全文获取类型
收费全文 | 75953篇 |
免费 | 828篇 |
国内免费 | 733篇 |
专业分类
测绘学 | 1904篇 |
大气科学 | 5733篇 |
地球物理 | 14177篇 |
地质学 | 30088篇 |
海洋学 | 6211篇 |
天文学 | 15669篇 |
综合类 | 276篇 |
自然地理 | 3456篇 |
出版年
2021年 | 584篇 |
2020年 | 665篇 |
2019年 | 731篇 |
2018年 | 3943篇 |
2017年 | 3654篇 |
2016年 | 3036篇 |
2015年 | 1043篇 |
2014年 | 1665篇 |
2013年 | 2956篇 |
2012年 | 2677篇 |
2011年 | 4549篇 |
2010年 | 4087篇 |
2009年 | 4705篇 |
2008年 | 3889篇 |
2007年 | 4392篇 |
2006年 | 2263篇 |
2005年 | 1938篇 |
2004年 | 1795篇 |
2003年 | 1826篇 |
2002年 | 1687篇 |
2001年 | 1332篇 |
2000年 | 1309篇 |
1999年 | 1061篇 |
1998年 | 1041篇 |
1997年 | 1045篇 |
1996年 | 912篇 |
1995年 | 851篇 |
1994年 | 816篇 |
1993年 | 684篇 |
1992年 | 687篇 |
1991年 | 669篇 |
1990年 | 711篇 |
1989年 | 601篇 |
1988年 | 582篇 |
1987年 | 708篇 |
1986年 | 624篇 |
1985年 | 753篇 |
1984年 | 799篇 |
1983年 | 790篇 |
1982年 | 741篇 |
1981年 | 687篇 |
1980年 | 675篇 |
1979年 | 607篇 |
1978年 | 631篇 |
1977年 | 545篇 |
1976年 | 507篇 |
1975年 | 515篇 |
1974年 | 556篇 |
1973年 | 540篇 |
1972年 | 357篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
A new approach to constraining seawater δ34S and sulphate concentration using francolite‐bound sulphate reveals an abrupt increase in δ34S to +50‰ around the Early–Middle Cambrian boundary. Such high δ34S values are best explained by increased rates of pyrite burial due to ocean anoxia coupled with an increased sensitivity of the ocean sulphate reservoir to perturbations due to low sulphate concentrations of 500–700 μgL?1. We argue that the spread of anoxic waters at this time was partly the result of greenhouse warming related to the eruption of the Kalkarindji Large Igneous Province of northern Australia and that it triggered the collapse of early metazoan reef ecosystems during the latest Early Cambrian. Mass extinctions of the last 260 Myr have all coincided with enhanced volcanic activity, while several are also associated with positive shifts in seawater δ34S. Extending this correlation back in time further implicates volcanically induced climate change as a major determining factor in biosphere evolution. Terra Nova, 18, 257–263, 2006 相似文献
22.
23.
Sediment successions in coastal cliffs around Mezen Bay, southeastern White Sea, record an unusually detailed history of former glaciations, interstadial marine and fluvial events from the Weichselian. A regional glaciation model for the Weichselian is based on new data from the Mezen Bay area and previously published data from adjacent areas. Following the Mikulinian (Eemian) interglacial a shelf‐centred glaciation in the Kara Sea is reflected in proglacial conditions at 100–90 ka. A local ice‐cap over the Timan ridge existed between 75 and 65 ka. Renewed glaciation in the Kara Sea spread southwestwards around 60 ka only, interrupted by a marine inundation, before it advanced to its maximum position at about 55–50 ka. After a prolonged ice‐free period, the Scandinavian ice‐sheet invaded the area from the west and terminated east of Mezen Bay about 17 ka. The previously published evidence of a large ice‐dammed lake in the central Arkhangelsk region, Lake Komi, finds no support in this study. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
24.
