全文获取类型
收费全文 | 5059篇 |
免费 | 258篇 |
国内免费 | 24篇 |
专业分类
测绘学 | 151篇 |
大气科学 | 368篇 |
地球物理 | 1919篇 |
地质学 | 1672篇 |
海洋学 | 331篇 |
天文学 | 621篇 |
综合类 | 28篇 |
自然地理 | 251篇 |
出版年
2022年 | 39篇 |
2021年 | 104篇 |
2020年 | 105篇 |
2019年 | 82篇 |
2018年 | 215篇 |
2017年 | 199篇 |
2016年 | 321篇 |
2015年 | 241篇 |
2014年 | 282篇 |
2013年 | 338篇 |
2012年 | 267篇 |
2011年 | 305篇 |
2010年 | 242篇 |
2009年 | 239篇 |
2008年 | 213篇 |
2007年 | 183篇 |
2006年 | 179篇 |
2005年 | 107篇 |
2004年 | 122篇 |
2003年 | 100篇 |
2002年 | 95篇 |
2001年 | 88篇 |
2000年 | 60篇 |
1999年 | 62篇 |
1998年 | 83篇 |
1997年 | 63篇 |
1996年 | 38篇 |
1995年 | 39篇 |
1994年 | 47篇 |
1993年 | 36篇 |
1992年 | 39篇 |
1991年 | 39篇 |
1990年 | 59篇 |
1989年 | 40篇 |
1988年 | 26篇 |
1987年 | 29篇 |
1986年 | 34篇 |
1984年 | 30篇 |
1983年 | 25篇 |
1982年 | 31篇 |
1981年 | 25篇 |
1980年 | 25篇 |
1979年 | 29篇 |
1978年 | 25篇 |
1977年 | 25篇 |
1976年 | 25篇 |
1975年 | 24篇 |
1974年 | 25篇 |
1973年 | 31篇 |
1971年 | 36篇 |
排序方式: 共有5341条查询结果,搜索用时 15 毫秒
991.
Remote sensing techniques are specially suitable to detect and to map areas affected by forest fires. In this work, Landsat 5 Thematic Mapper (TM) data has been used to study a number of forest fires that occurred in the province of Valencia (Spain) and to monitor the vegetation regeneration over burnt areas. A reference area (non‐burnt forest) was established to assess the change produced by fire. The radiance in the thermal band (10.4–12.5 μm) and the normalized difference in reflectance between near 1R (0.76–0.90 μm) and middle IR (2.08–2.35 μm) were the most suitable parameters to map burnt areas. This index can also be used for monitoring vegetation regeneration in burnt areas. About a month after the fire, the burns show temperatures of 5–6 °C higher than those found in the reference area, and the vegetation index shows negative values whereas the reference area values remain positive. The differences between the burns and the reference area for the vegetation index decrease with time as vegetation regenerates. 相似文献
992.
Vinícius dos SANTOS Peter MARSHALL FLEMING Luís HENRIQUE MANCINI Stela DALVA SANTOS COTA Graziele Beatriz de LIMA Rafaela RODRIGUES GOMES Roberto Eduardo KIRCHHEIM Ricardo SANCHéZ-MURILLO Didier GASTMANS 《大气科学进展》2022,39(10):1693-1708
Precipitation isotope ratios (O and H) record the history of water phase transitions and fractionation processes during moisture transport and rainfall formation. Here, we evaluated the isotopic composition of precipitation over the central-southeastern region of Brazil at different timescales. Monthly isotopic compositions were associated with classical effects (rainfall amount, seasonality, and continentality), demonstrating the importance of vapor recirculation processes and different regional atmospheric systems (South American Convergence Zone-SACZ and Cold Fronts-CF). While moisture recycling and regional atmospheric processes may also be observed on a daily timescale, classical effects such as the amount effect were not strongly correlated (δ18O-precipitation rate r ≤ –0.37). Daily variability revealed specific climatic features, such as δ18O depleted values (~ –6‰ to –8‰) during the wet season were associated with strong convective activity and large moisture availability. Daily isotopic analysis revealed the role of different moisture sources and transport effects. Isotope ratios combined with d-excess explain how atmospheric recirculation processes interact with convective activity during rainfall formation processes. Our findings provide a new understanding of rainfall sampling timescales and highlight the importance of water isotopes to decipher key hydrometeorological processes in a complex spatial and temporal context in central-southeastern Brazil. 相似文献
993.
Kowalski Andrew S. Serrano-Ortiz Penélope Miranda-García Gabriela Fratini Gerardo 《Boundary-Layer Meteorology》2021,179(3):347-367
Boundary-Layer Meteorology - An analysis based on the law of linear momentum conservation demonstrates unequivocally that the mass fraction is the scalar whose gradient determines gas diffusion,... 相似文献
994.
