全文获取类型
收费全文 | 152篇 |
免费 | 3篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 4篇 |
大气科学 | 10篇 |
地球物理 | 30篇 |
地质学 | 59篇 |
海洋学 | 12篇 |
天文学 | 6篇 |
综合类 | 1篇 |
自然地理 | 34篇 |
出版年
2022年 | 5篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 4篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 9篇 |
2015年 | 5篇 |
2014年 | 4篇 |
2013年 | 9篇 |
2012年 | 9篇 |
2011年 | 8篇 |
2010年 | 8篇 |
2009年 | 12篇 |
2008年 | 7篇 |
2007年 | 10篇 |
2006年 | 10篇 |
2005年 | 3篇 |
2004年 | 2篇 |
2003年 | 6篇 |
2002年 | 6篇 |
2001年 | 1篇 |
2000年 | 4篇 |
1997年 | 3篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有156条查询结果,搜索用时 15 毫秒
151.
Wei-Ning Xiang Wendy Lee Stratton 《International journal of geographical information science》2013,27(4):499-510
Abstract Mapping variable stream buffers in a vector environment in which buffer width values are delineated often yields inaccurate results. Possible vector solutions are either ineffective or inefficient, An alternate raster approach is presented here in which a buffer effectiveness-achievement function (b-function) is introduced to map desirable buffer zones at an individual cell level based upon areal differentiations in physical and ecological conditions. The implementation of b-function is made feasible by a GIS procedure devised in this article. This tested method can be extended to a variety of variable buffer studies, such as visual buffers, noise buffers, greenways, and urban natural buffers. ‘A robe can never be made of the fur from one fox's axillae’ (A Chinese idiom). 相似文献
152.
Nina Reuss Peter R. Leavitt Roland I. Hall Christian Bigler Dan Hammarlund 《Journal of Paleolimnology》2010,43(1):149-169
A surface-sediment survey of pigments in 100 lakes in the Scandes Mountains, northern Sweden, was combined with a reconstruction
of Holocene sedimentary pigments from Lake Seukokjaure to assess the major factors regulating phototrophic communities, and
how these controls may have changed during the period from the deglaciation (~9700 cal. years BP) to the present. The study
area covers a pronounced gradient of temperature and precipitation, and encompasses the subarctic tree line, an important
ecotonal boundary in this region. Lake Seukokjaure is located in a presently treeless basin close to the modern tree line.
The spatial survey of sedimentary pigments was analyzed using principle components analysis (PCA) and redundancy analysis
(RDA). PCA explained 73–83% of variance in pigment abundance and composition, whereas RDA explained 22–32% of variation in
fossil assemblages. Dissolved organic carbon (DOC) content of lake water, sediment δ13C, maximum lake depth, elevation and lake-water conductivity were all identified as environmental variables with significant
association with pigment abundances in the spatial survey, although phototrophic communities of lakes situated in different
vegetation zones (alpine, birch, conifer/birch) were incompletely distinguished by the ordinations. In the RDAs, the primary
pigment variability occurred along a production gradient that was correlated negatively to water-column DOC content and δ13C signature of sediments. This pattern suggested that the important controls of primary production were light regime and terrestrial
supplies of 13C-depleted carbon. In contrast, depth, elevation and conductivity were found to be more important for the differentiation
of the phototrophic community composition. Application of these spatial survey results to the Holocene sediment record of
Lake Seukokjaure demonstrated the importance of DOC for the temporal development of the lake, from an early state of high
production to a period of slight oligotrophication. In general, the algal changes were regulated by the interaction of DOC
and conductivity, although transitions in the phototrophic community during the late Holocene were less easily interpreted.
Terrestrial vegetation development thus appears to be of utmost importance for the regulation of primary production in oligotrophic
alpine and subarctic lakes and climate impacts on lakes, whereas other basin-specific factors may control the ontogeny of
algal community composition. 相似文献
153.
Martin Sharp Keith Richards Ian Willis Neil Arnold Peter Nienow Wendy Lawson Jean-Louis Tison 《地球表面变化过程与地形》1993,18(6):557-571
As part of an integrated study of the hydrology, meltwater quality and dynamics of the Haut Glacier d'Arolla, Switzerland, the glacier's drainage network structure was determined from patterns of dye recovery in 342 injection experiments conducted from 47 moulins distributed widely across the glacier. This structure was compared with theoretical predictions based upon reconstructed patterns of water flow governed by (a) the subglacial hydraulic potential surface, and (b) the subglacial bedrock surface. These reconstructions were based on measurements of ice surface and bedrock topography obtained by a combination of ground survey and radio-echo sounding techniques. The two reconstructions simulate the drainage system structures expected for (a) closed channels, in which water is pressurized by the overlying ice, and (b) gravity-driven, open-channel flow. The closed-channel model provides the best fit to the observed structure, even though theoretical calculations suggest that, under summer discharge conditions, open-channel flow may be widespread beneath the glacier. Possible reasons for this apparent discrepancy are discussed. 相似文献
154.
