全文获取类型
收费全文 | 26403篇 |
免费 | 427篇 |
国内免费 | 277篇 |
专业分类
测绘学 | 665篇 |
大气科学 | 2035篇 |
地球物理 | 5660篇 |
地质学 | 9240篇 |
海洋学 | 2113篇 |
天文学 | 5584篇 |
综合类 | 39篇 |
自然地理 | 1771篇 |
出版年
2020年 | 158篇 |
2019年 | 154篇 |
2018年 | 331篇 |
2017年 | 302篇 |
2016年 | 455篇 |
2015年 | 334篇 |
2014年 | 458篇 |
2013年 | 1252篇 |
2012年 | 559篇 |
2011年 | 841篇 |
2010年 | 686篇 |
2009年 | 971篇 |
2008年 | 894篇 |
2007年 | 853篇 |
2006年 | 879篇 |
2005年 | 748篇 |
2004年 | 795篇 |
2003年 | 762篇 |
2002年 | 746篇 |
2001年 | 608篇 |
2000年 | 626篇 |
1999年 | 585篇 |
1998年 | 560篇 |
1997年 | 575篇 |
1996年 | 474篇 |
1995年 | 472篇 |
1994年 | 451篇 |
1993年 | 425篇 |
1992年 | 391篇 |
1991年 | 332篇 |
1990年 | 391篇 |
1989年 | 308篇 |
1988年 | 347篇 |
1987年 | 384篇 |
1986年 | 328篇 |
1985年 | 487篇 |
1984年 | 531篇 |
1983年 | 541篇 |
1982年 | 430篇 |
1981年 | 423篇 |
1980年 | 443篇 |
1979年 | 391篇 |
1978年 | 406篇 |
1977年 | 348篇 |
1976年 | 386篇 |
1975年 | 347篇 |
1974年 | 383篇 |
1973年 | 368篇 |
1972年 | 238篇 |
1971年 | 189篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
181.
The Shamrocke ore body is a stratiform deposit of disseminated copper-iron sulphides found within lenses of calcareous meta-arkose occurring in Lomagundi Group graphitic schist (Late Precambrian) at Karoi, Rhodesia. Both sulphides and sediments were subjected to high grades of regional metamorphism. Argon/argon isochron ages indicate a major metamorphic event at 550 m. y. (Damaran orogeny) with later minor argon losses. Ore sulphide 34S values range from +3.0 to +14.8 CDT and a general decrease from footwall to hanging-wall reflects an original sedimentary environment where sulphides formed about the sea bed from hydrothermal fluids progressively mixing with sea water. Isotopically lighter sulphides formed syngenetically in the host rocks from bacterial reduction of sulphate. The pyrrhotite was probably formed from pyrite during metamorphism, and owing to reducing conditions maintained similar 34S values to the original pyrite. Oxygen and carbon isotopic analyses of mineralised and unmineralised carbonate lenses are consistent with deposition as marine limestones in an evaporitic environment and/or near hot spring vents.I. N. S. Contribution No 734 相似文献
182.
Kenneth D. Adams Ted Goebel Kelly Graf Geoffrey M. Smith Anna J. Camp Richard W. Briggs David Rhode 《Geoarchaeology》2008,23(5):608-643
The Great Basin of the western U.S. contains a rich record of Late Pleistocene and Holocene lake‐level fluctuations as well as an extensive record of human occupation during the same time frame. We compare spatial‐temporal relationships between these records in the Lahontan basin to consider whether lake‐level fluctuations across the Pleistocene‐Holocene transition controlled distribution of archaeological sites. We use the reasonably well‐dated archaeological record from caves and rockshelters as well as results from new pedestrian surveys to investigate this problem. Although lake levels probably reached maximum elevations of about 1230–1235 m in the different subbasins of Lahontan during the Younger Dryas (YD) period, the duration that the lakes occupied the highest levels was brief. Paleoindian and Early Archaic archaeological sites are concentrated on somewhat lower and slightly younger shorelines (_1220–1225 m) that also date from the Younger Dryas period. This study suggests that Paleoindians often concentrated their activities adjacent to large lakes and wetland resources soon after they first entered the Great Basin. © 2008 Wiley Periodicals, Inc. 相似文献
183.
184.