New microthermometric data combined with stable isotope geochemistry and paragenetic relationships support a previously suggested
cooling–mixing model for the iron oxide–copper–gold mineralization in the Mantoverde district. Fluid inclusions show characteristics
of a CO2-bearing aqueous NaCl ± CaCl2 salt system. The evolution of the Mantoverde hydrothermal system is characterized by (1) an early hypersaline, high to moderate
temperature fluid; (2) a moderate saline, moderate temperature fluid; and (3) a low saline, moderate to low temperature fluid.
Early magnetite formation took place at median temperatures of 435.0°C, whereas hematite formed at median temperatures of
334.4°C. The main sulfide mineralization texturally post-dates the iron oxides and occurred before late-stage calcite, which
developed at a median temperature of 244.8°C. Boiling occurs only locally and is of no relevance for the ore formation. The
microthermometric and stable isotope data are supportive for a fluid cooling and mixing model, and suggestive for a predominantly
magmatic–hydrothermal fluid component during the iron oxide and main sulfide mineralization. Thereafter, the incursion of
a nonmagmatic fluid of ultimately meteoric or seawater gains more importance. 相似文献
995.
Alexandre Raphael Cabral Michael Wiedenbeck Francisco Javier Rios Ant?nio Augusto Seabra Gomes Jr Orlando Garcia Rocha Filho Richard David Jones 《Mineralium Deposita》2012,47(4):411-424
Talc mineralisation occurs as hematite–talc schist between soft hematite ore and dolomitic itabirite at Gongo Soco, Quadrilátero
Ferrífero of Minas Gerais, Brazil. The hematite–talc schist and soft hematite have a prominent tectonic foliation of tabular
hematite. Tabular hematite without preferential orientation is superimposed on the tectonic foliation. The talcose schist
is enriched in F and has a constant Fe/S ratio. Electron-microprobe analyses indicate trace amounts of S in different generations
of hematite. The whole-rock Fe/S ratio possibly represents sulfate S from hematite-hosted fluid inclusions. Fluid inclusions
in foliation-overprinting hematite and chlorite geothermometry from talcose rocks suggest, respectively, temperatures from
<200°C to ~300°C. Tourmaline, a rarely observed mineral in the hematite–talc schist, belongs to the alkali group and falls
in the dravite compositional field. Boron-isotope determinations of tourmaline crystals, using secondary ion mass spectrometry,
vary from −20‰ to −12‰ δ11B. This compositional isotopic range and the tourmaline chemical composition suggest a meta-evaporitic origin. A non-marine
evaporitic setting is the most likely source of acidic, highly oxidising fluids, which resulted in the abundant F-bearing
talc and the presence of otherwise immobile Ti in hematite. Oxidising brines were channelled along shear zones and converted
dolomitic itabirite into the Gongo Soco soft hematite and the talc mineralisation. The latter is envisaged as the hydrothermal
wall-rock alteration of dolomitic itabirite, which gave rise to the soft hematite ore. 相似文献
996.
The Horní Slavkov–Krásno Sn–W ore district is hosted by strongly altered Variscan topaz–albite granite (Krudum granite body) on the northwestern margin of the Bohemian Massif. We studied the fluid inclusions on greisens, ore pockets, and ore veins from the Hub Stock, an apical expression of the Krudum granite. Fluid inclusions record almost continuously the post-magmatic cooling history of the granite body from ~500 to <50°C. Rarely observed highest-temperature (~500°C) highest-salinity (~30?wt.% NaCl eq.) fluid inclusions are probably the result of secondary boiling of fluids exsolved from the crystallizing magma during pressure release which followed hydraulic brecciation of the gneissic mantle above the granite cupola. The greisenization was related to near-critical low-salinity (0–7?wt.% NaCl eq.) aqueous fluids with low amount of CO2, CH4, and N2 (≤10?mol% in total) at temperatures of ~350–400°C and pressures of 300–530 bar. Crush-leach data display highly variable and negatively correlated I/Cl and Br/Cl values which are incompatible with both orthomagmatic and/or metamorphic origin of the fluid phase, but can be explained by infiltration of surficial and/or sedimentary fluids. Low fluid salinity indicates a substantial portion of meteoric waters in the fluid mixture that is in accordance with previous stable isotope data. The post-greisenization fluid activity associated with vein formation and argillitization is characterized by decreasing temperature (<350 to <50°C), decreasing pressure (down to ~50–100 bar), and mostly also decreasing salinity. 相似文献
997.
Juan Carlos Carracedo Francisco Pérez Torrado Alejandro Rodríguez González Vicente Soler José Luis Fernández Turiel Valentin R. Troll Sebastian Wiesmaier 《Geology Today》2012,28(2):53-58
Forty years after the Teneguía Volcano (La Palma, 1971), a submarine eruption took place off the town of La Restinga, south of El Hierro, the smallest and youngest island of the Canarian Archipelago. Precursors allowed an early detection of the event and its approximate location, suggesting it was submarine. Uncertainties derived from insufficient scientific information available to the authorities during the eruption, leading to disproportionate civil protection measures, which had an impact on the island's economy—based primarily on tourism—while residents experienced extra fear and distress. 相似文献
998.
