全文获取类型
收费全文 | 37275篇 |
免费 | 1106篇 |
国内免费 | 1050篇 |
专业分类
测绘学 | 961篇 |
大气科学 | 2875篇 |
地球物理 | 7662篇 |
地质学 | 13785篇 |
海洋学 | 3342篇 |
天文学 | 8156篇 |
综合类 | 240篇 |
自然地理 | 2410篇 |
出版年
2022年 | 292篇 |
2021年 | 477篇 |
2020年 | 459篇 |
2019年 | 497篇 |
2018年 | 913篇 |
2017年 | 876篇 |
2016年 | 1050篇 |
2015年 | 723篇 |
2014年 | 1049篇 |
2013年 | 1869篇 |
2012年 | 1347篇 |
2011年 | 1788篇 |
2010年 | 1564篇 |
2009年 | 2020篇 |
2008年 | 1699篇 |
2007年 | 1768篇 |
2006年 | 1699篇 |
2005年 | 1221篇 |
2004年 | 1139篇 |
2003年 | 1037篇 |
2002年 | 1005篇 |
2001年 | 845篇 |
2000年 | 825篇 |
1999年 | 673篇 |
1998年 | 716篇 |
1997年 | 693篇 |
1996年 | 573篇 |
1995年 | 565篇 |
1994年 | 480篇 |
1993年 | 422篇 |
1992年 | 419篇 |
1991年 | 386篇 |
1990年 | 457篇 |
1989年 | 373篇 |
1988年 | 356篇 |
1987年 | 438篇 |
1986年 | 346篇 |
1985年 | 430篇 |
1984年 | 531篇 |
1983年 | 451篇 |
1982年 | 452篇 |
1981年 | 403篇 |
1980年 | 419篇 |
1979年 | 360篇 |
1978年 | 345篇 |
1977年 | 340篇 |
1976年 | 309篇 |
1975年 | 296篇 |
1974年 | 312篇 |
1973年 | 340篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
John L. Largier 《Estuaries and Coasts》1992,15(1):26-39
A tidal intrusion front forms as a dense seawater inflow plunges (subducts) beneath ambient estuarine water during flood tide. The associated foam lines and color changes have been observed on many smaller estuaries with constricted mouths. Internal hydraulic theory and laboratory experiments are reviewed and expressions are obtained for the position of plunging and the amount of associated mixing. The existence of a tidal intrusion front and its structure are discussed in terms of densimetric Froude numbers. These fronts are particularly important in smaller estuaries in which the intrusion process may dominate wind and tidal mixing and thus determine the overall stratification of the estuary. Three classes of three-dimensional plunging flow are identified and discussed. In particular, it is suggested that the peculiar, cursive V-shape plunge line is characteristic of strongly plunging flow. 相似文献
993.
Probing the oxidation-reduction properties of terrestrially and microbially derived dissolved organic matter 总被引:5,自引:0,他引:5
Dissolved organic matter (DOM) has been shown to be an integral component in biogeochemical electron transfer reactions due to its demonstrated ability to facilitate redox reactions. While the role of DOM as a facilitator of electron transfer processes has been demonstrated, greater knowledge would lead to better understanding of the structural components responsible for redox behavior, such as quinones and nitrogen and sulfur (N/S) functional groups. This investigation uses direct scan voltammetry (DSV) coupled with fluorescence and NMR spectroscopy as well as thermochemolysis gas chromatography mass spectrometry (GC-MS) and X-ray photoelectron spectroscopy (XPS) to elucidate the organic moieties responsible for facilitating electron transfer reactions. We contrast electrochemical properties and structural details of three organic matter isolates from diverse sources; Great Dismal Swamp DOM (terrestrially derived, highly aromatic), Pony Lake DOM (microbially derived, highly aliphatic) and Toolik Lake (terrestrially derived, photochemically and microbially altered) with juglone (a redox-active model quinone). Aromatic and phenolic constituents were detected (by 13C NMR) and recovered (by thermochemolysis GC-MS) from all three fulvic acid samples, highlighting the ubiquity of these compounds and suggesting that the quinone-phenol redox couple is not limited to DOM derived from lignin precursors. The range of hydroxy-benzene and benzoic acid derivatives may explain the lack of a single pair of well-defined oxidation and reduction peaks in the DSV scans. The presence of a wide-range of hydroxylated benzoic acid isomers and other redox-active aromatic residues implies that native DOM possesses overlapping redox potentials analogous to their characteristic range of pKa values. 相似文献
994.
Epithermal deposits in North Xinjiang, NW China 总被引:23,自引:0,他引:23
Y. J. Chen F. Pirajno G. Wu J. P. Qi X. L. Xiong 《International Journal of Earth Sciences》2012,101(4):889-917
The North Xinjiang region (NW China) is an important part of the Central Asia Orogenic Belt, situated at the junction of Siberia, Tarim and Kazakhstan plates. It is an area characterized by multiple stages of Phanerozoic continental growth, during which several porphyry and epithermal systems were formed. The relationship of these mineral systems to the geodynamic evolution of the region has not yet been well understood. In this paper, we list the main geological characteristics of 21 significant epithermal precious and base metal deposits in North Xinjiang, and classify them into high-sulfidation and low-sulfidation styles, with the latter being predominant. We have selected seven epithermal deposits representing different styles formed under different tectonic regimes and discuss their geology and geochemistry in some detail. The deposit-scale geology and geochemistry of epithermal systems in North Xinjiang are essentially similar to those in other parts of the world. All epithermal deposits in North Xinjiang are hosted in volcanic rocks with ages ranging from Devonian to Triassic, with the Early Carboniferous volcanic sequences being the most important, followed by the Permian and Triassic. The Devonian–Early Carboniferous host rocks belong to the calc-alkaline series that developed in pre-collisional arc-back-arc basin systems; whereas the Permian–Triassic host volcanic rocks of shoshonite series formed in post-subduction regimes. Available isotopic ages of these epithermal systems cluster in two periods: Early Carboniferous (>320?Ma) and Late Carboniferous–Triassic (320–220?Ma), reflecting two metallogenic episodes that occurred during subduction-related accretion and post-subduction collision regimes, respectively. Accordingly, three groups of epithermal deposits in North Xinjiang can be recognized as (1) pre-collisional deposits without or with negligible collisional-related modification, (2) deposits formed in collision regime and (3) ore systems strongly overprinted by fluid flow in post-subduction collision regime. 相似文献
995.