Cordieritites and highly peraluminous granites within the ElPilón granite complex, Sierras Pampeanas, Argentina,were emplaced during a medium-P, high-T metamorphic event duringthe initial decompression of a Cambrian orogen along the southwesternmargin of Gondwana. Very fresh orbicular and massive cordierititebodies with up to 90% cordieritite are genetically associatedwith a cordierite monzogranite pluton and a larger body of porphyriticgranodiorite. The petrogenesis of this association has beenstudied using petrographical, mineralogical, thermobarometric,geochemical, geochronological and isotope methods. The graniticmagmas were formed by anatexis of mid-crustal metamorphic rocksformed earlier in the Pampean orogeny. The cordieritites appearat the top of feeder conduits that connected the source regionlocated at 相似文献
185.
J. M. Charnock C. M. B. Henderson J. F. W. Mosselmans R. A. D. Pattrick 《Physics and Chemistry of Minerals》1996,23(7):403-408
The X-ray absorption spectra of the transition metal L3,2-edges of the dichalcogenides FeS2 (pyrite), FeSe2, FeTe2, CoS2, CoSe2, CoTe2, NiS2, NiSe2 and NiTe2 have been studied. These spectra are compared with theoretical simulations make using the atomic multiplet approach assuming on octahedral site for the metal atoms. The Ni spectra can be closely simulated using this approach but the Fe and Co spectra show less good fits to the experimental data. The reasons for this and its implications for the use of L-edge spectra in the study of such minerals are discussed in terms of the covalency and the ground states of the compounds; these results indicate the mixing of the ground states increases in the order Ni<Co<Fe. 相似文献
186.
R. W. John 《Astronomische Nachrichten》1986,307(5):303-305
In the vacuum case, Einstein's equations generalized by additive terms containing derivatives of the metric up to the 4th order are applied to cosmological Bianchi-type I model space-times. 相似文献
187.
On 11 September 1973 a peculiar prominence was observed. The prominence displayed strong ( 50km s–1) systematic motions toward and away from the observer. The unusual spectrographic appearance of the prominence might have been due to downflowing material lifted into the corona during an earlier coronal transient.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 相似文献
188.
The seasonal variation of the nocturnal average intensity, and the typical variation of the sodium airglow intensity during the night have been deduced, using a data set that has been accumulated over a period of 5 years. The maximum intensity, which occurs at the equinoxes, is about 2–3 times as large as the intensity during winter, when the intensity of the D2 line is about 30 R. The nocturnal variation is symmetrical about midnight, where the intensity is some 30% less than just after (before) dusk (dawn) values. 相似文献
189.
W. Köhnlein 《Planetary and Space Science》1981,29(1):47-56
The electron temperature variations are investigated above Arecibo, Jicamarca, Millstone Hill, St. Santin and a polar area—located at the meridian of Millstone Hill. The data analyzed represent quiet geomagnetic conditions (Kp ≤ 3) during a solar maximum (1967–1970). Between 200 and 600 km the electron temperature data stem from incoherent scatter measurements and above 600 km from the ISIS-1 observations. A simple analytical model which includes Fourier terms and cubic splines (for approximating the height dependence of the coefficients) describes the diurnal and seasonal pattern of the electron temperature in the altitude interval 200–3500 km. Three height regions are particularly striking, i.e. near 200 km where the diurnal variations show a sinusoidal pattern, the altitude interval up to approximately 1000 km which exhibits strong temperature gradients and a complex diurnal and seasonal structure, and the upper region beyond 1000 km which reflects again sinusoidal pattern but with a very pronounced latitudinal dependence. 相似文献
190.
Falconer David A. Jordan Stuart D. Brosius Jeffrey W. Davila Joseph M. Thomas Roger J. Andreatta Vicenzo Hara Hirohisa 《Solar physics》1998,180(1-2):179-191
We investigate the possibility that strong EUV lines observed with the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS) provide good proxies for estimating the total coronal flux over shorter wavelength ranges. We use coordinated SERTS and Yohkoh observations to obtain both polynomial and power-law fits relating the broad-band soft X-ray fluxes to the intensities of Fexvi 335 Ú and 361 Ú, Fexv 284 Ú and 417 Ú, and Mgix 368 Ú measured with SERTS. We found that the power-law fits best cover the full range of solar conditions from quiet Sun through active region, though not surprisingly the cooler Mgix 368 Ú line proves to be a poor proxy. The quadratic polynomial fits yield fair agreement over a large range for all but the Mgix line. However, the linear fits fail conspicuously when extrapolated into the quiet-Sun regime. The implications of this work for the Heii 304 Ú line formation problem are also briefly considered. 相似文献