全文获取类型
收费全文 | 255篇 |
免费 | 18篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 18篇 |
地球物理 | 67篇 |
地质学 | 73篇 |
海洋学 | 54篇 |
天文学 | 37篇 |
综合类 | 2篇 |
自然地理 | 20篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 6篇 |
2020年 | 10篇 |
2019年 | 7篇 |
2018年 | 11篇 |
2017年 | 12篇 |
2016年 | 14篇 |
2015年 | 8篇 |
2014年 | 3篇 |
2013年 | 12篇 |
2012年 | 7篇 |
2011年 | 16篇 |
2010年 | 14篇 |
2009年 | 16篇 |
2008年 | 16篇 |
2007年 | 17篇 |
2006年 | 10篇 |
2005年 | 13篇 |
2004年 | 6篇 |
2003年 | 2篇 |
2002年 | 8篇 |
2001年 | 2篇 |
2000年 | 5篇 |
1999年 | 6篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1926年 | 1篇 |
排序方式: 共有274条查询结果,搜索用时 15 毫秒
51.
52.
Reinaldo L. Bozelli Adriano Caliman Rafael D. Guariento Luciana S. Carneiro Jayme M. Santangelo Marcos P. Figueiredo-Barros Joo J.F. Leal Adriana M. Rocha Letícia B. Quesado Paloma M. Lopes Vinicius F. Farjalla Claudio C. Marinho Fabio Roland Francisco A. Esteves 《Limnologica》2009,39(4):306-313
Human activities are exposing freshwater ecosystems to a wide range of stressors, whose direct and indirect effects can be alleviated or exacerbated through interactive effects with dynamic environmental drivers. This study used long-term data from two Neotropical lacustrine freshwater systems (Batata Lake, an Amazonian floodplain lake and Imboassica lagoon, an Atlantic coastal lagoon) subjected to different kinds of environmental fluctuations (i.e., flood pulse and sandbar opening) and anthropogenic impacts (i.e., siltation and eutrophication). Our objective was to determine whether the effects of human perturbations are contingent on modifications of important biotic and abiotic characteristics through environmental variability. For both ecosystems, environmental variability consistently interacted with anthropogenic perturbations to alter most of the variables analyzed, such as nutrient dynamics, chlorophyll-a concentration, zooplankton and benthic invertebrate species richness, and temporal community stability, which indicates that interactive effects between environmental variability and anthropogenic perturbations may impact a myriad of ecosystem properties. Furthermore, the nature of these interactive effects was highly dependent on the variable considered and on the ecosystem analyzed. For example, at Imboassica lagoon, sandbar openings interacted synergistically with trophic state to increase the phosphorus concentration in the water column. At Batata Lake, flooding generally alleviated the negative effects of siltation on species richness by both diluting inorganic suspended material concentration and by promoting local recruitment from the regional species pool. Such results indicate that our ability to understand and predict the outcome of anthropogenic impacts on inland aquatic systems can be hampered if we consider human stressors as “static” phenomena disconnected from dynamic interactions with major local environmental drivers. 相似文献
53.
Wagner José Barreto Dilson Norio Ishikawa Ieda Spacino Scarminio João de Souza Costa Paulo dos Santos Nora Miriam de Fatima Soares Romilaine Mansano Nicolau Adriana Celeste Esteves Gonçalves Sonia Regina Giancoli Barreto 《洁净——土壤、空气、水》2008,36(4):353-359
The conservation of large water resources is essential for the preservation of human life. The quantification and, more importantly, the speciation of chemical substances that indicate the presence of anthropogenic contamination in water resources are of great importance. This paper presents the results of analysis for the determination of organic, inorganic and total phosphorus, pseudo‐sulfur, and iron and manganese, in five fractions, in water sediments collected from the Capivara Hydroelectric Power Plant, Brazil. A study on the seasonal variation of these parameters was conducted, with data having been collected in the winter and in the summer, at two sites along the dam, 5 km apart, close to the city of Primeiro de Maio. Phosphorous was found in sediments and adjacent soil in the organic form (OP), and was used as an indicator of anthropogenic influence on the reservoir banks. Speciation of potentially toxic Mn showed that it is present in the exchangeable fraction of the 0–5 cm depth layer (sediment/water interface), making its transfer to the water column possible. Results from this study showed that domestic and industrial effluent treatment measures are needed for the preservation of the quality of aquatic environments. 相似文献
54.
55.
Cave clastic sediments as a tool for refining the study of human occupation of prehistoric sites: insights from the cave site of La Cala (Cilento,southern Italy) 下载免费PDF全文
56.
M. Merck A. Karle S. Martinez F. Arqueros K. H. Becker M. Bott-Bodenhausen R. Eckmann E. Faleiro J. Fernandez P. Fernandez V. Fonseca V. Haustein G. Heinzelmann I. Holl F. Just F. Krennrich M. Kühn E. Lorenz H. Meyer N. Müller R. Plaga J. Prahl M. Probst M. Rozanska M. Samorski H. Sander K. Sauerland C. Sesea W. Stamm 《Astroparticle Physics》1996,5(3-4):379-392
A precise knowledge of the angular resolution of scintillator arrays used to observe extended air showers (EAS) is of key importance in the search for VHE/UHE γ point sources. Four independent methods have been used to determine the mean resolution for which a value of ΔΘ63 of 0.8°(1.0°) at a proton threshold of 50 (40) TeV has been found for the HEGRA EAS-array. 相似文献
57.
