全文获取类型
收费全文 | 715篇 |
免费 | 27篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 60篇 |
地球物理 | 203篇 |
地质学 | 268篇 |
海洋学 | 51篇 |
天文学 | 75篇 |
自然地理 | 75篇 |
出版年
2023年 | 4篇 |
2021年 | 5篇 |
2020年 | 9篇 |
2019年 | 12篇 |
2018年 | 16篇 |
2017年 | 19篇 |
2016年 | 18篇 |
2015年 | 22篇 |
2014年 | 14篇 |
2013年 | 42篇 |
2012年 | 25篇 |
2011年 | 31篇 |
2010年 | 24篇 |
2009年 | 33篇 |
2008年 | 32篇 |
2007年 | 49篇 |
2006年 | 24篇 |
2005年 | 29篇 |
2004年 | 26篇 |
2003年 | 18篇 |
2002年 | 23篇 |
2001年 | 23篇 |
2000年 | 19篇 |
1999年 | 20篇 |
1998年 | 19篇 |
1997年 | 7篇 |
1996年 | 4篇 |
1995年 | 15篇 |
1994年 | 12篇 |
1993年 | 7篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 12篇 |
1984年 | 6篇 |
1983年 | 14篇 |
1982年 | 10篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1969年 | 3篇 |
1968年 | 3篇 |
排序方式: 共有748条查询结果,搜索用时 15 毫秒
71.
We investigated phase equilibria in the six-component systemNa2OK2OAl2O3SiO2F2O1H2Oat 100 MPa to characterize differentiation paths of naturalfluorine-bearing granitic and rhyolitic magmas. Topaz and cryoliteare stable saturating solid phases in calcium-poor systems.At 100 MPa the maximum solidus depression and fluorine solubilityin evolving silicic melts are controlled by the eutectics haplogranitecryoliteH2Oat 640°C and 4 wt % F, and haplogranitetopazH2Oat 640°C and 2 wt % F. Topaz and cryolite form a binaryperalkaline eutectic at 660°C, 100 MPa and fluid saturation.The low-temperature nature of this invariant point causes displacementof multiphase eutectics with quartz and alkali feldspar towardsthe topazcryolite join and enables the silicate liquidusand cotectic surfaces to extend to very high fluorine concentrations(more than 30 wt % F) for weakly peraluminous and subaluminouscompositions. The differentiation of fluorine-bearing magmasfollows two distinct paths of fluorine behavior, depending onwhether additional minerals buffer the alkali/alumina ratioin the melt. In systems with micas or aluminosilicates thatbuffer the activity of alumina, magmatic crystallization willreach either topaz or cryolite saturation and the system solidifiesat low fluorine concentration. In leucogranitic suites precipitatingquartz and feldspar only, the liquid line of descent will reachtopaz or cryolite but fluorine will continue to increase untilthe quaternary eutectic with two fluorine-bearing solid phasesis reached at 540°C, 100 MPa and aqueous-fluid saturation.The maximum water solubility in the haplogranitic melts increaseswith the fluorine content and reaches 12· 5 ±0· 5 wt % H2O at the quartzcryolitetopazeutectic composition. A continuous transition between hydrousfluorosilicate melts and solute-rich aqueous fluids is not documentedby this study. Our experimental results are applicable to leucocraticfluorosilicic magmas. In multicomponent systems, however, thepresence of calcium may severely limit enrichment of fluorineby crystallization of fluorite. KEY WORDS: granite; rhyolite; topaz; cryolite; magmatic differentiation 相似文献
72.
Khanal Manoj Adhikary Deepak Balusu Rao Wilkins Andy Belle Bharath 《Geotechnical and Geological Engineering》2022,40(4):1899-1920
Geotechnical and Geological Engineering - Most of the published research on goaf-hole failures are performed using 2D numerical analyses. However, 2D analyses of the goaf-hole failure mechanism,... 相似文献
73.
74.
