全文获取类型
收费全文 | 29232篇 |
免费 | 464篇 |
国内免费 | 319篇 |
专业分类
测绘学 | 616篇 |
大气科学 | 1817篇 |
地球物理 | 5233篇 |
地质学 | 11262篇 |
海洋学 | 2963篇 |
天文学 | 6858篇 |
综合类 | 68篇 |
自然地理 | 1198篇 |
出版年
2022年 | 297篇 |
2021年 | 455篇 |
2020年 | 509篇 |
2019年 | 556篇 |
2018年 | 1091篇 |
2017年 | 1040篇 |
2016年 | 1146篇 |
2015年 | 545篇 |
2014年 | 1041篇 |
2013年 | 1686篇 |
2012年 | 1152篇 |
2011年 | 1433篇 |
2010年 | 1282篇 |
2009年 | 1557篇 |
2008年 | 1329篇 |
2007年 | 1395篇 |
2006年 | 1301篇 |
2005年 | 755篇 |
2004年 | 705篇 |
2003年 | 659篇 |
2002年 | 685篇 |
2001年 | 619篇 |
2000年 | 567篇 |
1999年 | 460篇 |
1998年 | 478篇 |
1997年 | 453篇 |
1996年 | 390篇 |
1995年 | 349篇 |
1994年 | 378篇 |
1993年 | 294篇 |
1992年 | 294篇 |
1991年 | 286篇 |
1990年 | 322篇 |
1989年 | 220篇 |
1988年 | 225篇 |
1987年 | 268篇 |
1986年 | 209篇 |
1985年 | 314篇 |
1984年 | 276篇 |
1983年 | 258篇 |
1982年 | 275篇 |
1981年 | 203篇 |
1980年 | 243篇 |
1979年 | 195篇 |
1978年 | 211篇 |
1977年 | 165篇 |
1976年 | 163篇 |
1975年 | 172篇 |
1974年 | 167篇 |
1973年 | 166篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
881.
Assessing scleractinian corals as recorders for paleo-pH: Empirical calibration and vital effects 总被引:2,自引:0,他引:2
B. Hönisch N.G. Hemming A.G. Grottoli G.N. Hanson 《Geochimica et cosmochimica acta》2004,68(18):3675-3685
Laboratory experiments on the branching, symbiont-bearing coral genus Porites and Acropora have been carried out to determine the dependence of the skeletal boron isotopic composition (δ11B) on the pH of seawater. The results show a clear relationship similar to previously established empirical calibrations for planktonic foraminifera and inorganic calcite. A −0.6‰ offset exists between P. cylindrica and A. nobilis which is systematic over the pH range of 7.7-8.2. To test whether the δ11B of coral skeletons changes with physiological processes such as photosynthesis and respiration, corals were grown along a depth transect in their natural environment and under controlled conditions in the laboratory at varying light intensities and food supply. Although we also observe an isotopic offset between P. compressa and Montipora verrucosa, neither experimental treatment systematically changed the δ11B of the two species. These findings are encouraging for using the boron isotope paleo-pH proxy in corals, because it appears that seawater pH is the dominant control on the boron isotopic composition in corals. 相似文献
882.
A. B. Pushkarev Yu. Yu. Kovalev I. E. Molotov M. B. Nechaeva Yu. N. Gorshenkov G. Tuccari C. Stanghellini X. Hong J. Quick S. Dougherty X. Liu 《Astronomy Reports》2004,48(11):900-908
VLBI observations of several quasars and BL Lacertae objects were carried out at 1.66 GHz in November–December 1999 using six antennas (Medvezh'i Ozera, Svetloe, Pushchino, Noto, HartRAO, and Shanghai). Maps of six sources (0420+022, 0420-014, 1308+326, 1345+125, 1803+784, and DA 193) obtained with milliarcsecond resolution are presented and discussed, together with their broad-band (1–22 GHz) spectra obtained on the RATAN-600 radio telescope at epochs close to those of the VLBI observations. Comparison of the VLBI maps with maps of these sources obtained on standard VLBI networks and with the RATAN-600 quasisimultaneous total-flux measurements indicates the reliability of the results obtained on this Low Frequency VLBI Network and the good efficiency of this network. 相似文献
883.
