全文获取类型
收费全文 | 483篇 |
免费 | 36篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 11篇 |
大气科学 | 46篇 |
地球物理 | 148篇 |
地质学 | 186篇 |
海洋学 | 61篇 |
天文学 | 52篇 |
综合类 | 4篇 |
自然地理 | 16篇 |
出版年
2024年 | 2篇 |
2023年 | 3篇 |
2022年 | 9篇 |
2021年 | 17篇 |
2020年 | 11篇 |
2019年 | 9篇 |
2018年 | 21篇 |
2017年 | 25篇 |
2016年 | 34篇 |
2015年 | 25篇 |
2014年 | 18篇 |
2013年 | 34篇 |
2012年 | 33篇 |
2011年 | 32篇 |
2010年 | 29篇 |
2009年 | 33篇 |
2008年 | 35篇 |
2007年 | 29篇 |
2006年 | 18篇 |
2005年 | 18篇 |
2004年 | 13篇 |
2003年 | 12篇 |
2002年 | 12篇 |
2001年 | 12篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1968年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有524条查询结果,搜索用时 984 毫秒
91.
Pedro Manuel Anastácio Emília Pereira Leite Margarida Ferreira Luís Vicente Alexandra Marçal Correia 《Limnologica》2011,41(3):228-234
The red swamp crayfish, Procambarus clarkii, and the eastern mosquitofish, Gambusia holbrooki have been introduced worldwide, but the interactions between these invasive species are not well known. The effects of crayfish presence, crayfish size and water depth on the consumption of G. holbrooki by P. clarkii were analyzed, as were the effects of mosquitofish presence and water depth on the consumption of recently hatched crayfish (RHC) by mosquitofish. To better understand this subject we conducted aquaria and open air mesocosm experiments simulating a rice field section. Aquarium experiments showed that P. clarkii consumes G. holbrooki and that G. holbrooki consumes RHC. Adult crayfish size did not significantly affect the consumption of mosquitofish and water depth had no effect on the consumption of G. holbrooki by P. clarkii and vice versa. In the outdoor mesocosm experiments P. clarkii caused no statistically significant effect on G. holbrooki numbers or biomass when compared with controls without crayfish. In spite of this, the analysis of crayfish stomach contents clearly demonstrated that several G. holbrooki individuals were consumed, indicating that crayfish can profit from the presence of G. holbrooki. Results from our study suggest that in shallow pools there are strong interactions between the two invasive species since adult crayfish predate mosquitofish and mosquitofish predate recently hatched crayfish. Therefore, there are bidirectional trophic interactions between these invasive species which change depending on their relative size. We believe that these interactions should become more relevant in confined areas such as small shallow pools formed during the late summer and autumn. 相似文献
92.
While the presence of steroid estrogens in the environment has become a major environmental and health concern, their occurrence in coastal sediments remains poorly characterized. In this study, we measured the levels of three natural (estrone, 17β-estradiol, estriol) and one synthetic (17α-ethinylestradiol) estrogens in 54 coastal sediment samples collected from nine locations off central-southern Chile. Steroid estrogens were found in every sample. Remarkably high levels of 17α-ethinylestradiol were detected, reaching up to 48.14 ng/g dry weight. As a result, the global estrogenic loads were estimated to be high at all sites. Interestingly, they were found to correlate with the size of human populations served by sewage plants. Our study indicates that 17α-ethinylestradiol may accumulate in coastal sediments. The possible impact of this highly potent synthetic estrogen on the biota of the marine ecosystem off central-south Chile and on human health remains an open question. 相似文献
93.
Costa PM Caeiro S Lobo J Martins M Ferreira AM Caetano M Vale C DelValls TÁ Costa MH 《Marine pollution bulletin》2011,62(1):55-65
Juvenile Senegalese soles were exposed through 28-day laboratory and field (in situ) bioassays to sediments from three sites of the Sado estuary (W Portugal): a reference and two contaminated by metallic and organic contaminants. Fish were surveyed for ten hepatic histopathological alterations divided by four distinct reaction patterns and integrated through the estimation of individual histopathological condition indices. Fish exposed to contaminated sediments sustained more damage, with especial respect to regressive changes like necrosis. However, differences were observed between laboratory- and field-exposed animals, with the latest, for instance, exhibiting more pronounced fatty degeneration and hepatocellular eosinophilic alteration. Also, some lesions in fish exposed to the reference sediment indicate that in both assays unaccounted variables produced experimental background noise, such as hyaline degeneration in laboratory-exposed fish. Still, the field assays yielded results that were found to better reflect the overall levels of contaminants and physico-chemical characteristics of the tested sediments. 相似文献
94.