996.
中温UBF与UASB两相厌氧系统处理垃圾渗滤液的实验研究 总被引:1,自引:0,他引:1
针对石碑岭垃圾场渗滤液水质特性,采用以高效生物陶粒为填料的UBF反应器作酸化相,以UASB反应器作甲烷相的两相厌氧系统进行了处理垃圾渗滤液的实验研究.实验结果表明:在系统进水CODCr和BOD5质量浓度分别为3 887 mg/L和819 mg/L,UBF与UASB的HRT分别控制在10.3 h和61.7 h时,CODCr和BOD5总去除率分别为85.4%和90.1%,UBF酸化率达42.9%,BOD5/CODCr比值由0.21提高到0.39. 相似文献
997.
Caledonian high-pressure metamorphism in the Strona-Ceneri Zone (Southern Alps of southern Switzerland and northern Italy) 总被引:3,自引:0,他引:3
The Strona-Ceneri Zone comprises a succession of polymetamorphic, pre-Alpidic basement rocks including ortho- and paragneisses,
metasedimentary schists, amphibolites, and eclogites. The rock pile represents a Late Proterozoic or Palaeozoic subduction
accretion complex that was intruded by Ordovician granitoids. Eclogites, which occur as lenses within the ortho-paragneiss
succession and as xenoliths within the granitoids record a subduction related high-pressure event (D1) with peak metamorphic
conditions of 710 ± 30 °C at 21.0 ± 2.5 kbar. After isothermal uplift, the eclogites experienced a Barrowtype (D2) tectonometamorphic
overprint under amphibolite facies conditions (570-630 °C, 7-9 kbar). U-Pb dating on zircon of the eclogites gives a metamorphic
age of 457 ± 5 Ma, and syn-eclogite facies rutile gives a 206Pb/238U age of 443 ± 19 Ma classifying the subduction as a Caledonian event. These data show that the main tectonometamorphic evolution
of the Strona-Ceneri Zone most probably took place in a convergent margin scenario, in which accretion, eclogitization of
MOR-basalt, polyphase (D1 and D2) deformation, anatexis and magmatism all occurred during the Ordovician. Caledonian high-pressure
metamorphism, subsequent magmatism and Barrow-type metamorphism are believed to be related to subduction and collision within
the northern margin of Gondwana.
Editorial handling: Edwin Gnos 相似文献
998.
S. G. SONG L. F. ZHANG Y. NIU C. J. WEI J. G. LIOU G. M. SHU 《Journal of Metamorphic Geology》2007,25(5):547-563
Low‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km?1), and this cold subduction brought a large amount of H2O to the deep mantle in the Early Palaeozoic times. 相似文献
999.
Randall L. GresensHoward L. Stensrud 《Lithos》1971,4(1):63-69
A specimen of intergrown biotite and muscovite from a pegmatite was subjected to detailed chemical, optical, and X-ray analysis. The crystals are intergrown in such a way that certain structural elements are common to both and that parts of the crystal structure must pass unbroken through the contact between them. Numerous ‘edge dislocations’ must occur at the boundary because of the different basal spacings. Partitioning of elements between the minerals, along with the structural observations, suggest that the crystals are an equilibrium intergrowth. 相似文献
1000.
Abundant iron oxide deposits including banded iron formations, apatite iron oxide ores, and enigmatic marble/skarn-hosted magnetite deposits occur in the Palaeoproterozoic Bergslagen region, southern Sweden. During the last 100 years, the latter deposit class has been interpreted as contact metasomatic skarn deposits, metamorphosed iron formations, or metamorphosed carbonate replacement deposits. Their origin is still incompletely understood. At the Smältarmossen mine, magnetite was mined from a ca. 50-m-thick calcic skarn zone at the contact between rhyolite and stratigraphically overlying limestone. A syn-volcanic dacite porphyry which intruded the footwall has numerous apophyses that extend into the mineralized zone. Whole-rock lithogeochemical and mineral chemical analyses combined with textural analysis suggests that the skarns formed by veining and replacement of the dacite porphyry and rhyolite. These rocks were added substantial Ca and Fe, minor Mg, Mn, and LREE, as well as trace Co, Sn, U, As, and Sr. In contrast, massive magnetite formed by pervasive replacement of limestone. Tectonic fabrics in magnetite and skarn are consistent with ore formation before or early during Svecokarelian ductile deformation. Whereas a syngenetic–exhalative model has previously been suggested, our results are more compatible with magnetite formation at ca. 1.89 Ga in a contact metasomatic skarn setting associated with the dacite porphyry. 相似文献