全文获取类型
收费全文 | 1352篇 |
免费 | 338篇 |
国内免费 | 282篇 |
专业分类
测绘学 | 7篇 |
大气科学 | 5篇 |
地球物理 | 401篇 |
地质学 | 981篇 |
海洋学 | 125篇 |
天文学 | 8篇 |
综合类 | 38篇 |
自然地理 | 407篇 |
出版年
2024年 | 6篇 |
2023年 | 27篇 |
2022年 | 55篇 |
2021年 | 49篇 |
2020年 | 47篇 |
2019年 | 64篇 |
2018年 | 55篇 |
2017年 | 64篇 |
2016年 | 58篇 |
2015年 | 53篇 |
2014年 | 75篇 |
2013年 | 102篇 |
2012年 | 61篇 |
2011年 | 82篇 |
2010年 | 46篇 |
2009年 | 101篇 |
2008年 | 80篇 |
2007年 | 106篇 |
2006年 | 102篇 |
2005年 | 72篇 |
2004年 | 73篇 |
2003年 | 61篇 |
2002年 | 78篇 |
2001年 | 63篇 |
2000年 | 65篇 |
1999年 | 42篇 |
1998年 | 34篇 |
1997年 | 32篇 |
1996年 | 36篇 |
1995年 | 42篇 |
1994年 | 31篇 |
1993年 | 20篇 |
1992年 | 15篇 |
1991年 | 6篇 |
1990年 | 7篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1979年 | 5篇 |
1978年 | 1篇 |
1977年 | 2篇 |
排序方式: 共有1972条查询结果,搜索用时 31 毫秒
51.
在西南极菲尔德斯半岛长城站地区发现有大量古海蚀龛 ,龛内保存有完整沉积剖面 ,通过对其中一个沉积剖面的粒度特征及元素组成的分析 ,确认该剖面是后期的湖相沉积 ,并据此恢复了距今 460 0年以来该地区古环境演化历史。结果表明 ,距今 40 0 0— 30 0 0年长城站区地表岩石化学风化作用较强烈 ,处于一个相对温暖的时期 ,这与湖泊沉积物、海洋沉积物等古环境记录载体的研究结果是一致的 ,证明古海蚀龛沉积可以作为研究南极无冰区和冰缘地区古环境演化历史的一种新的地质载体。 相似文献
52.
The karst landforms distributed on the Qinghai-Xizang (Tibet) Plateau can be genetically classed with the Tertiary underground karst, which were gradually exhumed to the surface with the uplift of the plateau during Quaternary period. The relative deposits of the Tertiary palaeokarst processes, such as the residuum and speleothem, were discovered recently in the southern and southeastern fringe areas of the plateau, where has geological-currently been disintegrated by the headward erosion processes of the modern river systems. The major chemical components of the clay portion of the residuum consist mainly of SiO2C, Al2CO3 and Fe2O3. The clay minerals composition of the clay portion belongs to illite-kaolinite pattern for most of the residuum samples, and kaolinite-illite pattern for a few of the samples. It can be judged from the silicic acid index and the clay minerals composition that the formation of the residuum of the Plateau was in its initial phase. However, such a lower chemical weathering index only reflected the weathering degree in the bottom or lower parts of the lateritic weathering crust. The relatively intensive chemical weathering processes of the surface layers of the lateritic weathering crust could be logically speculated. The surface feature textures of quartz grains in the residuum were formed mainly by the chemical erosion, which revealed a long-term humid-tropical environment when the residuum and the palaeokarst formed. 相似文献
53.
Martin Roberge 《The Professional geographer》2002,54(2):175-189
In‐stream gravel mining, massive bridge piers, and channelization have all contributed to the geomorphic instability of the Lower Salt River channel in Arizona. Dam closure, changing dam operating rules, and the frequent modification of the channel bed have decreased our ability to predict the Salt River hydrology. Engineering practice has adapted to this situation and to a public that is increasingly intolerant of service disruptions by constructing larger bridges and extending levees. Building these larger structures may be counterproductive; future construction should not constrict the channel and should re‐establish a braided river to decrease the energy available to the system. 相似文献
54.
Microcoleus vaginatus isolated from a desert algal crust of Shapotou was cultured in BG-11 medium containing 0.2 mol l−1 NaCl or 0.2 mol l−1 NaCl plus 100 mg l−1 sucrose, extracellular polymeric substances (EPS) or hot water-soluble polysaccharides (HWP), respectively. Photosynthetic oxygen evolution rates, photosystem II activity (Fv/Fm) and dark respiration of NaCl-stressed cells were enhanced significantly by the added sucrose or EPS under salt stress conditions (0.2 mol l−1 NaCl). Compared with cells treated with salt alone, sodium contents in cells reduced significantly; the content of cellular total carbohydrate did not change, and intracellular sucrose, water-soluble sugar increased significantly following the addition of exogenous carbohydrates. Sucrose synthase (SS) activity of NaCl-stressed cells increased following the addition of sucrose, and sucrose phosphate synthase (SPS) activity of NaCl-stressed cells increased following the addition of exogenous sucrose, EPS or HWP compared with cells stressed with NaCl only. The results suggested that the extruded EPS might be re-absorbed by cells of M. vaginatus as carbon source, they could increase salt tolerance of M. vaginatus through the changes of carbohydrate metabolism and the selective uptake of sodium ions. 相似文献
55.