There are compelling reasons for policy makers to be interested in the low-carbon agenda. More than half of the world's population lives in, and more than half of the world's economic output comes from, cities. Up to 70% of global carbon emissions can also be attributed to consumption that takes place in cities. Recent research has shown that cost-effective investments in low-carbon options could deliver a 40% reduction in GHG emissions from cities by 2020, while also providing wider economic benefits such as enhanced competitiveness and increased employment. As yet, however, investments in low-carbon cities have not been made at scale due mainly to the scale of the finance required, local government budgetary constraints, and perceptions about their costs and benefits. With a focus on the UK, a contemporary account is provided of what local authorities see as the major financial risks associated with funding low-carbon cities. Practical proposals – which also have more general relevance to the future financing of low-carbon cities around the world – are offered on how local authorities, in conjunction with central government, the private sector, and institutional investors, can effectively manage these risks. Policy relevance Cities house more than half of the world's population, generate more than half of the world's economic output, and produce between 40% and 70% of all anthropogenic GHG emissions. In the UK, 70% of such emissions are under the influence of its local authorities. Thus, one of the key public policy challenges for the low-carbon transition is how it should be financed. There are several obstacles and related risks to this transition, including financial and legal obstacles and the differing views and perceptions of stakeholders. These can be attenuated, somewhat, by national government support at scale, local authority leadership, and cooperation between other authorities and the private sector, and the development of tools and guidance to reduce transaction costs. 相似文献
75.
J. L. Culhane D. H. Brooks L. van Driel-Gesztelyi P. Démoulin D. Baker M. L. DeRosa C. H. Mandrini L. Zhao T. H. Zurbuchen 《Solar physics》2014,289(10):3799-3816
Seeking to establish whether active-region upflow material contributes to the slow solar wind, we examine in detail the plasma upflows from Active Region (AR) 10978, which crossed the Sun’s disc in the interval 8 to 16 December 2007 during Carrington rotation (CR) 2064. In previous work, using data from the Hinode/EUV Imaging Spectrometer, upflow velocity evolution was extensively studied as the region crossed the disc, while a linear force-free-field magnetic extrapolation was used to confirm aspects of the velocity evolution and to establish the presence of quasi-separatrix layers at the upflow source areas. The plasma properties, temperature, density, and first ionisation potential bias [FIP-bias] were measured with the spectrometer during the disc passage of the active region. Global potential-field source-surface (PFSS) models showed that AR 10978 was completely covered by the closed field of a helmet streamer that is part of the streamer belt. Therefore it is not clear how any of the upflowing AR-associated plasma could reach the source surface at 2.5 R⊙ and contribute to the slow solar wind. However, a detailed examination of solar-wind in-situ data obtained by the Advanced Composition Explorer (ACE) spacecraft at the L1 point shows that increases in O7+/O6+, C6+/C5+, and Fe/O – a FIP-bias proxy – are present before the heliospheric current-sheet crossing. These increases, along with an accompanying reduction in proton velocity and an increase in density are characteristic of both AR and slow-solar-wind plasma. Finally, we describe a two-step reconnection process by which some of the upflowing plasma from the AR might reach the heliosphere. 相似文献
76.
The geoid of Mars is dominated by its equilibrium figure and by the effect of the Tharsis rise. To investigate the rotational stability of Mars prior to the rise of Tharsis, we produced a residual non-hydrostatic geoid without Tharsis. First the hydrostatic component of the present-day flattening was removed. This procedure was performed using a 6% non-hydrostatic component of flattening, a value set by the spin axis precession rate of Mars. Then zonal spherical harmonics up to degree 6 centered on Tharsis were removed. Finally, the resultant residual geoid was evaluated for rotational stability by comparing polar and equatorial moments at 4050 trial pole positions. If the spin axis of ancient Mars was secularly stable, our analysis indicates that substantial polar wander has occurred with the rise of Tharsis. Stable spin axis positions on the non-hydrostatic residual figure of Mars are 15° to 90° from the present-day poles. This result is consistent with previously proposed paleopoles based on magnetic anomalies, geomorphology, and grazing impacts. 相似文献
77.