Richard F. Wendlandt Wendy J. Harrison 《Contributions to Mineralogy and Petrology》1979,69(4):409-419
Melting relations at 5 and 20 kbar on the composition join sanidine-potassium carbonate are dominated by a two-liquid region that covers over 60% of the join at 1,300 ° C. At this temperature, the silicate melt contains approximately 19 wt% carbonate component at 5 kbar and 32 wt% carbonate component at 20 kbar. The conjugate carbonate melt contains less than 5 wt% silicate component, and it varies less as a function of temperature than does the silicate melt.Partition coefficients for Ce, Sm, and Tm between the immiscible carbonate and silicate melts at 1,200 ° and 1,300 ° C at 5 and 20 kbar are in favor of the carbonate melt by a factor of 2–3 for light REE and 5–8 for heavy REE. The effect of pressure on partitioning cannot be evaluated independently because of complementary changes in melt compositions.Minimum REE partition coefficients for CO2 vapor/carbonate melt and CO2 vapor/silicate melt can be calculated from the carbonate melt/silicate melt partition coefficients, the known proportions of melt, and maximum estimates of the proportion of CO2 vapor. The vapor phase is enriched in light REE relative to both melts at 20 kbar and enriched in all REE, especially the light elements, at 5 kbar. The enrichment of REE in CO2 vapor relative to both melts is 3–4 orders of magnitude in excess of that in water vapor (Mysen, 1979) at 5 kbar and is approximately the same as that in water vapor at 20 kbar.Mantle metasomatism by a CO2-rich vapor enriched in light REE, occurring as a precursor to magma genesis, may explain the enhanced REE contents and light REE enrichment of carbonatites, alkali-rich silicate melts, and kimberlites. Light REE enrichment in fenites and the granular suite of nodules from kimberlites attests to the mobility of REE in CO2-rich fluids under both mantle and crustal conditions. 相似文献
155.
The partitioning of samarium and thulium between garnets and melts in the systems Mg3Al2-Si3O12-H2O and Ca3Al2Si3O12-H2O has been studied as a function of REE concentration in the garnets at 30 kbar pressure. Synthesis experiments of variable time under constant P, T conditions indicate that garnet initially crystallizes rapidly to produce apparent values of D
Sm (D
Sm=concentration of Sm in garnet/concentration of Sm in liquid) which are too large in the case of pyrope and too small in the case of grossular. As the experiment proceeds, Sm diffuses out of or into the garnet and the equilibrium value of D
Sm is approached. Approximate values of diffusion coefficients for Sm in pyrope garnet obtained by this method are 6 × 10–13 cm2 s–1 at 1,300 ° C and 2 × 10–12 cm2 s–1 at 1,500 ° C, and for grossular, 8.3 × 10–12 cm2 s–1 at 1,200 ° C and 4.6 × 10–11 cm2 s–1 at 1,300 ° C. The equilibrium values of D
Sm have been reversed by experiments with Sm-free pyrope and Sm-bearing glass, and with Sm-bearing grossular and Sm-free glass.Between 12 ppm and 1,000 ppm Sm in pyrope at 1,300 ° C and between 80 ppm and >2 wt.% Tm in pyrope at 1,500 ° C, partition coefficients are constant and independent of REE concentration. Above 100 ppm of Sm in garnet at 1,500 ° C, partition coefficients are independent of Sm concentration. At lower concentrations, however, D
Sm is dependent upon the Sm content of the garnet. The two regions may be interpreted in terms of charge-balanced substitution of Sm3Al5O12 in the garnet at high Sm concentrations and defect equilibria involving cation vacancies at low concentrations. At very low REE concentrations (< 1 ppm Tm in grossular at 1,300 ° C) DREE garnet/liquid again becomes constant with an apparent Henry's Law value greater than that at high concentrations. This may be interpreted in terms of a large abundance of cation vacancies relative to the number of REE ions.The importance of defects in the low concentration region has been confirmed by adding other REE (at 80 ppm level) to the system Mg3Al2Si3O12-H2O at low Sm concentrations. These change D
Sm in the defect region, demonstrating their role in the production of vacancies.Experiments on a natural pyropic garnet indicate that defect equilibria are of importance to REE partitioning within the concentration ranges found in nature. 相似文献
156.
In groundwater of the Trans-Pecos region of West Texas, unexpectedly high levels of nitrate (NO3 ?) are documented in four basins: Red Light Draw, Eagle Flats, Wild Horse and Michigan Flats, and Lobo and Ryan Flats. NO3 ? concentrations are changing over time in the majority (82.8 %) of wells and are increasing in most (69.8 %). The temporal change raises questions about the potential sources of NO3 ? and about flow dynamics in these basins. Presence of NO3 ? and temporal variability in concentration has implications beyond contamination risk because it indicates relatively rapid recharge (<60 years) to the basin groundwaters which was not expected based on previous estimates from chloride mass balance models and groundwater age-dating techniques. This research combines existing data ranging back to the 1940s with data collected in 2011 to document a multi-decadal trend of overall increasing NO3 ? concentration in deep basin groundwaters. Chlorofluorocarbon analyses of groundwater collected during 2011 indicate the presence of young (<70 years) water in the basins. The authors infer from these data that there are mechanism(s) by which relatively rapid and widespread recharge occurs on the basin floors; that recharge is spatially and temporally variable and that it results from both anthropogenic (irrigated agriculture) and natural (precipitation) sources. In light of these observations, fundamental conceptual models of flow in these basins should be re-evaluated. 相似文献