全文获取类型
收费全文 | 3818篇 |
免费 | 111篇 |
国内免费 | 52篇 |
专业分类
测绘学 | 100篇 |
大气科学 | 232篇 |
地球物理 | 920篇 |
地质学 | 1302篇 |
海洋学 | 354篇 |
天文学 | 699篇 |
综合类 | 18篇 |
自然地理 | 356篇 |
出版年
2022年 | 27篇 |
2021年 | 43篇 |
2020年 | 56篇 |
2019年 | 52篇 |
2018年 | 94篇 |
2017年 | 86篇 |
2016年 | 100篇 |
2015年 | 84篇 |
2014年 | 102篇 |
2013年 | 203篇 |
2012年 | 109篇 |
2011年 | 179篇 |
2010年 | 149篇 |
2009年 | 169篇 |
2008年 | 151篇 |
2007年 | 153篇 |
2006年 | 147篇 |
2005年 | 131篇 |
2004年 | 127篇 |
2003年 | 94篇 |
2002年 | 129篇 |
2001年 | 66篇 |
2000年 | 74篇 |
1999年 | 65篇 |
1998年 | 56篇 |
1997年 | 56篇 |
1996年 | 48篇 |
1995年 | 60篇 |
1994年 | 45篇 |
1993年 | 43篇 |
1992年 | 48篇 |
1991年 | 46篇 |
1990年 | 36篇 |
1989年 | 34篇 |
1988年 | 42篇 |
1987年 | 47篇 |
1986年 | 41篇 |
1985年 | 58篇 |
1984年 | 68篇 |
1983年 | 68篇 |
1982年 | 64篇 |
1981年 | 54篇 |
1980年 | 52篇 |
1979年 | 54篇 |
1978年 | 33篇 |
1977年 | 48篇 |
1976年 | 47篇 |
1975年 | 33篇 |
1974年 | 28篇 |
1973年 | 28篇 |
排序方式: 共有3981条查询结果,搜索用时 328 毫秒
971.
972.
Steven Emerson Richard Jahnke Michael Bender Philip Froelich Gary Klinkhammer Carl Bowser George Setlock 《Earth and Planetary Science Letters》1980,49(1):57-80
Interstitial waters were extracted from cores at three locations in the eastern equatorial Pacific and analyzed for nutrients, dissolved carbonate species, Mn and Fe. From the depth variation in pore water chemistry, we infer that organic matter oxidation reactions occur with depth in the following sequence: O2 reduction, NO3? and MnO2 reduction, and then ferric iron reduction. From NO3? results we infer that O2 is largely or totally consumed within the top few centimeters of sediment. NO3? is completely reduced at a sediment depth of 20 cm at a site near the crest of the East Pacific Rise, but is preserved at levels of 20–30 μmol/kg at 40 cm depth at a Guatemala Basin site.We have calculated the alkalinity for pore water samples assuming ions diffuse according to relative ionic diffusion coefficients, that the stoichiometry of organic matter oxidation reactions is that of “Redfield” organic matter, and that the pore waters are saturated throughout with respect to CaCO3. The measured alkalinity increase is only about half of the predicted value. The difference is probably a result of either enhanced mixing of the pore water in the top few centimeters of sediments by biological or physical processes, or the occurrence of an inorganic reaction which consumes alkalinity.At depths of oxygen and nitrate reduction in the sediments, the ion concentration product of CaCO3 is the same, within the analytical error, as the solubility product of Ingle et al. [34] at 1 atm and 4°C. This result indicates CaCO3 resaturation on pressure change during coring. Where pore water Mn concentrations become measurable, the ion concentration product increases, indicating either supersaturation with respect to calcite or that another phase is controlling the carbonate solubility. 相似文献
973.
Gary E. Thomas Charles A. Barth Elaine R. Hansen Charles W. Hord George M. Lawrence George H. Mount Gary J. Rottman David W. Rusch A. Ian Stewart Ronald J. Thomas Julius London Paul L. Bailey Paul J. Crutzen Robert E. Dickinson John C. Gille Shaw C. Liu John F. Noxon Crofton B. Farmer 《Pure and Applied Geophysics》1980,118(1):591-615
The 1981–82 Solar Mesosphere Explorer (SME) mission is described. The SME experiment will provide a comprehensive study of mesospheric ozone and the processes which form and destroy it. Five instruments will be carried on the spinning spacecraft to measure the ozone density and its altitude distribution from 30 to 80 km, monitor the incoming solar ultraviolet radiation, and measure other atmospheric constituent which affect ozone. The polar-orbiting spacecraft will be placed into a 3pm-3 am Sun-synchronous orbit. The atmospheric measurements will scan the Earth's limb and measure: (1) the mesospheric and stratospheric ozone density distribution by inversion of Rayleigh-scattered ultraviolet limb radiance, and the thermal emission from ozone at 9.6 m; (2) the water vapor density distribution by inversion of thermal emission at 6.3 m; (3) the ozone photolysis rate by inversion of the O2(1g) 1.27 m limb radiance; (4) the temperature profile by a combination of narrow-band and wide-band measurements of the 15 m thermal emission by CO2; and, (5) theNO2 density distribution by inversion of Rayleighscattered limb radiance at 0.439 m. The solar ultraviolet monitor will measure both the 0.2–0.31 m spectral region and the Lyman-alpha (0.1216 m) contribution to the solar irradiance. This combination of measurements will provide a rigorous test of the photochemical equilibrium theory of the mesospheric oxygen-hydrogen system, will determine what changes occur in the ozone distribution as a result of changes in the incoming solar radiation, and will detect changes that may occur as a result of meteorological disturbances. 相似文献
974.