A comparative analysis of oscillatory spectra based on 66 time series for 14 active regions observed in 2001 shows that, although the chromospheric and photospheric oscillations in the Evershed flow zone possess many common features, there is no firm evidence that the direct and inverse flows have the same physical origin. The interactions between the various oscillation modes and stationary flows results in a complex pattern of wave motions in a sunspot. We studied the Doppler-velocity variations in the sunspot NOAA 0051 during its motion over the disk. The spatial-temporal distribution of the line-of-sight velocity in the chromospheric umbra displays a chevron structure, clearly indicating the presence of propagating waves. These waves move from the center of the umbra to outer regions with a phase speed of 45–60 km/s, a period of 2.8 min, and a measured Doppler speed of 2 km/s. The amplitude of these oscillations decreases abruptly at the boundary between the umbra and penumbra, and the observed waves are not directly related to propagating penumbral waves. Furthermore, the observed pattern of the photospheric velocities shows periodic motions (with a period of 5 min) directed from the inner boundary of the penumbra and superpenumbra toward the line of maximum Evershed velocity. 相似文献
884.
V. S. Bychkova N. S. Kardashev V. V. Vlasyuk O. I. Spiridonova 《Astronomy Reports》2004,48(10):840-852
We monitored five active galactic nuclei in the R optical band with a CCD mounted on the 1-m Zeiss-1000 telescope of the Special Astrophysical Observatory from April 2001 to August 2003. Three sources displayed intraday variability on several nights. Stronger variability was detected on time scales from two days to a week. The two-year light curves are presented for four of the observed objects. 相似文献
885.
The paper presents the first quantitative results of a laboratory study of the velocity field in a two-arm spiral-wave pattern generated in a steady-state fashion by a hydrodynamical instability in a differentially rotating, thin layer of liquid. The liquid layer has a free surface, and the rotational profile includes an interval where the velocity drops abruptly, as in the gaseous disks of spiral galaxies. The properties of anticyclonic vortices observed between the arms of this pattern at the corotation radius are considered. 相似文献
886.
A possible mechanism for the formation of near-polar magnetic spots on some stars with convective envelopes is proposed. The mechanism is based on the idea that the maximum of the dynamo waves that are excited in thin convective shells by the dynamo mechanism is shifted appreciably from the maximum of the magnetic-field sources in the direction of motion of the dynamo wave. If there is no region of super-rotation near the equator for some reason (as a consequence of disruption due to tidal interaction with a companion in a binary system, for example) and the wave of stellar activity propagates toward the poles rather than toward the equator, this maximum will be in the near-polar regions. 相似文献
887.
