全文获取类型
收费全文 | 173篇 |
免费 | 22篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 19篇 |
地球物理 | 49篇 |
地质学 | 55篇 |
海洋学 | 15篇 |
天文学 | 43篇 |
自然地理 | 8篇 |
出版年
2022年 | 1篇 |
2021年 | 6篇 |
2020年 | 3篇 |
2019年 | 9篇 |
2018年 | 8篇 |
2017年 | 4篇 |
2016年 | 15篇 |
2015年 | 7篇 |
2014年 | 7篇 |
2013年 | 15篇 |
2012年 | 8篇 |
2011年 | 6篇 |
2010年 | 4篇 |
2009年 | 12篇 |
2008年 | 5篇 |
2007年 | 6篇 |
2006年 | 7篇 |
2005年 | 1篇 |
2004年 | 4篇 |
2003年 | 7篇 |
2002年 | 8篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1999年 | 1篇 |
1998年 | 5篇 |
1997年 | 5篇 |
1994年 | 1篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 6篇 |
1970年 | 2篇 |
1925年 | 1篇 |
排序方式: 共有197条查询结果,搜索用时 15 毫秒
91.
92.
Greg G. Sass Stephen R. Carpenter Jereme W. Gaeta James F. Kitchell Tyler D. Ahrenstorff 《Aquatic Sciences - Research Across Boundaries》2012,74(2):255-266
Lakeshore residential development (LRD) reduces coarse woody habitat (CWH) in lakes resulting in negative effects on fishes.
We tested whether the addition of CWH could reverse those effects. We added CWH to Camp Lake, a lake with CWH abundances similar
to developed lakes, following 2 years of study of the fish populations in the reference and treatment basins. Both basins
were monitored for 4 years following the manipulation. Specifically, we tested for changes in the population dynamics (densities,
size-structure, growth), diet, and behavior (habitat use) of bluegill (Lepomis macrochirus) and largemouth bass (Micropterus salmoides). CWH addition had no discernible effect on fish population dynamics. Diet and behavioral responses were more pronounced
in the treatment basin. Prey diversity and availability increased. Piscivory increased, with decreased reliance upon terrestrial
prey, for largemouth bass. Habitat use was positively correlated with CWH branching complexity and abundance. Our study suggests
that negative effects observed in fish populations through CWH reductions cannot be reversed in the short-term by adding CWH.
We recommend that regulations governing the LRD process be protective of CWH. 相似文献
93.
Very Large Array (VLA) observations at wavelengths of 20 and 91 cm have been combined with data from the SOHO and RHESSI solar
missions to study the evolution of transequatorial loops connecting active regions on the solar surface. The radio observations
provide information about the acceleration and propagation of energetic electrons in these large-scale coronal magnetic structures
where energy release and transport take place. On one day, a long-lasting Type I noise storm at 91 cm was seen to intensify
and shift position above the northern hemisphere region following an impulsive hard X-ray burst in the southern hemisphere
footpoint region. VLA 20-cm observations as well as SOHO EIT EUV images showed evolving coronal plasma that appeared to move
across the solar equator during this time period. This suggests that the transequatorial loop acted as a conduit for energetic
particles or fields that may have triggered magnetic changes in the corona where the northern noise storm region was seen.
On another day, a hard X-ray burst detected at the limb was accompanied by impulsive 20- and 91-cm burst emission along a
loop connecting to an active region in the same hemisphere but about 5′ away, again suggesting particle propagation and remote
flare triggering across interconnecting loops. 相似文献
94.
95.
96.
Natural Hazards - This study systematically reviews the diverse body of research on community flood risk management in the USA to identify knowledge gaps and develop innovative and practical... 相似文献
97.