Loren Shure Robert L. Parker George E. Backus 《Physics of the Earth and Planetary Interiors》1982,28(3):215-229
Which features of a geomagnetic field model on the surface of the core are really necessary in order to fit, within observational error, the field observations at and above the Earth's surface? To approach this question, we define ‘roughness’ in various ways as a norm on an appropriate Hilbert space of field models which is small when the field is smooth on the core surface. Then, we calculate the model with least norm (the smoothest model) which fits the data, sources outside the core being treated as noise. Sample calculations illustrate the effects of noise, of the choice of norm and of an uneven distribution of observing stations. 相似文献
975.
Douglas G. Carson George R. Rossman Robert W. Vaughan 《Physics and Chemistry of Minerals》1982,8(1):14-19
Conventional and solid state proton nuclear magnetic resonance (NMR) techniques have been used to examine water molecules in the channels of a single crystal of cordierite, (Mg, Fe)2Al4Si5O18, as a function of temperature, magnetic field, and orientation. Only one type of water was found rather than water in two distinct rigid orientations which were indicated by earlier infrared spectral studies. However, the measured dipolar splittings indicate that this water is in rapid motion. Shifts in the dipolar doublet due to Fe2+ impurities indicate that the water molecules are not moving among adjacent channel sites along a channel cavity. A two-site hopping model is proposed involving the major residence time spent with the hydrogen-hydrogen vector parallel to the channels, a minor residence time spent with the hydrogen-hydrogen vector perpendicular to the channels, and a short time (<1 μs) in transit. This model fits both the present NMR data and previously reported infrared absorption data and is compared to previously reported neutron diffraction data. 相似文献
976.
Young-of-the-year alewife, American shad, blueback herring, and striped bass were analyzed for Kepone contamination. Samples were collected from the Potomac, Rappahannock, Pamunkey, Mattaponi, Chickahominy, and James rivers during the period 1977–79. Concentrations of Kepone ≥0.3 ppm occurred in all four species collected in the James River nursery zone between km 65 and 120, and in the lower Chickahominy River. Concentrations of Kepone <0.3, ppm were also present in samples from the Mattaponi and Pamunkey rivers. Four possible explanations, for the occurrence of Kepone in these samples from the upper York River system (Mattaponi and Pamunkey rivers) were, evaluated. Kepone was not detectable in samples from the Rappahannock and Potomac rivers. 相似文献
977.
978.
George Purcaru 《Physics of the Earth and Planetary Interiors》1979,18(4):274-287
The available data on the destructive intermediate earthquakes () in the Vrancea, Romania, region have been examined with the aim of revealing some time-magnitude regularities. The basic idea is that the total sequence (? 1100–1973 yr.), which appears as random, could be decomposed in some regular source-components which, by extrapolation, are superimposed to predict the future total sequence.The common nature of faulting (reverse dip-slip) and inferred regularities in the time-magnitude pattern of destructive Romanian earthquakes — (a) three active (seismic) time-bands alternating with quiet periods, the existence of (b) “quasicycles” and of (c) “supercycles” — led to the following predictions: (1) the occurrence of a shock with in 1980 ± 13 years; and (2) later earthquakes are predicted in 2005, in 2030–2040 (), and one with nearly maximum magnitude () in 2070–2090.In every century, about 40 years represent a time interval of high seismic danger for Romania and, according to the proposed long-term time-magnitude model, three destructive earthquakes arc to occur in (and/or near) the evidenced seismic periods P1, P2 and P3.It is shown that, taking into account the actual difficulties involved in the earthquake prediction, the Vrancea destructive earthquake of March 4, 1977 (M = 7.1) was quite successfully predicted. 相似文献
979.
Electronic and Mössbauer absorption spectra and electron microprobe data are correlated for iron-bearing orthopyroxenes. The correlation provides a means of quantitatively determining the distribution of Fe2+ between the M(1) and M(2) sites of orthopyroxene crystals from electronic spectra and electron microprobe analysis. The electronic spectra are used to analyze the changes in the Fe2+ distribution produced during heating experiments and confirm earlier results from Mössbauer spectra. Two components of the spin-allowed transition of Fe2+ in the M(1) site are identified at about 13,000 cm?1 and 8,500 cm?1 in γ. Molar absorptivity (?) values for all spin-allowed Fe2+ absorption bands in the near-infrared region are determined. The M(2) Fe2+ band at ~5,000 cm?1 in β is the analytically most useful for site occupancy determinations. It remains linear with concentration (?=9.65) over the entire compositional range. The band at ~10,500 cm?1 in α is the most sensitive to M(2) Fe2+ concentration (?=40.8), but deviates from linearity at high iron concentrations. The origins of spin-forbidden transitions in the visible region are examined. 相似文献
980.
Joseph J. Morley 《Quaternary Research》1979,12(3):381-395
A quantitative analysis of radiolarian species in 57 deep-sea surface sediment samples from the South Atlantic Ocean produced four geographically distinct assemblages (tropical, polar, gyre margin, and subtropical). The distributions of these assemblages or factors coincide with present-day patterns of sea-surface temperatures or water masses.These four assemblages were used to construct a transfer function relating radiolarian distribution in the surface sediments to present-day winter and summer temperatures using standard regression techniques. As a test of the quality of this transfer function, temperatures were estimated on surface sediment samples from the eastern South Pacific. The temperatures produced by the transfer function compared favorably with the observed (present-day) winter and summer sea-surface temperatures at these sites. 相似文献