Sarah J. Nelson Kenneth B. Johnson Kathleen C. Weathers Cynthia S. Loftin Ivan J. Fernandez Jeffrey S. Kahl David P. Krabbenhoft 《Applied Geochemistry》2008
Atmospheric mercury (Hg) is delivered to ecosystems via rain, snow, cloud/fog, and dry deposition. The importance of snow, especially snow that has passed through the forest canopy (throughfall), in delivering Hg to terrestrial ecosystems has received little attention in the literature. The snowpack is a dynamic system that links atmospheric deposition and ecosystem cycling through deposition and emission of deposited Hg. To examine the magnitude of Hg delivery via snowfall, and to illuminate processes affecting Hg flux to catchments during winter (cold season), Hg in snow in no-canopy areas and under forest canopies measured with four collection methods were compared: (1) Hg in wet precipitation as measured by the Mercury Deposition Network (MDN) for the site in Acadia National Park, Maine, USA, (2) event throughfall (collected after snowfall cessation for accumulations of >8 cm), (3) season-long throughfall collected using the same apparatus for event sampling but deployed for the entire cold season, and (4) snowpack sampling. Estimates (mean ± SE) of Hg deposition using these methods during the 91-day cold season in 2004–2005 at conifer sites showed that season-long throughfall Hg flux (1.80 μg/m2) < snowpack Hg (2.38 ± 0.68 μg/m2) < event throughfall flux (5.63 ± 0.38 μg/m2). Mercury deposition at the MDN site (0.91 μg/m2) was similar to that measured at other no-canopy sites in the area using the other methods, but was 3.4 times less than was measured under conifer canopies using the event sampling regime. This indicates that snow accumulated under the forest canopy received Hg from the overstory or exhibited less re-emission of Hg deposited in snow relative to open areas. The soil surface of field-scale plots were sprayed with a natural rain water sample that contained an Hg tracer (202Hg) just prior to the first snowfall to explore whether some snowpack Hg might be explained from soil emissions. The appearance of the 202Hg tracer in the snowpack (0–64% of the total Hg mass in the snowpack) suggests that movement of Hg from the soil into the snowpack is possible. However, as with any tracer study the 202Hg tracer may not precisely represent the reactivity and mobility of natural Hg in soils. 相似文献
58.
Gary L Rowe Jr Shinji Ohsawa Bokuichiro Takano Susan L Brantley Jose F Fernandez Jorge Barquero 《Bulletin of Volcanology》1992,54(6):494-503
Monitoring of crater lake chemistry during the recent decline and disappearance of the crater lake of Poás Volcano revealed that large variations in SO4/Cl, F/Cl, and Mg/Cl ratios were caused by the enhanced release of HCl vapor from the lake surface due to increasing lake temperature and solution acidity. Variation in the concentration of polythionic acids (H2SxO6, x=4–6) was the most reliable predictor of renewed phreatic eruptive activity at the volcano, exhibiting sharp decreases three months prior to the initiation of phreatic eruptions in June 1987. Polythionic acids may offer a direct indicator of changing subsurface magmatic activity whereas chloride-based element ratios may be influenced by surface volatilization of HCl and subsequent recycling of acidic fluids in crater lake volcanoes. 相似文献
59.
A time series of K3 spectroheliograms taken at the Coimbra Observatory exhibits an erupting loop on the east limb on July 9, 1982 in active region NOAA 3804. The Goddard SMM Hard X-Ray Burst Spectrometer (HXRBS) observations taken during this period reveal a hard X-ray flare occurring just before the loop eruption is observed, and SMS-GOES soft X-ray observations reveal a strong long-duration event (LDE) following the impulsive phase of the flare. A Solwind coronagram exhibits a powerful coronal mass ejection (CME) associated with the erupting loop. H flare and prominence observations as well as centimeter and decimeter radio observations of the event are also reviewed. A large, north–south-oriented quiescent prominence reported within the upper part of the CME expansion region may play a role in the eruption as well. The spatial and temporal correlations among these observations are examined in the light of two different current models for prominence eruption and CME activation: (1) The CME is triggered by the observed hard X-ray impulsive flare. (2) The CME is not triggered by a flare, and the observed soft X-ray flare is an LDE due to reconnection within the CME bubble. It is concluded that this event is probably of a mixed type that combines characteristics of models (1) and (2). The July 9 event is then compared to three other energetic CME and flare eruptions associated with the same active-region complex, all occurring in the period July 9 through September 4, 1982. It is noted that these four energetic events coincide with the final evolutionary phase of a long-lasting active-region complex, which is discussed in a companion paper (Bumba, Garcia, and Jordan, 1997). The paper concludes by addressing the solar flare myth controversy in the light of this work. 相似文献