全文获取类型
收费全文 | 588篇 |
免费 | 17篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 68篇 |
地球物理 | 154篇 |
地质学 | 179篇 |
海洋学 | 61篇 |
天文学 | 52篇 |
自然地理 | 72篇 |
出版年
2020年 | 6篇 |
2019年 | 6篇 |
2018年 | 6篇 |
2017年 | 11篇 |
2016年 | 11篇 |
2015年 | 16篇 |
2014年 | 11篇 |
2013年 | 36篇 |
2012年 | 19篇 |
2011年 | 27篇 |
2010年 | 20篇 |
2009年 | 30篇 |
2008年 | 25篇 |
2007年 | 22篇 |
2006年 | 29篇 |
2005年 | 17篇 |
2004年 | 20篇 |
2003年 | 18篇 |
2002年 | 14篇 |
2001年 | 9篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 11篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 10篇 |
1993年 | 10篇 |
1992年 | 5篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 8篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 18篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 11篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 6篇 |
1972年 | 7篇 |
1971年 | 3篇 |
1969年 | 3篇 |
1958年 | 2篇 |
排序方式: 共有607条查询结果,搜索用时 15 毫秒
131.
Barry R. Bickmore Justin C. Wheeler Kathryn L. Nagy 《Geochimica et cosmochimica acta》2008,72(18):4521-4536
In light of recent work on the reactivity of specific sites on large (hydr)oxo-molecules and the evolution of surface topography during dissolution, we examined the ability to extract molecular-scale reaction pathways from macroscopic dissolution and surface charge measurements of powdered minerals using an approach that involved regression of multiple datasets and statistical graphical analysis of model fits. The test case (far-from-equilibrium quartz dissolution from 25 to 300 °C, pH 1-12, in solutions with [Na+] ? 0.5 M) avoids the objections to this goal raised in these recent studies. The strategy was used to assess several mechanistic rate laws, and was more powerful in distinguishing between models than the statistical approaches employed previously. The best-fit model included three mechanisms—two involving hydrolysis of Si centers by H2O next to neutral (>Si-OH0) and deprotonated (>Si-O−) silanol groups, and one involving hydrolysis of Si centers by OH−. The model rate law is
132.
Careful site characterization is critical for successful geologic storage of carbon dioxide (CO2) because of the many physical and chemical processes impacting CO2 movement and containment under field conditions. Traditional site characterization techniques such as geological mapping,
geophysical imaging, well logging, core analyses, and hydraulic well testing provide the basis for judging whether or not
a site is suitable for CO2 storage. However, only through the injection and monitoring of CO2 itself can the coupling between buoyancy flow, geologic heterogeneity, and history-dependent multi-phase flow effects be
observed and quantified. CO2 injection and monitoring can therefore provide a valuable addition to the site-characterization process. Additionally, careful
monitoring and verification of CO2 plume development during the early stages of commercial operation should be performed to assess storage potential and demonstrate
permanence. The Frio brine pilot, a research project located in Dayton, Texas (USA) is used as a case study to illustrate
the concept of an iterative sequence in which traditional site characterization is used to prepare for CO2 injection and then CO2 injection itself is used to further site-characterization efforts, constrain geologic storage potential, and validate understanding
of geochemical and hydrological processes. At the Frio brine pilot, in addition to traditional site-characterization techniques,
CO2 movement in the subsurface is monitored by sampling fluid at an observation well, running CO2-saturation-sensitive well logs periodically in both injection and observation wells, imaging with crosswell seismic in the
plane between the injection and observation wells, and obtaining vertical seismic profiles to monitor the CO2 plume as it migrates beyond the immediate vicinity of the wells. Numerical modeling plays a central role in integrating geological,
geophysical, and hydrological field observations. 相似文献
133.
