全文获取类型
收费全文 | 27674篇 |
免费 | 604篇 |
国内免费 | 298篇 |
专业分类
测绘学 | 711篇 |
大气科学 | 2222篇 |
地球物理 | 6024篇 |
地质学 | 9595篇 |
海洋学 | 2250篇 |
天文学 | 5748篇 |
综合类 | 46篇 |
自然地理 | 1980篇 |
出版年
2020年 | 185篇 |
2019年 | 178篇 |
2018年 | 353篇 |
2017年 | 339篇 |
2016年 | 517篇 |
2015年 | 383篇 |
2014年 | 518篇 |
2013年 | 1356篇 |
2012年 | 608篇 |
2011年 | 938篇 |
2010年 | 749篇 |
2009年 | 1046篇 |
2008年 | 975篇 |
2007年 | 934篇 |
2006年 | 952篇 |
2005年 | 810篇 |
2004年 | 837篇 |
2003年 | 795篇 |
2002年 | 788篇 |
2001年 | 635篇 |
2000年 | 660篇 |
1999年 | 613篇 |
1998年 | 584篇 |
1997年 | 590篇 |
1996年 | 490篇 |
1995年 | 489篇 |
1994年 | 456篇 |
1993年 | 431篇 |
1992年 | 395篇 |
1991年 | 344篇 |
1990年 | 394篇 |
1989年 | 328篇 |
1988年 | 350篇 |
1987年 | 398篇 |
1986年 | 338篇 |
1985年 | 505篇 |
1984年 | 546篇 |
1983年 | 548篇 |
1982年 | 435篇 |
1981年 | 438篇 |
1980年 | 449篇 |
1979年 | 399篇 |
1978年 | 410篇 |
1977年 | 355篇 |
1976年 | 384篇 |
1975年 | 351篇 |
1974年 | 389篇 |
1973年 | 372篇 |
1972年 | 239篇 |
1971年 | 189篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
J. Schäfer H. Neuroth H. Ahrendt W. Dörr W. Franke 《International Journal of Earth Sciences》1997,86(3):599-611
The Saxothuringian flysch basin, on the north flank of the Central European Variscides, was fed and eventually overthrust by the northwestern, active margin of the Tepla-Barrandian terrane. Clast spectra, mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge. The earliest clastic sediments preserved are of early Famennian age (ca. 370?Ma). They are exposed immediately to the NW of the suture, and belong to the par-autochthon of the foreland. Besides ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposited at the NW margin of the Saxothuringian basin. These findings, together with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian narrow ocean had been closed by that time. Microprobe analyses of detrital hornblendes suggest derivation from the “Randamphibolit” unit, now present in the middle part of the Saxothuringian allochthon (Münchberg nappes). Detrital zircons of metamorphic rocks formed a little earlier (ca. 380?Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Viséan), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, together with debris of Cadomian basement, 500-Ma granitoids and 380?Ma (early Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Viséan), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shortening well in excess of 10–30?mm per year, and rates of exhumation of ca. 3?mm/a, and possibly more. 相似文献
902.
W. G. Ernst 《Contributions to Mineralogy and Petrology》1997,128(1):30-44
Thin mafic dikes, possibly correlative with the Independence dike swarm of SE California, transect uppermost Proterozoic–Cambrian
metasedimentary strata in the White-Inyo Range. Textures and bulk-rock chemistry indicate that the protoliths were diabases
and microdiorites, accompanied by Ca + Mg + Fe +Ni + Cr-rich hornblende (± minor augite) cumulates. Analytical data suggest
crystal settling and fractionation at shallow depths. Most of the dikes lie in the mapped aureoles of – and were metamorphosed
by – voluminous Late Jurassic granitoid plutons; however, a few metadikes cut these plutons and must have been recrystallized
during the emplacement of Cretaceous granitic stocks. The mafic metadikes thus include members of two or more temporally distinct
suites, pre-Late Jurassic, and latest Jurassic–Cretaceous. Neoblastic mineral assemblages and element partitioning within
these nonfoliated mafic metadikes reflect lower-to-upper greenschist facies overprints; metamorphic parageneses, coincident
with those developed in the metasedimentary wallrocks, are defined by the production of chlorite, biotite, white mica, epidote,
and actinolite, and by albitization of the igneous plagioclase. Based on analytical and mineralogic data obtained in this
study, the following conclusions regarding subsolidus recrystallization of the mafic metadikes are advanced: (1) Newly grown
minerals and phase assemblages are systematic in their areal distributions. (2) Metamorphic grade increases chiefly toward
the north and east, toward the Late Jurassic granitoids. (3) Element fractionation among coexisting neoblastic phases is regular,
and compatible with a close approach to chemical equilibrium. (4) Assemblages 3–5 km from the granitic intrusive contacts
reflect lowermost greenschist facies physical conditions. (5) Investigated mafic dikes exhibit mineral parageneses isofacial
with the regional/contact metamorphic assemblages previously documented for the enclosing pre-Mesozoic clastic country rocks.
