全文获取类型
收费全文 | 590篇 |
免费 | 16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 | 159篇 |
地质学 | 179篇 |
海洋学 | 57篇 |
天文学 | 52篇 |
自然地理 | 72篇 |
出版年
2020年 | 6篇 |
2019年 | 5篇 |
2018年 | 7篇 |
2017年 | 10篇 |
2016年 | 10篇 |
2015年 | 14篇 |
2014年 | 12篇 |
2013年 | 35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 | 10篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 14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 | 9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608条查询结果,搜索用时 31 毫秒
571.
Barry J. Katz Christopher G.A. Harrison Eugene H. Man 《Earth and Planetary Science Letters》1979,44(2):279-286
The “second region” of the kinetic curve for the racemization (epimerization) of isoleucine in foraminifera is defined from previously published data corrected for the thermal history experienced by the samples. These kinetic parameters are applied to racemization data from Deep Sea Drilling Project Sites 332A and 333 from the Deep Drill Valley, Mid-Atlantic Ridge, and Site 148 from the Aves Ridge, in order to determine whether there are any observable effects due to the geothermal gradient. The data for Site 148 clearly show an increase of temperature with depth. The data for the two sites of the Mid-Atlantic Ridge do not show this clear relationship. When bottom water temperature variations are taken into consideration, the effects of the geothermal gradient become apparent. Since the degree of racemization is dependent upon both age and temperature, a knowledge of the age of a sample places constraints upon its thermal history, and hence on the heat flow at the location since deposition of the sample. The crude heat flow models thus developed are compatible with present geologic and geophysical information. It appears probable that detailed heat flow models may be developed by improving analytical precision. 相似文献
572.
Barry Kean Atkinson 《Physics and Chemistry of Minerals》1978,2(4):305-315
Stress relaxation experiments were performed on hot-pressed cylinders of synthetic, polycrystalline galena at temperatures of 500°, 600°, 700° and 800°C at atmospheric pressure after various levels of axial strain, in the range from 2% to 25%, had been imposed at a constant strain rate (?) of 3×10?4 s?1. For this study a new apparatus was built which can measure small changes in applied stress (σ), better than ±0.5 × 105 Pa, and which incorporates a facility whereby the composition of the test specimen can be fixed through equilibration with a gas atmosphere of controlled composition. Deformation mechanism maps constructed largely on theoretical grounds predict that, under the conditions studied here, high-temperature dislocation creep, for which ?∝σ5, gives way on lowering the applied stress to diffusion creep, for which ?∝σ. The experimental results agree in large measure with these theoretical predictions. However, it is not possible in this preliminary study to conclusively identify the dominant deformation mechanisms. 相似文献
573.
Barry Saltzman 《Pure and Applied Geophysics》1968,69(1):237-259
Summary Equations governing the axially-symmetric time-average state of the atmosphere and the transient departures from this mean state are set down. As a first step toward a solution of this system for seasonal average conditions, a model is formulated based on the thermodynamical energy equation for the vertical average of the mean state, and on the perturbation solutions of the linearized equations governing the baroclinic growth of transient eddies. All forms of non-adiabatic heating within the atmosphere and at the earth's surface are parameterized. The resulting differential equation governing the axially-symmetric mean potential temperature distribution takes the form of a steadystate diffusion equation in surface spherical coordinates, with a variable Austausch coefficient which is to be determined iteratively as a dependent variable.Global solutions, for winter and summer equilibrium conditions, are obtained for the thermal structure, the heat balance components, the transient eddy variances of temperature and meridional wind speed, and the covariance representing the meridional eddy heat transport. These solutions are for different types of surface conditions (ocean, land), and for a successively more complete variety of modes of heat transfer ranging from pure radiation to a combination of radiation, latent heat processes, and conduction and convection within the atmosphere and the subsurface layers. The results for this latter complete case seem to be a reasonable first order approximation to the observed distributions. Suggestions are made for improving and generalizing the study. 相似文献
574.
Within the ‘glimmerite’ nodules occurring within kimberlite pipes we recognize the MARID suite consisting of varying proportions of mica, amphibole, rutile, ilmenite and diopside. Banding of some specimens is interpreted as cumulate layering. All specimens were deformed either before incorporation into the host kimberlite or during intrusion. Compared with minerals in peridotite xenoliths, the MARID ones are lower in Al2O3 and Cr2O3, but richer in total iron. The MARID micas, amphiboles, diopsides, ilmenites and probably rutiles contain substantial Fe2O3 indicative of oxidizing conditions. The amphibole is potassic richterite. Micas of the megacryst suite in kimberlite have less total iron and Fe2O3 than micas of the MARID suite. We suggest that the rocks of the MARID suite crystallized under oxidizing conditions from a magma, chemically similar to kimberlite, within the higher parts of the upper mantle: the presence of amphibole restricts the depth to less than ~ 100 km. A xenolith containing olivine and orthopyroxene as well as minerals similar to but not the same compositionally as MARID-types is interpreted as a metasomite, possibly representing wall-rock of a magma body from which MARID-suite rocks crystallized. 相似文献
575.
In July 1971 when approximately 25% of the clams in Long Cove, Searsport, Maine, had been killed by the March 1971 oil spill, collections of surviving clams for histological examination were made. These studies were continued through 1974 and revealed a high incidence of gonadal tumours in clams contaminated by the oil. The area of highest oil impact correlated with the highest per cent of tumours. The tumours were found to be malignant neoplasms. 相似文献
576.