This paper adopts standard tests developed in temperate catchment research to determine the total phosphorus (TP) and the algal available (base‐extractable) phosphorus (NaOH–P) content of a wide range of glaciofluvial sediments from the Northern Hemisphere. We find that the TP content of these sediments is broadly similar to the P content of major rock types in Earth's crust (230–670 µgP/g) and so the TP yields of glacier basins may be high owing to the efficacy of suspended sediment evacuation by glacial meltwaters. We show that this is best achieved where subglacial drainage systems are present. The NaOH–P pool of the sediments is found to be low (1–23 µgP/g) relative to the TP pool and also to the NaOH–P pool of suspended sediments in temperate, non‐glacierized catchments. This most probably reflects the restricted duration of intimate contact between dilute meltwaters and glacial suspended sediments during the ablation season. Thus, despite the high surface‐area:volume ratio of glacial suspended sediments, the potential for P adsorption to mineral surfaces following release by dissolution is also low. Further, sorption experiments and sequential extraction tests conducted using glacial suspended sediments from two Svalbard catchments indicate that the generation of reactive secondary minerals (e.g. Fe‐ and other hydroxides) with a strong capacity to scavenge P from solution (and thereby promote the continued dissolution of P) may also be limited by the short residence times. Most P is therefore associated with poorly weathered, calcite/apatite‐rich mineral phases. However, we use examples from the Svalbard glacier basins (Austre Brøggerbreen and Midre Lovénbreen) to show that the high sediment yields of glaciers may result in appreciable NaOH–P loading of ice‐marginal receiving waters. Again, the importance of subglacial drainage is highlighted, as it produces a major, episodic release of NaOH–P at Midre Lovénbreen that results in a yield (8·2 kg NaOH–P/km2/year) more than one order of magnitude greater than that at Austre Brøggerbreen (where subglacial drainage is absent and the yield is 0·48 kg NaOH‐P/km2/year). Therefore, as since both detrimental and beneficial effects of sediment‐bound P loading in ice marginal receiving waters are possible (i.e. either reduced primary productivity owing to increased turbidity or P fertilization following desorption) there is a pressing need to assess the ambient P status of such environments and also the capacity for ice‐marginal ecosystems to adapt to such inputs. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
78.
A grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath ~140 km2 of Rutford Ice Stream, West Antarctica. The subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water-saturated sediments. The two troughs are filled with deforming sediment, whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream. The deforming bed is very flat along flow, but undulates across flow. Sliding areas show rougher bed topography. Cross-stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction. These bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation. In places, they form finger-like mounds of material, which extend into the sliding region further downstream. Mean bedform height is 22 m, mean width is 267 m, and many of them extend for at least 1–2 km along flow. We interpret most of these bedforms as drumlins and one as a mega-scale glacial lineation. The juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas, variable both spatially and temporally. Any theory of subglacial sediment rheology must also be able to account for our conclusion that, at any given time, pervasive deformation extends at least a few metres into the bed and can persist over a considerable area (many km2). Bedform geometry and basal conditions concur with interpretations of former ice streams, with evidence for increasing elongation ratio with distance downstream. However, those studies also identified bedrock cropping out at the ice-bed interface, for which there is no evidence on Rutford Ice Stream. 相似文献
79.
Robert R. J. Mohler Wesley F. Palmer Thomas C. Baker Robert M. Bizzell 《The Professional geographer》1986,38(4):375-382
The development and testing of a computerized technique for utilizing multitemporal Landsat multispectral scanner data to estimate spring small grains acreage are described. The Kauth-Thomas green number transformation is applied to the data prior to application of the pixel-level standardized logic. Four years of data from the U.S. and Canada are used in testing. A comparison of the test results to those of the analyst-intensive procedures indicates a potential for increase in near-harvest estimation efficiency with little sacrifice in accuracy. 相似文献
80.