Petrogenesis of Volcanic Rocks from Saipan and Rota, Mariana Islands, and Implications for the Evolution of Nascent Island Arcs 总被引:2,自引:0,他引:2
Reagan Mark K.; Hanan Barry B.; Heizler Matthew T.; Hartman Brian S.; Hickey-Vargas Rosemary 《Journal of Petrology》2008,49(3):441-464
An 40Ar/39Ar age of 45·1 Ma determined for lavas fromnorthern Saipan confirms that these high-silica rhyolites eruptedduring the proto-arc stage of volcanism in theIzu–Bonin–Mariana system, which is characterizedelsewhere by eruption of boninitic lavas. Incompatible traceelement concentrations and Sr, Hf, Nd, and Pb isotope ratiosfor these rhyolites are transitional between those of c. 48Ma boninitic lavas and post-38 Ma first-arc andesitesand dacites from Saipan and Rota that have typical subduction-relatedcompositions. These transitional compositions are modeled bycrystal fractionation of parental tholeiitic basalt combinedwith assimilation of young boninitic crust. A second stage ofRayleigh fractionation in the upper crust is required by SiO2concentrations that exceed 77 wt % and near-zero compatibleelement concentrations. First-arc magma compositions are consistentwith fractionation of basalt and assimilation of crust similarin composition to the first-arc magmas themselves. The mantlesources of the proto-arc and first-arc lavas from Saipan andRota are similar to those of Philippine back-arc basin basaltsbased on Nd and Hf isotopic compositions. The Pb isotope compositionsof these lavas are between those of Pacific sea-floor basaltsand Jurassic and younger cherty and clay-rich sediments. Thiscontrasts with the boninitic proto-arc volcanic rocks from Guamand Deep Sea Drilling Project Sites 458 and 459 that have Pbisotope compositions similar to Pacific basin basalts and volcaniclasticsediments. The preferred explanation for the difference in thenature of proto-arc volcanism between Saipan and other fore-arclocations is that the crust ceased extending 3–4 Myr earlierbeneath Saipan. This was caused by a change from mantle upwelling,fore-arc extension, and shallow melting to an environment dominatedby more normal mantle wedge convection, stable crust, and deepermelting. KEY WORDS: rhyolite; andesite; Mariana arc; isotope ratios; trace elements 相似文献
134.
This paper explores the implications of changes in climatic averages and a range in precipitation levels on food production opportunities at the farm, regional and provincial levels for the province of Ontario. A doubling in atmospheric CO2 or its equivalent would contribute to extended frost-free seasons throughout Ontario, and possibly increase year-to-year variability in precipitation. At the farm level, these changes imply greater fluctuations in annual farm profits. The benefits of longer frost-free seasons would be impaired during years with relatively low precipitation, and under these conditions, the security of the province's food supply would be at risk.LRRC Contribution No. 90-06. 相似文献
135.
The origin of ocean island basalt end-member compositions: trace element and isotopic constraints 总被引:33,自引:0,他引:33
Barry L. Weaver 《Earth and Planetary Science Letters》1991,104(2-4)
Ocean island basalt (OIB) suites display a wide diversity of major element, trace element, and isotopic compositions. The incompatible trace element and isotopic ratios of OIB reflect considerable heterogeneity in the mantle source regions. In addition to the distinctive Sr, Nd and Pb isotopic signatures of the HIMU, EMI and EMII OIB end-members, differences in incompatible trace element ratios among these end-members are of great help in identifying the nature and origin of their sources. Examination of trace element and isotopic constraints for the three OIB end-members suggests a relatively simple model for their origin. The dominant component in all OIB is ancient recycled basaltic oceanic crust which has been processed through a subduction zone, and which carries the trace element and isotopic signature of a dehydration residue (enrichment in HFSE relative to LILE and LREE, low Rb/Sr, but high U/Pb and Th/Pb ratios leading to the development of radiogenic Pb isotope compositions). HIMU OIB are derived from such a source, with little contamination from other components. Both the EMI and EMII OIB end-members are also dominantly derived from this source, but they contain significant proportions (up to 5–10%) of sedimentary components derived from the continental crust. In the case of EMI OIB, ancient pelagic sediment with high LILE/HFSE, LREE/HFSE, Ba/Th and Ba/La ratios, and low U/Pb ratios, is the contaminant. EMII OIB are also contaminated by a sedimentary component, in the form of ancient terrigenous sediment with high LILE/HFSE and LREE/HFSE ratios, but lacking relative Ba enrichment, and with higher U/Pb and Rb/Sr ratios. A model whereby the source for all OIB is ancient (1–2 Ga old) subducted oceanic crust ± entrained sediment (pelagic and/or terrigenous) is therefore consistent with the trace element and isotopic data. Although subducted oceanic lithosphere will accumulate and be dominantly concentrated within the mesosphere boundary layer, forming the source for hot-spots, such material may also become convectively dispersed within the depleted upper mantle as blobs or streaks, giving rise to small-scale chemical heterogeneities in the upper mantle. 相似文献
136.