Black dimensional stones: geology, technical properties and deposit characterization of the dolerites from Uruguay 总被引:1,自引:1,他引:0
Manuela Morales Demarco Pedro Oyhantçabal Karl-Jochen Stein Siegfried Siegesmund 《Environmental Earth Sciences》2011,63(7-8):1879-1909
Dimensional stones with a black color occupy a prominent place on the international market. Uruguayan dolerite dikes of andesitic and andesitic–basaltic composition are mined for commercial blocks of black dimensional stones. A total of 16 dikes of both compositions were studied and samples collected for geochemical and petrographical analysis. Color measurements were performed on different black dimensional stones in order to compare them with the Uruguayan dolerites. Samples of the two commercial varieties (Absolute Black and Moderate Black) were obtained for petrophysical analysis (e.g. density, porosity, uniaxial compressive strength, tensile strength, etc.). Detailed structural analyses were performed in several quarries. Geochemistry and petrography determines the intensity of the black color. The Uruguayan dolerite Absolute Black is the darkest black dimensional stone analyzed in this study. The petrophysical properties of Uruguayan dolerites make them one of the highest quality black dimensional stones. Structural analyses show that five joint sets have been recognized: two sub-vertical joints, one horizontal and two diagonal. These joint sets are one of the most important factors that control the deposits, since they control the block size distribution and the amount of waste material. 相似文献
95.
Analysis of Mississippian coral assemblages from the Khenifra region of Central Morocco together with data from foraminiferal/algal microfossils has established new age dating of 5 localities within the Azrou–Khenifra Basin: Souk El Had and Sidi Lamine, where corals occur mainly in biostromes protected by oolitic shoals, Tabainout, where corals have been recorded in different environments related to microbial mounds, Alhajra Almatkouba, where corals occur in biostromal reworked beds and Tiouinine, where corals occur in a well structured, fringing reef. This study demonstrates the presence of richer more diverse coral assemblages than previously recorded, in a variety of environmental settings. These coral assemblages strengthen correlations with the Adarouch area in the NE part of the Azrou–Khenifra Basin. It is emphasised that in the upper Viséan there are close similarities with rugose coral assemblages in other parts of the Western Palaeotethys including North Africa, SW Spain and NW Europe, and that all belong to the same biogeographic province. 相似文献
96.
Anthony Hildenbrand Pedro Madureira Fernando Ornelas Marques Inês Cruz Bernard Henry Pedro Silva 《Earth and Planetary Science Letters》2008,273(3-4):289-298
New K/Ar dating and geochemical analyses have been carried out on the WNW–ESE elongated oceanic island of S. Jorge to reconstruct the volcanic evolution of a linear ridge developed close to the Azores triple junction. We show that S. Jorge sub-aerial construction encompasses the last 1.3 Myr, a time interval far much longer than previously reported. The early development of the ridge involved a sub-aerial building phase exposed in the southeast end of the island and now constrained between 1.32 ± 0.02 and 1.21 ± 0.02 Ma. Basic lavas from this older stage are alkaline and enriched in incompatible elements, reflecting partial melting of an enriched mantle source. At least three differentiation cycles from alkaline basalts to mugearites are documented within this stage. The successive episodes of magma rising, storage and evolution suggest an intermittent re-opening of the magma feeding system, possibly due to recurrent tensional or trans-tensional tectonic events. Present data show a gap in sub-aerial volcanism before a second main ongoing building phase starting at about 750 ka. Sub-aerial construction of the S. Jorge ridge migrated progressively towards the west, but involved several overlapping volcanic episodes constrained along the main WNW–ESE structural axis of the island. Mafic magmas erupted during the second phase have been also generated by partial melting of an enriched mantle source. Trace element data suggest, however, variable and lower degrees of partial melting of a shallower mantle domain, which is interpreted as an increasing control of lithospheric deformation on the genesis and extraction of primitive melts during the last 750 kyr. The multi-stage development of the S. Jorge volcanic ridge over the last 1.3 Myr has most likely been greatly influenced by regional tectonics, controlled by deformation along the diffuse boundary between the Nubian and the Eurasian plates, and the increasing effect of sea-floor spreading at the Mid-Atlantic Ridge. 相似文献
97.
98.