Ignacio Díaz-Martínez José María Hernández Salvador García Fernández Xabier Murelaga Félix Pérez-Lorente 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(3):464-470
A new bird ichnotaxon found in Cenicero (La Rioja, Ebro Basin, Spain) is described here. The footprints are preserved in sandstone beds in a central-distal alluvial fringe with a mud-dominated floodplain, located in the transition unit between the Nájera and Haro formations. This level is positioned between the Y and Z local Agenian biozones (lower Miocene). The footprints were preliminarily studied in another work and considered as an indeterminate ichnotaxon. Uvaichnites riojana ichnogen. nov. and ichnosp. nov. is a tridactyl footprint characterized by a prominent central pad, very large to enormous sized, with unjointed toes at the proximal end. These features differ from Aquatilavipes, Aviadactyla, Avipeda, Ludicharadripodiscus, Fuscinapeda, and Ornithotarnocia of the Avipedidae morphofamily. U. riojana is considered to be similar to common crane (Grus grus) footprints in the Gruidae family. There are a few references about this family in the Iberian Peninsula and Balearic Islands and this find could confirm the presence of Gruidae since at least the lower Miocene. 相似文献
999.
Silvia Barredo Farid Chemale Claudia Marsicano Janaína N. Ávila Eduardo G. Ottone Victor A. Ramos 《Gondwana Research》2012,21(2-3):624-636
Extensional processes that followed the Gondwanan Orogeny rise to the development of a series of rift basins along the continental margin over older accreted Eopaleozoic terranes. Stratigraphic, structural, paleontological, and isotopic studies are presented in this work in order to constrain the ages of the sedimentary infilling and to analyze the tectosedimentary evolution of one of the Cuyo basin depocenters, known as Rincón Blanco. This asymmetrical half-graben was filled by continental sediments under a strong tectonic control. The infilling was strongly controlled by tectonics which in term produced distinctive features along the whole sedimentary sequence. Using a combination of lithological and structural data the infilling was subdivided into packages of genetically linked units bounded by regional extended surfaces. Several tuffs and acid volcanic rocks have been collected across the whole section of the Rincon Blanco sub-basin for SHRIMP and LA-MC-ICPMS U–Pb zircon dating. The ages obtained range from 246.4 ± 1.1 Ma to 230.3 ± 1.5 Ma which is the time elapsed for the deposition of three tectono-sequence units separated by regional unconformities and mainly constrained to the Middle Triassic. They are interpreted as a result of a reactivation of the extensional system that has evolved along strike as segments of faults that linked together and/or as laterally propagating faults. Regional correlation with coeval rift basins permits to establish north-south propagation in the extensional regime along the western margin of SW Gondwana. This trending started in the lowermost Triassic and extended until the latest Triassic. Two of them were precisely correlated with Cerro Puntudo and Cacheuta half-graben systems. The new data indicate that the three sequences were mostly deposited during the Middle Triassic (246 to 230 Ma), with no evidence of sedimentation during Norian and Rhaetian, which is in conflict with some previous biostratigraphic studies. 相似文献
1000.
Juan M. Jiménez Kazimierz Więski Laurie B. Marczak Chuan-Kai Ho Steven C. Pennings 《Estuaries and Coasts》2012,35(2):475-485
Top–down and bottom–up effects interact to structure communities, especially in salt marshes, which contain strong gradients
in bottom–up drivers such as salinity and nutrients. How omnivorous consumers respond to variation in prey availability and
plant quality is poorly understood. We used a mesocosm experiment to examine how salinity, nutrients, an omnivore (the katydid
Orchelimum fidicinium) and an herbivore (the planthopper Prokelisia spp.) interacted to structure a simplified salt marsh food web based on the marsh grass Spartina alterniflora. Bottom–up effects were strong, with both salinity and nutrients decreasing leaf C/N and increasing Prokelisia abundance. Top–down effects on plants were also strong, with both the herbivore and the omnivore affecting S. alterniflora traits and growth, especially when nutrients or salt were added. In contrast, top–down control by Orchelimum of Prokelisia was independent of bottom–up conditions. Orchelimum grew best on a diet containing both Spartina and Prokelisia, and in contrast to a sympatric omnivorous crab, did not shift to an animal-based diet when prey were present, suggesting
that it is constrained to consume a mixed diet. These results suggest that the trophic effects of omnivores depend on omnivore
behavior, dietary constraints, and ability to suppress lower trophic levels, and that omnivorous katydids may play a previously
unrecognized role in salt marsh food webs. 相似文献