Clearly, mafic dikes of several ages of injection and recrystallization are present in the central White-Inyo Range, making
correlation with the Independence dike swarm problematic. In any case, the dikes record localized contact metamorphism that
took place sporadically over portions of an approximately 100 million year interval.
Received: 13 March 1996 / Accepted: 24 December 1996 相似文献
903.
The Olmedo bauxite deposit occurs in the Nurra district of northwest Sardinia. It forms a stratiform horizon in Cretaceous
limestone and marl. Uplift in mid-Cretaceous had exposed recently deposited limestone to karst weathering, and a layer of
argillaceous debris accumulated on its surface and was partly converted to bauxite. Intermediate products were desiccated
marl, bauxitic argillite and argillaceous bauxite. Subsidence followed, and the bauxite was preserved by the deposition of
late Cretaceous limestone and other sediments. Uplift in Oligocene-Miocene time, with ensuing erosion, exposed the bauxite
horizon to its present configuration. Concentrations of normative minerals illustrate chemical processes and the build-up
of Al in the bauxite horizon. Plots of chemical data and correlation coefficients show that Al, Ti, Zr, Nb, Th, Cr and V were
immobile during the bauxitization process. Mass changes point to large net removal of Si, Mg and K from the system, although
some of this material and slightly mobile Al were reprecipitated in the underlying argillite and altered marl. Immobile element
ratios trace the source of the bauxite to the underlying argillaceous limestone. Al in the bauxite was accumulated from the
degradation of 25 to 50 m of the argillaceous limestone.
Received: 10 June 1996 / Accepted: 15 April 1997 相似文献
904.
Estimation of groundwater evaporation and salt flux from Owens Lake, California, USA 总被引:3,自引:0,他引:3
S. W. Tyler S. Kranz M. B. Parlange J. Albertson G. G. Katul G. F. Cochran B. A. Lyles G. Holder 《Journal of Hydrology》1997,200(1-4):110-135
Groundwater evaporation and subsequent precipitation of soluble salts at Owens Lake in eastern California have created one of the single largest sources of airborne dust in the USA, yet the evaporation and salt flux have not been fully quantified. In this study, we compare eddy correlation, microlysimeters and solute profiling methods to determine their validity and sensitivity in playa environments. These techniques are often used to estimate evaporative losses, yet have not been critically compared at one field site to judge their relative effectiveness and accuracy. Results suggest that eddy correlation methods are the most widely applicable for the variety of conditions found on large playa lakes. Chloride profiling is shown to be highly sensitive to thermal and density-driven fluxes in the near surface and, as a result, appears to underestimate yearly groundwater evaporation. Yearly mean groundwater evaporation from the playa surface estimated from the three study areas was found to range from 88 to 104 mm year−1, whereas mean evaporation from the brine-covered areas was 872 mm year−1. Uncertainties on these mean rates were estimated to be ±25%, based on comparisons between eddy correlation and lysimeter estimates. On a yearly basis, evaporation accounts for approximately 47 × 106 m3 of water loss from the playa surface and open-water areas of the lake. Over the playa area, as much as 7.5 × 108 kg (7.5 × 105 t) of salt are annually concentrated by evaporation at or near the playa surface, much of which appears to be lost during dust storms in area. 相似文献
905.
Jürg W. Schlatter Alfred Wüest Dieter M. Imboden 《Aquatic Sciences - Research Across Boundaries》1997,59(3):225-242
The artificial tracer sulphur hexafluoride (SF6) has been used to study the density-driven deep water exchange between two sill-separated basins of Lake Lucerne, Gersauersee and Urnersee. The sources of the density gradients between the two basins are (1) salinity differences between the major inlets due to the different geology of their drainage areas, and (2) temperature differences due to spatial variation of wind forcing. Wind speeds are generally larger in Urnersee, especially in spring during the so-called Föhn events, when winds blow from the south. In contrast, Gersauersee is protected form these winds. In spring 1989, a total of 630 g of SF6 was released at 80 to 120 m depth in the small Treib Basin located between Urnersee and Gersauersee. During about 100 days the distribution of SF6 in the lake was determined by gaschromatography. Two models are used to quantify the exchange flow, (1) a one-box mass balance model for SF6 in the deep part of Treib Basin, and (2) a one-dimensional diffusion/advection model describing the temporal and vertical temperature variation in Urnersee. According to the first model, the flow into the deep hypolimnion of Urnersee, decreases from 21·106 m3·d?1 at the end of March to about 8·106 m3·d?1 in late April. The second model yields similar flow rates. The decrease of the flow rate during spring, confirmed by both approaches, is consistent (1) with the decreasing strength of the density gradient above the sill during spring and early summer, and (2) with hydrographic information collected in Lake Lucerne during other years. 相似文献
906.
