全文获取类型
| 收费全文 | 3727篇 |
| 免费 | 544篇 |
| 国内免费 | 623篇 |
专业分类
| 测绘学 | 679篇 |
| 大气科学 | 310篇 |
| 地球物理 | 584篇 |
| 地质学 | 1471篇 |
| 海洋学 | 640篇 |
| 天文学 | 620篇 |
| 综合类 | 297篇 |
| 自然地理 | 293篇 |
出版年
| 2024年 | 13篇 |
| 2023年 | 39篇 |
| 2022年 | 129篇 |
| 2021年 | 163篇 |
| 2020年 | 156篇 |
| 2019年 | 205篇 |
| 2018年 | 132篇 |
| 2017年 | 164篇 |
| 2016年 | 169篇 |
| 2015年 | 185篇 |
| 2014年 | 219篇 |
| 2013年 | 238篇 |
| 2012年 | 210篇 |
| 2011年 | 239篇 |
| 2010年 | 170篇 |
| 2009年 | 230篇 |
| 2008年 | 210篇 |
| 2007年 | 241篇 |
| 2006年 | 177篇 |
| 2005年 | 212篇 |
| 2004年 | 204篇 |
| 2003年 | 168篇 |
| 2002年 | 150篇 |
| 2001年 | 112篇 |
| 2000年 | 122篇 |
| 1999年 | 111篇 |
| 1998年 | 86篇 |
| 1997年 | 87篇 |
| 1996年 | 54篇 |
| 1995年 | 63篇 |
| 1994年 | 44篇 |
| 1993年 | 36篇 |
| 1992年 | 30篇 |
| 1991年 | 20篇 |
| 1990年 | 21篇 |
| 1989年 | 21篇 |
| 1988年 | 10篇 |
| 1987年 | 4篇 |
| 1986年 | 4篇 |
| 1985年 | 4篇 |
| 1984年 | 5篇 |
| 1983年 | 6篇 |
| 1982年 | 4篇 |
| 1981年 | 4篇 |
| 1978年 | 2篇 |
| 1976年 | 3篇 |
| 1954年 | 2篇 |
| 1905年 | 3篇 |
| 1880年 | 2篇 |
| 1877年 | 3篇 |
排序方式： 共有4894条查询结果，搜索用时 15 毫秒
911.
Early Miocene (ca.?21–18 Ma) volcanism in the Karacada? area comprises three groups of volcanic rocks: (1) calcalkaline suite (andesitic to rhyolitic lavas and their pyroclastics), (2) mildly-alkaline suite (alkali basalt, hawaiite, mugearite, benmoreite and trachydacite), and (3) a single trachyandesitic flow unit. Field observations, 40Ar/39Ar ages and geochemical data show that there was a progressive temporal transition from group 1 to 3 in a post-collisional tectonic setting. The calcalkaline suite rocks with medium-K in composition resemble those of subduction-related lavas, whereas the mildly-alkaline suite rocks having a sodic tendency (Na2O/K2O=1.5–3.2) resemble those of within-plate lavas. Incompatible element and Sr-Nd isotopic characteristics of the suites suggest that the lithospheric mantle beneath the Karacada? area was heterogeneously enriched by two processes before collision: (1) enrichment by subduction-related processes, which is important in the genesis of the calcalkaline volcanism, (2) enrichment by small degree melts from the astenosphere, which dominates the mildly alkaline volcanism. Perturbation of the enriched lithosphere by either delamination following collision and uplift or removal of the subducted slab following subduction and collision (i.e., slab breakoff) is the likely mechanism for the initiation of the post-collision volcanism. 相似文献
912.