CLIFFORD TOM N.; BARTON ERIKA S.; STERN RICHARD A.; DUCHESNE JEAN-CLAIR 《Journal of Petrology》2004,45(4):669-691
The O'okiep Copper District is underlain by voluminous 10351210Ma granite gneiss and granite with remnants of metamorphosedsupracrustal rocks. This assemblage was intruded by the 1030Ma copper-bearing Koperberg Suite that includes jotunite, anorthosite,biotite diorite and hypersthene-bearing rocks ranging from leuconoriteto hypersthenite. New sensitive high-resolution ion microprobeage data demonstrate the presence of 17002000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published SmNd model ages ofc. 1700 Ma (TCHUR) and c. 2000 Ma (TDM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, UThPb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (11801210Ma), represented by regional granite plutonism, notably theNababeep and Modderfontein Granite Gneisses and the Concordiaand Kweekfontein Granites that accompanied and outlasted (e.g.Kweekfontein Granite) regional tectonism [F2(D2)] and granulite-faciesmetamorphism (M2); (2) the Klondikean Episode (10201040Ma), which includes the intrusion of the porphyritic RietbergGranite and of the Koperberg Suite that are devoid of regionalplanar or linear fabrics. Klondikean tectonism (D3) is reflectedby major eastwest-trending open folds [F3(D3a)], andby localized eastwest-trending near-vertical ductilefolds [steep structures; F4(D3b)] whose formationwas broadly coeval with the intrusion of the Koperberg Suite.A regional, largely thermal, amphibolite- to granulite-faciesmetamorphism (M3) accompanied D3. This study demonstrates, interalia, that the complete spectrum of rock-types of the KoperbergSuite, together with the Rietberg Granite, was intruded in ashort time-interval (<10 Myr) at c. 1030 Ma, and that therewere lengthy periods of about 150 Myr of tectonic quiescencewithin the Namaquan Orogeny: (1) between the O'okiepian andKlondikean Episodes; (2) from the end of the latter to the formalend of Namaquan Orogenesis 800850 Ma ago. KEY WORDS: UPb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny 相似文献
888.
KADIK A.; PINEAU F.; LITVIN Y.; JENDRZEJEWSKI N.; MARTINEZ I.; JAVOY M. 《Journal of Petrology》2004,45(7):1297-1310
Studies of iron-bearing silicate melt (ferrobasalt) + iron metallicphase + graphite + hydrogen equilibria show that carbon andhydrogen solubilities in melts are important for the evolutionof the upper mantle. In a series of experiments conducted at3·7 GPa and 15201600°C, we have characterizedthe nature (oxidized vs reduced) and quantified the abundancesof C- and H-compounds dissolved in iron-bearing silicate melts.Experiments were carried out in an anvil-with-hole apparatuspermitting the achievement of equal chemical potentials of H2in the inner Pt capsule and outer furnace assembly. The fO2for silicate meltiron equilibrium was 2·32 ±0·04 log units below ironwüstite (IW). Theferrobasalt used as starting material experienced a reductionof its iron oxides and silicate network. The counterpart wasa liberation of oxygen reacting with the hydrogen entering thecapsule. The amount of H2O dissolved in the glasses was measuredby ion microprobe and by step-heating and was found to be between1 and 2 wt %. The dissolved carbon content was found to be 1600ppm C by step-heating. The speciation of C and H componentswas determined by IR and Raman spectroscopy. It was establishedthat the main part of the liberated oxygen was used to formOH and to a much lesser extent H2O, and only traces ofH2, CO2 and 相似文献
889.
Patyk-Kara N. G. Gorelikova N. V. Bardeeva E. G. 《Lithology and Mineral Resources》2004,39(6):509-523
Based on the study of the Tsentral'noe deposit, specific features of the formation of mineral assemblages of complex titanium–zirconium placers are considered. The placers formed during the multiple redeposition of clastogenic minerals from source rocks and younger sedimentary rocks (intermediate collectors of titanium–zirconium minerals). The location of erosion and sedimentation zones significantly varied in the Phanerozoic in the adjacent region, resulting in the development of intricate relationships between different-aged terrigenous rocks (possible intermediate collectors) that provided the formation of new mineral assemblages of clastogenic ore minerals. In addition, erosional processes during the continental evolution of the study region could promote the exposure of more ancient rock complexes, the local washout of crystalline basement rocks, and the delivery of ore minerals from the latter rocks to the coastal zone of sedimentary basins. The aim of this communication is to attract the attention of researchers to the issue of the formation of mineral assemblages of complex placers of heavy minerals with similar hydraulic grain dimension and migration capacity for concentration in a rather narrow grain size range. Such mineral assemblages only slightly inherit the primary compositional features of provenances and primarily reflect changes in the sedimentation environment. 相似文献
890.
Lithology and Mineral Resources - 相似文献