M. W. Kemblowski C-M. Chang I. Kamil 《Stochastic Environmental Research and Risk Assessment (SERRA)》1997,11(3):255-266
It has been observed that the field biodegradation rates for soluble hydrocarbon plumes are significantly smaller than the
aerobic rates observed in the laboratory. It is believed that this difference is related to the fact that in the field oxygen
and hydrocarbon must be mixed before the biodegradation reaction can occur, and that the effective degradation rate is controlled
by the actual, not mean, concentrations of oxygen and hydrocarbon. In this work, we present a conceptual model of oxygen-mixing
limited biodegradation, which indicates that the effective degradation rate should depend on the cross correlation between
the oxygen and hydrocarbon concentration fluctuations. This is followed by a development of a rigorous, field-scale model. 相似文献
907.
Testing an earthquake prediction algorithm 总被引:1,自引:0,他引:1
Vladimir G. Kossobokov John H. Healy James W. Dewey 《Pure and Applied Geophysics》1997,149(1):219-232
A test to evaluate earthquake prediction algorithms is being applied to a Russian algorithm known asM8 TheM8 algorithm makes intermediate term predictions for earthquakes to occur in a large circle, based on integral counts of transient seismicity in the circle. In a retroactive prediction for the period January 1, 1985 to July 1, 1991 the algorithm as configured for the forward test would have predicted eight of ten strong earthquakes in the test area. A null hypothesis, based on random assignment of predictions, predicts eight earthquakes in 2.87% of the trials. The forward test began July 1, 1991 and will run through December 31, 1997. As of July 1, 1995, the algorithm had forward predicted five out of nine earthquakes in the test area, which success ratio would have been achieved in 53% of random trials with the null hypothesis. 相似文献
908.
W. K. Hocking 《Annales Geophysicae》1997,15(9):1111-1122
Radars have been used successfully for many years to measure atmospheric motions over a wide range of altitudes, from ground level up to heights of several hundred kilometres into the ionosphere. In this paper we particularly wish to concentrate on the accuracy of these measurements for winds in the middle atmosphere (i.e. 10–100–km altitude). We begin by briefly reviewing the literature relating to comparisons between radar methods and other techniques. We demonstrate where the radar data are most and least reliable and then, in parallel with a discussion about the basic principles of the method, discuss why these different regimes have the different accuracies and precisions they do. This discussion is used to highlight the strengths and weaknesses of radar methods. Issues like radar volume, aspect sensitivity, gravity wave effects and scatterer intermittency in producing wind biases, and the degree by which the intermittent generation of scatterers at quasi-random points in space could skew the radar measurements, are all considered. We also investigate the possibility that MF radar techniques can be contaminated by E-region scatter to heights as low as 92–95–km altitude (i.e. up to 8–10 km below the ionospheric peak echo). Within all these comments, however, we also recognize that radar methods still represent powerful techniques which have an important future at all levels of the atmosphere. 相似文献
909.
Effects of convection instability due to incompatibility between ocean dynamics and surface forcings
The study demonstrates that an incompatibility between a surface temperature climatology and a given ocean model, into which the climatology is assimilated via Haney restoration, can cause model ocean climate drift and interdecadal oscillations when the ocean is switched to a weaker restoration. This is made using an idealized Atlantic Ocean model driven by thermal and wind forcing only. Initially, the temperature climatology is forcefully assimilated into the model, and an implied heat flux field is diagnosed. During this stage any incompatibility is suppressed. The restoring boundary condition is then switched to a new forcing consisting of a part of the diagnosed flux and a part of the restoring forcing in such a way that at the moment of the switching the heat flux is identical to that prior to the switching. Under this new forcing condition, the incompatibility becomes manifest, causing changes in convection patterns, and producing drift and interdecadal oscillations. The mechanisms are described. 相似文献
910.
A. Belehaki E. T. Sarris G. Tsiropoula R. W. McEntire S. Kokubun T. Yamamoto 《Annales Geophysicae》1997,15(12):1515-1531
Geotail energetic particle, magnetic field data and plasma observations (EPIC, MGF and CPI experiments) have been examined for a number of energetic particle bursts in the distant tail (120Re < |XGSM| < 130 Re), associated with moving magnetic field structures, following substorm onsets. The features obtained from this data analysis are consistent with the distant magnetotail dynamics determined first by ISEE3 observations and explained in terms of the neutral line model. At the onset of the bursts, before plasma sheet entrance, energetic electrons appear as a field-aligned beam flowing in the tailward direction, followed by anisotro-pic ions. Within the flux rope region, suprathermal ions exhibit a convective anisotropy, which allows determination of the plasma flow velocity, assuming that the anisotropy arises from the Compton-Getting effect. The velocities thus determined in the plasma sheet are estimated to be 200–650 km/s, and compare favourably with the velocities derived from the CPI electron and proton experiment. The estimated length of magnetic field structures varies between 28 and 56 Re and depends on the strength of the westward electrojet intensification. Finally, the three structures reported here show clear magnetic field signatures of flux rope topology. The existence of a strong magnetic field aligned approximately along the Y-axis and centred on the north-to-south excursion of the field, and the bipolar signature in both By and/or Bz components, is consistent with the existence of closed field lines extending from Earth and wrapping around the core of the flux rope structure. 相似文献