This paper proposes a multi‐level parallelized substructuring–frontal combined algorithm for the analysis of the problem of thermo/hydraulic/mechanical behaviour of unsaturated soil. Temperature, displacement, pore water pressure and pore air pressure are treated as the primary variables in a non‐linear analysis. Details are given firstly of the substructuring–frontal combined approach. The incorporation of the algorithm in a multi‐level parallel strategy is then discussed. The parallel processing can thus be carried out at different substructural levels. The method thus developed impacts, in a positive way, on both computer storage requirement and execution time. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
25.
Kunde Yang Yuanliang Ma Chao Sun Miller J.H. Potty G.R. 《Oceanic Engineering, IEEE Journal of》2004,29(4):964-972
This paper discusses the results of geoacoustic inversion carried out using explosive charge data from the Asian Seas International Acoustic Experiment (ASIAEX) East China Sea (ECS) Experiment. A multifrequency incoherent matched-field inversion processor and a genetic algorithm (GA) are used for the inversion. A multistep matched field inversion approach is presented, which makes use of the varying sensitivities of wave fields at various frequencies to reduce the inversion problem into a sequence of smaller inversions with fewer unknowns to estimate at each stage. Different parameters are estimated using data at different frequencies according to their sensitivities. Inversion results for different areas in the ECS region are summarized and compared with core data. 相似文献
26.
27.
N.J.P. Owens 《Estuarine, Coastal and Shelf Science》1985,20(4):505-510
In this study, the 14N:15N ratio of suspended particulate material collected from the Tamar river estuary, south-west England, is described. Three populations of particles, distinguishable by their 15N content, were observed. This investigation has shown that populations of estuarine particles are generated by biological transformations in situ and that the 15N content of estuarine particles does not merely reflect hydrodynamic mixing of the freshwater and seawater source particulate material. 相似文献
28.
The lensing properties of the Plummer model with a central point mass and external shear are derived, including the image multiplicities, critical curves and caustics. This provides a simple model for a flattened galaxy with a central supermassive black hole. For the Plummer model with black hole, the maximum number of images is four, provided the black hole mass is less than an upper bound which is calculated analytically. This introduces a method to constrain black hole masses by counting images, thus applicable at cosmological distance. With shear, the maximum number of images is six and we illustrate the occurrence of an astroid caustic and two metamorphoses. 相似文献
29.
In the coastal and estuarine waters of Goa, particulate organic carbon (POC) varied from 0.52 to 2.51 mg l?1 and from 0.28 to 5.24 mg l?1 and particulate phosphorus (PP) varied from 0.71 to 5.18 μg l?1 and from 0.78 to 20.34 μg l?1, respectively. The mean values of chlorophyll and primary productivity were 1.94 mg m?3 and 938.1 mg C m?2 day?1 in the coastal waters and 4.3 mg m?3 and 636.5 mg C m?1 day?1 in the estuarine waters, respectively. ratios were low in June and October even when POC values were quite high. The POC in surface waters was linearly correlated with the chlorophyll content. Also PP increased when chlorophyll and primary productivity remained high. The results suggest that the phytoplankton was sharply increasing and contributed to POC and PP content. The percentage of detritus calculated from the intercept values of chlorophyll on POC varied from 46 to 76% depending on season. Results indicate that the major portion of POC and PP during postmonsoon (October–January) is derived from phytoplankton production while the allochthonous matter predominate during monsoon (June–September). 相似文献
30.
The great continental ice sheets of the Pleistocene represented a significant topographic obstacle to the westerly winds
in northern midlatitudes. This work explores how consequent changes in the atmospheric stationary wave pattern might have
affected the shape and growth of the ice sheets themselves. A one dimensional (1-D) model is developed which permits an examination
of the types and magnitudes of the feedbacks that might be expected. When plausible temperature perturbations are introduced
at the ice-sheet margin which are proportional to the stationary wave amplitude, the equilibrium shape of the ice sheet is
significantly altered, and depends on the sign of the perturbation. The proposed feedback also affects the response of the
ice sheet to time-varying climate forcing. The results suggest that the evolution of a continental-scale ice sheet with a
land-based margin may be significantly determined by the changes it induces in the atmospheric circulation.
Received: 1 October 1999 / Accepted: 17 July 2000 相似文献