Cléa Araújo da Silva Pedro Walfir M. Souza-FilhoSuzan W.P. Rodrigues 《Continental Shelf Research》2009
The northern Brazilian coast, east of the Amazon River is characterized by several macrotidal estuarine systems that harbor large mangrove areas with approximately 7600 km2. The Marapanim Estuary is influenced by macrotidal regime with moderate waves influence. Morphologic units were investigated by using remote sensing images (i.e., Landsat-7 ETM+, RADARSAT- 1 Wide and SRTM) integrated with bathymetric data. The modern sedimentary deposits were analyzed from 67 cores collected by Vibracore and Rammkersonde systems. Analysis of morphology and surface sedimentary deposits of the Marapanim River reveal they are strongly influenced by the interaction of tidal, wave and fluvial currents. Based on these processes it was possible to recognize three distinct longitudinal facies zonation that revels the geological filling of a macrotidal estuary. The estuary mouth contain fine to medium marine sands strongly influenced by waves and tides, responsible for macrotidal sandy beaches and estuarine channel development, which are characterized by wave-ripple bedding and longitudinal cross-bedding sands. The estuary funnel is mainly influenced by tides that form wide tidal mudflats, colonized by mangroves, along the estuarine margin, with parallel laminations, lenticular bedding, root fragments and organic matter lenses. The upstream estuary contains coarse sand to gravel of fluvial origin. Massive mud with organic matter lenses, marks and roots fragments occur in the floodplain accumulates during seasonal flooding providing a slowly aggrading in the alluvial plain. This morphologic and depositional pattern show easily a tripartite zonation of a macrotidal estuary, that are in the final stage of filling. 相似文献
99.
Brent P. Stafford Natalie L. Cápiro Pedro J.J. Alvarez William G. Rixey 《Ground Water Monitoring & Remediation》2009,29(3):93-104
Neat ethanol (75.7 L) was released into the upper capillary zone in a continuous-flow, sand-packed aquifer tank (8.2 m3 ) with an average seepage velocity of 0.75 m/day. This model aquifer system contained a residual nonaqueous phase liquid (NAPL) that extended from the capillary zone to 10 cm below the water table. Maximum aqueous concentrations of ethanol were 20% v/v in the capillary zone and 0.08% in the saturated zone at 25 and 30 cm downgradient from the emplaced NAPL source, respectively. A bench-scale release experiment was also conducted for a similar size spill (scaled to the plan area). The concentrations of ethanol in ground water for both the bench- and pilot-scale experiments were consistent with advective–dispersive limited mass transfer from the capillary to the saturated zone. Concentrations of monoaromatic hydrocarbons and isooctane increased in the pore water of the capillary zone as a result of both redistribution of residual NAPL (confirmed by visualization) and enhanced hydrocarbon dissolution due to the cosolvent effect exerted by ethanol. In the tank experiment, higher hydrocarbon concentrations in ground water were also attributed to decreased hydrocarbon biodegradation activity caused by preferential microbial utilization of ethanol and the resulting depletion of oxygen. These results infer that spills of highly concentrated ethanol will be largely confined to the capillary zone due to its buoyancy, and ethanol concentrations in near-source zone ground water will be controlled by mass transfer limitations and hydrologic conditions. Furthermore, highly concentrated ethanol releases onto pre-existing NAPL will likely exacerbate impacts to ground water, due to NAPL mobilization and dissolution, and decreased bioattenuation of hydrocarbons. 相似文献
100.
Shear effects on hollow section piers under seismic actions: experimental and numerical analysis 总被引:1,自引:0,他引:1
Raimundo Delgado Pedro Delgado Nelson Vila Pouca António Arêde Patrício Rocha Aníbal Costa 《Bulletin of Earthquake Engineering》2009,7(2):377-389
Shear effects are often a very important issue on the seismic behaviour of piers, particularly for hollow section bridge piers.
In fact, for this type of piers the cyclic response is similar to that of a structural wall in which both the transverse reinforcement
ratio and the detailing can play an important role on its performance, even likely to be determinant in terms of the failure
mechanism. On the other hand, codes and design guidelines are usually very conservative concerning shear capacity in order
to avoid any shear failure mechanism likely to trigger well known catastrophic consequences. Therefore, research studies on
this topic are still needed for a better understanding of pier cyclic shear response and also for improvement of the performance
under seismic actions. Pursuing this general objective, this paper partially reports on an experimental/numerical campaign
carried out on 1:4 reduced scale bridge piers in order to highlight and investigate shear-type problems. Within the scope
of this paper, two specimens types were selected having equal rectangular hollow section (900 × 450 mm2, 75 mm thick) but different transverse reinforcement detailing, namely one with a single stirrup per wall (representative
of typical bridge construction without seismic design requirements) and another with multiple stirrups, according to Eurocode
8 provisions. Numerical simulations of the experimental results were also conducted aiming at contributing for complete and
consistent interpretations of experimental results. Detailed modelling was performed allowing for realistic simulations of
the non linear behaviour, particularly suitable when a significant shear component is involved. Therefore, the numerical strategy
was based on a detailed 3D FEM discretization using a two-scalar variable damage model for the concrete constitutive law and
a suitable cyclic behaviour law for steel bars represented by truss elements. Results have shown that shear deformation and
failure modes are well simulated, while providing detailed insight concerning concrete damage pattern and distribution of
yielding on the transverse and longitudinal reinforcement. 相似文献