James P. Howington Gordon A. McFeters James P. Barry James J. Smith 《Marine pollution bulletin》1992,25(9-12):324-327
The spatial distribution and movement of the sewage plume from McMurdo Station, Antarctica, was investigated in the ocean under the early summer ice. Samples of seawater were obtained via holes drilled through the ice and analysed for coliform bacteria. Ocean currents were also examined to determine their effect on the movement of the plume. High densities of coliform bacteria were found along the ca. 1 km shoreline of McMurdo Station and the plume extended 200–300 m seaward. The relocation of the outfall from a surface configuration to the subsurface (11 m deep) had little influence on the distribution of the plume that sometimes reached the seawater intake station, 400 m to the south. Ocean current measurements in the study area confirmed that, while the prevailing advection was to the north and away from the intake area, episodic reversals of flow at some current meter stations coincided with pulses of sewage that moved into the intake. These findings support the use of bacterial indicators as one means to map the distribution and movement of recent sewage contamination in cold (−1.8°C) seawater and provides evidence that the disposal and movement of domestic wastes in coastal polar environments deserves attention. 相似文献
137.
138.
Joanne H. Shorter J. Barry Mcmanus Charles E. Kolb Eugene J. Allwine Brian K. Lamb Byard W. Mosher Robert C. Harriss Uwe Partchatka Horst Fischer Geoffrey W. Harris Paul J. Crutzen Hans-Josef Karbach 《Journal of Atmospheric Chemistry》1996,24(2):121-140
We have investigated methane emissions from urban sources in the former East Germany using innovative measurement techniques including a mobile real-time methane instrument and tracer release experiments. Anthropogenic and biogenic sources were studied with the emphasis on methane emissions from gas system sources, including urban distribution facilities and a production plant. Methane fluxes from pressure regulating stations ranged from 0.006 to 24. l/min. Emissions from diffuse sources in urban areas were also measured with concentration maps and whole city flux experiments. The area fluxes of the two towns studied were 0.37 and 1.9 g/m2/s. The emissions from individual gas system stations and total town emissions of this study are comparable to results of similar sites examined in the United States. 相似文献
139.
Despite rapidly growing interest in the effects of ocean acidification on marine animals, the ability of deep-sea animals to acclimate or adapt to reduced pH conditions has received little attention. Deep-sea species are generally thought to be less tolerant of environmental variation than shallow-living species because they inhabit relatively stable conditions for nearly all environmental parameters. To explore whether deep-sea hermit crabs (Pagurus tanneri) can acclimate to ocean acidification over several weeks, we compared behavioral “boldness,” measured as time taken to re-emerge from shells after a simulated predatory attack by a toy octopus, under ambient (pH ~7.6) and expected future (pH ~7.1) conditions. The boldness measure for crab behavioral responses did not differ between different pH treatments, suggesting that future deep-sea acidification would not influence anti-predatory behavior. However, we did not examine the effects of olfactory cues released by predators that may affect hermit crab behavior and could be influenced by changes in the ocean carbonate system driven by increasing CO2 levels. 相似文献
140.
Radar speckle reduction and derived texture measures for land cover/use classification: a case study
This study examined the appropriateness of radar speckle reduction for deriving texture measures for land cover/use classifications. Radarsat-2 C-band quad-polarised data were obtained for Washington, DC, USA. Polarisation signatures were extracted for multiple image components, classified with a maximum-likelihood decision rule and thematic accuracies determined. Initial classifications using original and despeckled scenes showed despeckled radar to have better overall thematic accuracies. However, when variance texture measures were extracted for several window sizes from the original and despeckled imagery and classified, the accuracy for the radar data was decreased when despeckled prior to texture extraction. The highest classification accuracy obtained for the extracted variance texture measure from the original radar was 72%, which was reduced to 69% when this measure was extracted from a 5 × 5 despeckled image. These results suggest that it may be better to use despeckled radar as original data and extract texture measures from the original imagery. 相似文献