56.
57.
大兴安岭阿龙山地区流纹岩风化的地球化学特征 总被引:5,自引:1,他引:5
通过大兴安岭地区两个流纹岩风化剖面的研究发现,大兴安岭地区的岩石风化过程具有特殊性。风化过程分为两个阶段,即以化学风化为主导的基质风化阶段和以机械风化为主导的斑晶风化阶段,第二阶段显示出与正常风化剖面相反的风化趋势。低温和长冬使第二风化阶段延长,并呈现逆风化趋势。风化过程中Fe和Al始终处于流失状态。上述特征的产生是由原岩的结构及成分、气候因素和生态系统等多种因素作用的结果。 相似文献
58.
Quartz and quartzite are thought to be resistant as a mineral and a rock respectively; however, we have shown that the presence of small amounts of pyrite in the quartzites makes them vulnerable to weathering. We observe that weathering of Proterozoic quartzite in the semi‐arid conditions around Delhi proceeded from fractures towards the inside and produced weathering rinds. The chemical index of alteration (CIA), which is actually a measure of weathering of aluminosilicate minerals, increases from the core outwards, through the rinds. Although aluminosilicate minerals occur only as minor phases (<2 per cent), their weathering indicates a movement of the weathering front from the periphery towards the core. We have suggested a coupled mechanism in which the dissolution of pyrites by moving water produced a sulphate‐bearing acidic solution and ferrous iron, which reacted with aluminosilicate minerals and quartz, respectively. This initially makes the Delhi quartzite porous and subsequently friable. The total disintegration of grain to grain contacts imparted friability to this quartzite to produce silica sand. Subsequent physical erosion of loose sand, produced during rind development in the outermost zones, has given rise to features like tors, spheroids, gullies, cavities and small‐scale caves on these quartzites. Thus, the terrain has acquired ruggedness in semi‐arid conditions. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
59.
Rivers, chemical weathering and Earth's climate 总被引:4,自引:0,他引:4
Bernard Dupr Cline Dessert Priscia Oliva Yves Goddris Jrme Viers Louis Franois Romain Millot Jrme Gaillardet 《Comptes Rendus Geoscience》2003,335(16):1141-1160
We detail the results of recent studies describing and quantifying the large-scale chemical weathering of the main types of continental silicate rocks: granites and basalts. These studies aim at establishing chemical weathering laws for these two lithologies, describing the dependence of chemical weathering on environmental parameters, such as climate and mechanical erosion. As shown within this contribution, such mathematical laws are of primary importance for numerical models calculating the evolution of the partial pressure of atmospheric CO2 and the Earth climate at geological timescales. The major results can be summarized as follow: (1) weathering of continental basaltic lithologies accounts for about 30% of the total consumption of atmospheric CO2 through weathering of continental silicate rocks. This is related to their high weatherability (about eight times greater than the granite weatherability); (2) a simple weathering law has been established for basaltic lithologies, giving the consumption of atmospheric CO2 as a function of regional continental runoff, and mean annual regional temperature; (3) no such simple weathering law can be proposed for granitic lithologies, since the effect of temperature can only be identified for regions displaying high continental runoff; (4) a general law relating mechanical erosion and chemical weathering has been validated on small and large catchments. The consequences of these major advances on the climatic evolution of the Earth are discussed. Particularly, the impacts of the onset of the Deccan trapps and the Himalayan orogeny on the global carbon cycle are reinvestigated. To cite this article: B. Dupré et al., C. R. Geoscience 335 (2003). 相似文献
60.
Field investigations at Dugway Proving Ground in western Utah have produced new data on the chronology and human occupation of late Pleistocene and early Holocene lakes, rivers, and wetlands in the Lake Bonneville basin. We have classified paleo-river channels of these ages as “gravel channels” and “sand channels.” Gravel channels are straight to curved, digitate, and have abrupt bulbous ends. They are composed of fine gravel and coarse sand, and are topographically inverted (i.e., they stand higher than the surrounding mudflats). Sand channels are younger and sand filled, with well-developed meander-scroll morphology that is truncated by deflated mudflat surfaces. Gravel channels were formed by a river that originated as overflow from the Sevier basin along the Old River Bed during the late regressive phases of Lake Bonneville (after 12,500 and prior to 11,000 14C yr B.P.). Dated samples from sand channels and associated fluvial overbank and wetland deposits range in age from 11,000 to 8800 14C yr B.P., and are probably related to continued Sevier-basin overflow and to groundwater discharge. Paleoarchaic foragers occupied numerous sites on gravel-channel landforms and adjacent to sand channels in the extensive early Holocene wetland habitats. Reworking of tools and limited toolstone diversity is consistent with theoretical models suggesting Paleoarchaic foragers in the Old River Bed delta were less mobile than elsewhere in the Great Basin. 相似文献