 20th century. A particular attention was given to the changes in the recent decades and their links with the larger scale climatic and circulation changes over Europe and the Atlantic.The statistical significancesof systematic changes are controlled by linear trend analysis and the Mann–Kendall test. The long term fluctuations are illustrated applying a 15-point Gaussian filter on the time series. The Standardised Precipitation Index is used to evaluate the changes in the drought event frequency. The relationships with larger scale changes are mostly discussed relying on contemporary papers, and the Grosswetterlagen Catalogue is used as well.The annual precipitation total decreased by 15–20% in Hungary during the 20th century. The decline is substantial in both halves of the century, but the precipitation sums in the transition seasons declined in the first 50 years, and the winter precipitation decreased in the latest decades. The precipitation total of the period November–February declined significantly in the last 50 years. In the same time the mean winter value of the North Atlantic Oscillation Index (NAOI) increased, the positions of the main pressure patterns over the Atlantic are shifted northeastward, and lot of other coherent changes detected in the winter climate of the European–Atlantic region. The mean summer precipitation total has hardly changed, but the frequency of summer drought events increased. There are some signs of a shift of the Hungarian summer climate towards a Mediterranean like climate. 相似文献
176.
Feature Environmental geophysics: a site characterization tool for urban regeneration in the post-mining era 总被引:1,自引:0,他引:1
Peter Styles 《Geology Today》2003,19(5):173-178
The closure and decay of industrial activity involving mining, chemical or light industrial operations has scarred the landscape of urban areas. The Government has major initiatives in urban regeneration to remediate these 'brown-field sites'. 相似文献
177.
Establishing relative and absolute time frameworks for the sedimentary, magmatic, tectonic and gold mineralisation events in the Norseman-Wiluna Belt of the Archean Yilgarn Craton of Western Australia, has long been the main aim of research efforts. Recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events. In this paper the assumptions underlying this rationale are scrutinized, and it is shown that they are the source of significant misinterpretations. A revised time chart for the deformation events of the belt is established. The first shortening phase to affect the belt, D1, was preceded by an extensional event D1e and accompanied by a change from volcanic-dominated to plutonic-dominated magmatism at approximately 2685–2675 Ma. Later extension (D2e) controlled deposition of the ca 2655 Ma Kurrawang Sequence and was followed by D2, a major shortening event, which folded this sequence. D2 must therefore have started after 2655 Ma—at least 20 Ma later than previously thought and after the voluminous 2670–2655 Ma high-Ca granite intrusion. Younger transcurrent deformation, D3–D4, waned at around 2630 Ma, suggesting that the crustal shortening deformation cycle D2–D4 lasted approximately 20–30 Ma, contemporaneous with low-volume 2650–2630 Ma low-Ca granites and alkaline intrusions. Time constraints on gold deposits suggest a late mineralisation event between 2640–2630 Ma. Thus, D2–D4 deformation cycle and late felsic magmatism define a 20–30 Ma long tectonothermal event, which culminated with gold mineralisation. The finding that D2 folding took place after voluminous high-Ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models. Results suggest that buoyancy-driven doming of pre-tectonic competent bodies trigger growth of antiforms, whereas non-buoyant, competent granite bodies trigger growth of synforms. The conspicuous presence of pre-folding granites in the cores of anticlines may be a result from active buoyancy doming during folding. 相似文献
178.
179.
Susan Glasauer Peter G Weidler Terry J Beveridge 《Geochimica et cosmochimica acta》2003,67(7):1277-1288
The reductive dissolution of FeIII (hydr)oxides by dissimilatory iron-reducing bacteria (DIRB) could have a large impact on sediment genesis and Fe transport. If DIRB are able to reduce FeIII in minerals of high structural order to carry out anaerobic respiration, their range could encompass virtually every O2-free environment containing FeIII and adequate conditions for cell growth. Previous studies have established that Shewanella putrefaciens CN32, a known DIRB, will reduce crystalline Fe oxides when initially grown at high densities in a nutrient-rich broth, conditions that poorly model the environments where CN32 is found. By contrast, we grew CN32 by batch culture solely in a minimal growth medium. The stringent conditions imposed by the growth method better represent the conditions that cells are likely to encounter in their natural habitat. Furthermore, the expression of reductases necessary to carry out dissimilatory Fe reduction depends on the method of growth. It was found that under anaerobic conditions CN32 reduced hydrous ferric oxide (HFO), a poorly crystalline FeIII mineral, and did not reduce suspensions containing 4 mM FeIII in the form of poorly ordered nanometer-sized goethite (α-FeOOH), well-ordered micron-sized goethite, or nanometer-sized hematite (α-Fe2O3) crystallites. Transmission electron microscopy (TEM) showed that all minerals but the micron-sized goethite attached extensively to the bacteria and appeared to penetrate the outer cellular membrane. In the treatment with HFO, new FeII and FeIII minerals formed during reduction of HFO-Fe in culture medium containing 4.0 mmol/L Pi (soluble inorganic P), as observed by TEM with energy-dispersive X-ray spectroscopy, selected area electron diffraction, and X-ray diffraction. The minerals included magnetite (Fe3O4), goethite, green rust, and vivianite [Fe3(PO4)2 · 8H2O]. Vivianite appeared to be the stable end product and the mean coherence length was influenced by the rate of FeIII reduction. When Pi was 0.4 mol/L under otherwise identical conditions, goethite was the only mineral observed to form, and less Fe2+ was produced overall. Hence, the ability of DIRB to reduce Fe (hydr)oxides may be limited when the bacteria are grown under nutrient-limited conditions, and the minerals that result depend on the vigor of FeIII reduction. 相似文献
180.
Modelling nitrogen and oxygen isotope fractionation during denitrification in a lacustrine redox-transition zone 总被引:1,自引:0,他引:1
Moritz F. Lehmann Peter Reichert Alberto Barbieri 《Geochimica et cosmochimica acta》2003,67(14):2529-2542
The stable isotope composition (δ15N and δ18O) of nitrate was measured during Summer 1999 in the anaerobic hypolimnion of eutrophic Lake Lugano (Switzerland). Denitrification was demonstrated by a progressive nitrate depletion coupled to increasing δ15N and δ18O values for residual nitrate. Maximum δ15N and δ18O values amounted to 27.2 and 15.7‰, respectively.15N and 18O enrichment factors for denitrification (ε) were estimated using a closed-system model and a dynamic diffusion-reaction model. Using the Rayleigh equation (closed-system approach), we obtained ε values of −11.2 and −6.6‰ for nitrogen and oxygen, respectively. The average ε values derived using the diffusion-reaction model were determined to be −20.7 ± 3.8 for nitrogen and −11.0 ± 1.7 for oxygen. Both N and O isotope fractionation appeared to be lower when denitrification rates where high, possibly in association with high organic carbon availability. In addition, variations in the isotope effects may be attributed to the variable importance of sedimentary denitrification having only a small isotope effect on the water column. The combined measurement of N and O isotope ratios in nitrate revealed that coupled nitrification-denitrification in the open-water was of minor importance. This is the first study of nitrogen and oxygen isotope effects associated with microbial denitrification in a natural lake. Moreover, this study confirms the high potential of δ18O of nitrate as a valuable biogeochemical tracer in aquatic systems, complementing nitrate δ15N. 相似文献