Peter M. D. Bradshaw Ian Thomson Barry W. Smee Jan O. Larsson 《Journal of Geochemical Exploration》1974,3(3):209-225
This paper deals briefly with the principles of geochemical migration in the secondary (soil, sediment) environment, a knowledge of which is essential to a correct interpretation of exploration geochemical data. Examples are given which illustrate that the principles which apply in the more easily interpreted tropical areas, also apply in the more complicated glaciated regions. Any person employing exploration geochemistry in geomorphologically complicated areas, is well advised to study data from strictly residual soil areas where the fundamentals of geochemical migration are more easily observed. From this base it is easier to understand the additional complications of geochemistry in mountainous and glaciated terrain. Of the variety of exploration geochemical techniques which can be used, this paper deals specifically with two: soil profile sampling, and different strengths of acid extraction of metal from samples. Examples from the different environments are compared and contrasted. 相似文献
577.
Barry L. Weaver 《Contributions to Mineralogy and Petrology》1990,105(5):502-515
The volcanic rocks of the South Atlantic Ocean islands of Fernando de Noronha and Trindade comprise a diverse magmatic series ranging from nephelinites and basanites to phonolites and, on Fernando de Noronha, trachytes. All rock types are highly silica undersaturated with the exception of Fernando de Noronha trachytes_, and have high abundances of incompatible trace elements and strongly LREE (light rare earth element)-enriched REE patterns. Crystal fractionation of parental basanitic magmas produced evolved phonolites and trachytes which display severe trace-element fractionation, even among trace elements (Nb, Ta, Zr, Hf) which normally behave highly incompatibly during crystallisation of alkaline magmas. Moderately to highly evolved compositions develop strongly MREE (middle REE)-depleted REE patterns, and become increasingly depleted in elements such as Nb and, in particular, Ta. Ratios of Nb/Ta and Zr/Hf are highly fractionated in phonolites (60–65, 64–77 respectively in Fernando de Noronha phonolites) compared to ratios in basanites (14, 45 respectively). The compatibility of Nb, Ta, and the REE, and the strong fractionation of Nb/Ta and Zr/Hf ratios and the MREE, during crystallisation from basanite to phonolite are attributable to the crystallisation of small amounts (<5%) of sphene. Trace-element behaviour is relatively insensitive to the major phenocryst phases, and is controlled by minor phases in highly undersaturated alkaline suites. Incompatible trace-element ratios (e.g. La/Nb, Th/Ta) in nephelinites and basanites from Fernando de Noronha and Trindade are generally comparable with those in basaltic and hawaiitic OIB (ocean island basalt) lavas from other South Atlantic islands, but are distinct from those of Gough and Tristan da Cunha OIB. The mantle source for the highly undersaturated volcanism on Fernando de Noronha and Trindade is similar in trace-element characteristics to the typical OIB source which produces alkaline lavas with significant relative enrichment in Nb and Ta compared to other trace elements (as expressed by low La/Nb, Ba/Nb and Th/Ta ratios). The highly undersaturated nature of the magmas and the slight fractionation of some incompatible-element ratios (elevated Ba/Nb, Ba/Rb, Ba/Th etc.) is consistent with a smaller degree of melting of a typical OIB source, but with residual phlogopite in the source to account for significant K depletion and LIL-element fractionation. 相似文献
578.
579.
Presence of springs in karst terranes provides a unique opportunity to study the rather complex, multi-porosity, and multi-permeability
system. When springs are used to evaluate the integrity of storage facilities for hazardous materials or waste disposal facilities
constructed in karst areas, the spatial heterogeneity of karst aquifers makes intra-spring comparisons preferred statistical
tests. One of the commonly used statistical tests is water quality control procedure such as Shewhart-CUSUM control charts.
Appropriate application of the water quality control procedure to intra-spring monitoring depends on whether the assumptions
can be justified about the aquifer that drains to the spring and the dataset collected at the spring. Violation of the assumptions
would render the statistical tests invalid, which may result in a failure of the groundwater monitoring program. In intra-spring
monitoring, it is the temporal variations of water quality at a karst spring need to be addressed, whereas the water quality
at the spring is closely associated with the characteristics of the aquifer. The example datasets presented in the paper indicate
that both false negative and false positive detections can occur if the temporal variation is not well characterized. 相似文献
580.
Jennifer L. Druhan James F. Hogan Christopher J. Eastoe Barry J. Hibbs William R. Hutchison 《Hydrogeology Journal》2008,16(2):281-296
Identification of hydrogeologic controls on groundwater flowpaths, recharge, and salinization is often critical to the management of limited arid groundwater resources. One approach to identifying these mechanisms is a combined analysis of hydrogeologic and hydrochemical data to develop a comprehensive conceptual model of a groundwater basin. To demonstrate this technique, water samples were collected from 33 discrete vertical zone test holes in the Hueco Bolson aquifer, located within the Trans-Pecos Texas region and the primary water resource for El Paso, Texas, USA and Juárez, Mexico. These samples were analyzed for a suite of geochemical tracers and the data evaluated in light of basin hydrogeology. On the basis of δ2H and δ18O data, two regional recharge sources were recognized, one originating from western mountain-fronts and one from through-flow of the adjacent Tularosa aquifer. Chloride concentrations were strongly correlated with lithologic formations and both Cl/Br and 36Cl ratios suggested the primary chloride source is halite dissolution within a specific lithologic unit. In contrast, sulfur isotopes indicated that most sulfate originates from Tularosa basin Permian gypsum sources. These results yielded a more comprehensive conceptual model of the basin, which suggested that chloride salinization of wells is the result of upconing of waters from the Fort Hancock formation. 相似文献