Assuming steady state of carbon dioxide levels in a “pressure-cooker” atmosphere/ocean system (10–20 bars, near 100°C) produced by a land weathering sink and volcanic source (BLAG model), an abiotic Earth model for 3.8 Ga requires present biotic enhancements of weathering to be on the order of 100 or greater, consistent with the limit inferred from experimental and field studies. Using a plausible ratio of the present biotic enhancement (from higher plants) to enhancements produced by microbial activity alone, along with models for continental growth and outgassing rates consistent with geologic evidence, we find that computed surface temperatures hover near 20°C over geologic time, slowly decreasing to present, after a rapid initial decline as a result of microbial colonization of land. Results are consistent with the first possibility for glaciation in the late Archean/early Proterozoic. Useful modeling of climatic evolution, taking into account biotic enhancement of weathering, can now apparently be extended into the Precambrian, assuming operation of the carbonate-silicate buffer. 相似文献
98.
Tyler COPLEN Carol KENDALL I. Lynus BARNES 《Geostandards and Geoanalytical Research》1982,6(2):257-258
Friedman et al (1) propose that NBS 19 calcium carbonate substitute for PDB. This is in apparent conflict with the recommendation of the September 1976 consultants' meeting convened by the International Atomic Energy Agency on stable isotope standards. Furthermore, Friedman et al suggest ɛ-values be adopted for NBS 19 that do not take into account the ɛ-values of NBS 20 (Solenhofen limestone). We believe that adoption of these proposals may lead to confusion in the stable isotope literature and to lack of correspondence between historic and future isotopic results.
In sharp contrast to the results of Friedman et al (1), we find NBS 20 to be a satisfactory reference material if it is handled properly. 相似文献
In sharp contrast to the results of Friedman et al (1), we find NBS 20 to be a satisfactory reference material if it is handled properly. 相似文献
99.
Graphitic pelites of the western Moinian were metamorphosed at the time of emplacement of the Strontian Granodiorite intrusion, at a late stage of the Caledonian Orogeny, producing a metamorphic zonation. The Sillimanite Zone (in which K feldspar does not occur with sillimanite) is succeeded by the Muscovite-Sillimanite-K feldspar Zone, Sillimanite-K feldspar Zone (without primary muscovite) and Cordierite-K feldspar Zone. Secondary muscovite from retrograde hydration of sillimanite+K feldspar is distinguished texturally from primary muscovite, but is compositionally similar. Primary porphyroblastic muscovite, inherited from the regional metamorphic textural evolution of the rocks, disappears abruptly at the muscovite-out isograd. Migmatites of earlier regional origin, with recrystallized textures, are distinguished from those associated with the late Caledonian metamorphism, which are confined to the Sillimanite-K feldspar and Cordierite-K feldspar Zones. Muscovite compositions are inferred to be very low in Fe3+. There are no marked changes in muscovite composition at the entry of sillimanite+K feldspar. Higher Na contents than in some other muscovites coexisting with sillimanite+K feldspar are interpreted in terms of relatively low P in the Strontian area. Andalusite is found at two localities. From cordierite-garnet-sillimanite-biotite-K feldspar-quartz assemblages, a P estimate of 4.1±0.4 kbar is obtained, with the aqueous fluid having
, and the T at the cordierite-K feldspar isograd is estimated as 690° C. T at the muscovite-out isograd is inferred to the maximum for muscovite-quartz-sillimanite-K feldspar equilibrium with graphite at P4.1 kbar: T 645° C, with
. The well-defined lower boundary of the Muscovite-Sillimanite-K feldspar Zone is attributed to regionally rather homogeneous fluid composition at
. The low P prevented melting in the Muscovite-Sillimanite-K feldspar Zone. Migmatites in the higher zones are attributed to partial melting, which accounts for the low
necessary to produce the cordierite assemblages. Obscurity of these migmatites is attributed to deformation during and after the migmatization. A breccia structure in and near the Cordierite-K feldspar Zone, where pelites have flowed around disrupted pieces of psammite and amphibolite, can be explained by presence of grain-boundary melts in the pelites. 相似文献
100.
Deep seismic studies are rather difficult in Arctic seas; it is therefore necessary to find other sources of information on the structure of the earth's crust. In this paper a number of crustal sections across the Arctic seas of Eurasia are presented. They are based on the analysis of magnetic field anomalies. Upper and lower boundaries of anomalous magnetic bodies, as well as zones where the magnetization changes rapidly in the vertical direction,indicate the position of the main seismic discontinuities. This inference has been confirmed by comparison with seismic data for the region of Okhotsk. 相似文献