全文获取类型
收费全文 | 268163篇 |
免费 | 4422篇 |
国内免费 | 3417篇 |
专业分类
测绘学 | 7051篇 |
大气科学 | 19720篇 |
地球物理 | 55966篇 |
地质学 | 93664篇 |
海洋学 | 22393篇 |
天文学 | 57644篇 |
综合类 | 1065篇 |
自然地理 | 18499篇 |
出版年
2021年 | 2298篇 |
2020年 | 2647篇 |
2019年 | 2897篇 |
2018年 | 4526篇 |
2017年 | 4233篇 |
2016年 | 6279篇 |
2015年 | 4283篇 |
2014年 | 6991篇 |
2013年 | 14317篇 |
2012年 | 6925篇 |
2011年 | 8769篇 |
2010年 | 7853篇 |
2009年 | 10388篇 |
2008年 | 9081篇 |
2007年 | 8794篇 |
2006年 | 9789篇 |
2005年 | 7999篇 |
2004年 | 7855篇 |
2003年 | 7349篇 |
2002年 | 6875篇 |
2001年 | 6074篇 |
2000年 | 6035篇 |
1999年 | 5258篇 |
1998年 | 5270篇 |
1997年 | 5063篇 |
1996年 | 4704篇 |
1995年 | 4455篇 |
1994年 | 4135篇 |
1993年 | 3880篇 |
1992年 | 3651篇 |
1991年 | 3603篇 |
1990年 | 3782篇 |
1989年 | 3534篇 |
1988年 | 3315篇 |
1987年 | 3859篇 |
1986年 | 3426篇 |
1985年 | 4247篇 |
1984年 | 4756篇 |
1983年 | 4440篇 |
1982年 | 4330篇 |
1981年 | 3942篇 |
1980年 | 3655篇 |
1979年 | 3523篇 |
1978年 | 3499篇 |
1977年 | 3292篇 |
1976年 | 3060篇 |
1975年 | 2977篇 |
1974年 | 2929篇 |
1973年 | 3097篇 |
1972年 | 2036篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
碳酸岩Sr、Nd、Pb 同位素地球化学研究评述 总被引:3,自引:0,他引:3
碳酸岩是出露相对较少的幔源岩石,其中Sr与Nd是研究地幔物质组成的主要对象之一。本文统计了世界上主要碳酸岩的锶、钕、铅同位素组成特征;研究显示,碳酸岩源区主要是洋岛玄武岩高U/Pb的HIMU端员和富集端员(EM1或EM2)的混合作用;此外大部分碳酸岩的锶、钕同位素落在大洋玄武岩范围内;这些均表明其成因与地慢柱有密切联系。碳酸岩及与之共生的硅酸岩的同源或独立源区模式部很难充分解释两者同位素组成特征,逭反映碳酸岩的演化模式涉及更复杂的过程。可能是俯冲作用使碳酸岩源区经历不同时间和程度的富集、亏损过程导致地幔源区成分不均一。 相似文献
992.
993.
994.
Isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the Atacama Desert in northern Chile has shown anomalous 17O enrichments in both minerals. Δ17O values of 14-21 ‰ in nitrate and 0.4 to 4 ‰ in sulfate are the most positive found in terrestrial minerals to date. Modeling of atmospheric processes indicates that the Δ17O signatures are the result of photochemical reactions in the troposphere and stratosphere. We conclude that the bulk of the nitrate, sulfate and other soluble salts in some parts of the Atacama Desert must be the result of atmospheric deposition of particles produced by gas to particle conversion, with minor but varying amounts from sea spray and local terrestrial sources. Flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200,000 to 2.0 M years during hyper-arid conditions similar to those currently found in the Atacama Desert. Correlations between Δ17O and δ18O in nitrate salts from the Atacama Desert and Mojave Desert, California, indicate varying fractions of microbial and photochemical end-member sources. The photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless, whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture, biologic activity, and nitrogen cycling. These isotopic signatures have important implications for paleoclimate, astrobiology, and N cycling studies. 相似文献
995.
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
996.
997.
998.
Feeding habits of harp and hooded seals in drift ice waters along the east coast of Greenland in summer and winter 总被引:2,自引:0,他引:2
Results of analyses of stomach and intestinal contents from hooded ( Cystophora cristata ) and harp ( Phoca groenlandica ) seals captured in the pack ice belt of the Greenland Sea in summer (July-August) in 2000 and winter (February-March) in 2001 revealed that the diet of both species were comprised of relatively few prey taxa. Pelagic amphipods of the genus Parathemisto , the squid Gonatus fabricii , polar cod ( Boreogadus saida ) and capelin ( Mallotus villosus ) constituted 63-99% of the observed diet biomass in both seal species, irrespective of sampling period, but their relative contribution to the diet varied both with species and sampling period/area. For hooded seals, G. fabricii and capelin were the dominant food items in winter 2001, but the summer 2000 diet comprised a mixture of this squid and polar cod. Parathemisto was most important for the harp seals during summer 2000; in winter 2001 the contribution from krill and capelin were comparable to that of Parathemisto . Multivariate analyses revealed differences in the intestinal contents of hooded and harp seals in areas where the two species' occurrence spatially overlapped. Different foraging depths of the two species may have contributed to the observed differences in diets. 相似文献
999.
1000.