SUN Xiaoming 《《地质学报》英文版》2011,85(3):656-660
In the present study, the modified Sverjensky–Molling equation, derived from a linear-free energy relationship, is used to predict the Gibbs free energies of formation of crystalline phases of α-MOOH (with a goethite structure) and α-M2O3 (with a hematite structure) from the known thermodynamic properties of the corresponding aqueous trivalent cations (M3+). The modified equation is expressed as ΔG0f,MVX=aMVXΔG0n,M3++bMVX+βMVXγM3+, where the coefficients aMVX, bMVX, and βMVX characterize a particular structural family of MvX (M is a trivalent cation [M3+] and X represents the remainder of the composition of solid); γ3+ is the ionic radius of trivalent cations (M3+); ΔG0f,MVX is the standard Gibbs free energy of formation of MvX; and ΔG0n,M3+ is the non-solvation energy of trivalent cations (M3+). By fitting the equation to the known experimental thermodynamic data, the coefficients for the goethite family (α-MOOH) are aMVX=0.8838, bMVX=?424.4431 (kcal/mol), and βMVX=115 (kcal/mol.?), while the coefficients for the hematite family (α-M2O3) are aMVX=1.7468, bMVX=?814.9573 (kcal/mol), and βMVX=278 (kcal/mol.?). The constrained relationship can be used to predict the standard Gibbs free energies of formation of crystalline phases and fictive phases (i.e. phases that are thermodynamically unstable and do not occur at standard conditions) within the isostructural families of goethite (α-MOOH) and hematite (α-M2O3) if the standard Gibbs free energies of formation of the trivalent cations are known. 相似文献
913.
Plant macrofossils from 38 packrat middens spanning the last ~ 33,000 cal yr BP record vegetation between ~ 650 and 900 m elevation along the eastern escarpment of the Sierra San Pedro Mártir, northern Baja California. The middens span most of the Holocene, with a gap between ~ 4600 and 1800 cal yr BP, but coverage in the Pleistocene is uneven with a larger hiatus between 23,100 and 14,400 cal yr BP. The midden flora is relatively stable from the Pleistocene to Holocene. Exceptions include Pinus californiarum, Juniperus californica and other chaparral elements that were most abundant > 23,100 cal yr BP and declined after 14,400 cal yr BP. Despite being near the chaparral/woodland-desertscrub ecotone during glacial times, the midden assemblages reflect none of the climatic reversals evident in the glacial or marine record, and this is corroborated by a nearby semi-continuous pollen stratigraphy from lake sediments. Regular appearance of C4 grasses and summer-flowering annuals since 13,600 cal yr BP indicates occurrence of summer rainfall equivalent to modern (JAS average of ~ 80–90 mm). This casts doubt on the claim, based on temperature proxies from marine sediments in the Guaymas Basin, that monsoonal development in the northern Gulf and Arizona was delayed until after 6200 cal yr BP. 相似文献
914.
915.
916.
917.
918.
ZOU Yongliao ZHANG Liyan LIU Jianzhong MU Lingli REN Xin ZHANG Guangliang CHANG Jin YAN Jun ZHANG Nan ZHANG Hongbo Lü Chang LIU Jianjun ZUO Wei SU Yan WEN Weibin BIAN Wei WANG Min XU Chun LI Chunlai OUYANG Ziyuan 《《地质学报》英文版》2011,85(6):1299-1309
Gamma-ray spectrometer (GRS) is one of the main payloads on the Chang’E-1 (CE-1) lunar probe, mainly aimed to detect the elemental abundances and distributions on the lunar surface. At 03:58 on 28 November 2007, it performed the first observation of the lunar gamma rays. As of 24 October 2008, 2105?h of effective gamma rays spectra had been acquired by CE-1 GRS, which covers the whole surface of the moon. This paper mainly describes the data processing procedures and methods of deriving the elemental abundances by using the CE-1 GRS time series corrected spectra: first, to bin data into pixels for mapping; then, to perform a background deduction of the cumulative spectra and obtain a peak area of the elements; and finally, to use the elemental abundances inversion model to produce the elemental abundances. Based on these processing methods, the global abundance maps of U, K, and Th at a 5°×5° equal-area pixel are acquired by CE-1 GRS data. The paper gives a preliminary analysis of the uncertainties of the elemental abundances 相似文献
919.
920.
Samples were continuously collected from aerosol, fresh snow, and snow pits on Glacier No. 1 at Urumqi River source in eastern Tianshan (天山) Mountains. The deposition processes and the characteristics of mineral dust microparticles from aerosol to fresh snow, and then evolution to the snow pit were determined. Total dust microparticle concentration in the surface snow and aerosol showed a similar temporal variation trend, which was strongly associated with regional and local at-mospheric circulation in the ... 相似文献