首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1235篇
  免费   78篇
  国内免费   14篇
测绘学   56篇
大气科学   199篇
地球物理   284篇
地质学   455篇
海洋学   76篇
天文学   151篇
综合类   15篇
自然地理   91篇
  2024年   3篇
  2023年   6篇
  2022年   6篇
  2021年   29篇
  2020年   47篇
  2019年   31篇
  2018年   51篇
  2017年   40篇
  2016年   72篇
  2015年   52篇
  2014年   61篇
  2013年   81篇
  2012年   76篇
  2011年   93篇
  2010年   81篇
  2009年   90篇
  2008年   83篇
  2007年   55篇
  2006年   48篇
  2005年   45篇
  2004年   34篇
  2003年   37篇
  2002年   15篇
  2001年   23篇
  2000年   12篇
  1999年   8篇
  1998年   12篇
  1997年   6篇
  1996年   6篇
  1995年   8篇
  1994年   12篇
  1993年   6篇
  1992年   3篇
  1991年   8篇
  1990年   6篇
  1989年   9篇
  1988年   8篇
  1987年   3篇
  1986年   5篇
  1985年   3篇
  1984年   9篇
  1982年   4篇
  1981年   3篇
  1980年   5篇
  1979年   4篇
  1978年   4篇
  1973年   3篇
  1970年   2篇
  1969年   2篇
  1967年   3篇
排序方式: 共有1327条查询结果,搜索用时 62 毫秒
991.
992.
ABSTRACT

In this study, water level models are constructed to characterize mangrove flooding across urban gradients in Puerto Rico. The most urban sites exhibited 95% longer hydroperiods, 23% lower flood frequencies, and 110% lower depths than the least urban sites. Rainfall importance was explained more by geomorphology and tidal connectivity than by urbanization, but there was evidence for changes in tidal amplitudes along the urban gradient. Relationships between surface water chemical metrics and land cover contradicted previous studies by suggesting lower nutrients and biochemical oxygen demand with increasing urbanization. However, much of this changed with the exclusion of potential outlier sites, as well as under different statistical comparisons. These results reinforce the understanding that the most important drivers of urban mangrove hydrology and water quality in Puerto Rico are likely geomorphology and tidal connectivity, with some influence from surrounding land cover. Results should be considered alongside the reported errors stemming from digital elevation and rainfall response models.  相似文献   
993.
994.
995.
996.
Considerable controversy has been generated by the observation that the Earth's climate has warmed over the last century. Public policy decisions hinge on the question of whether this trend is natural climate variability or the result of the increase in atmospheric concentrations of greenhouse gases. The strength of the enhanced greenhouse effect depends, in large part, on the uncertain value of climate sensitivity. In this paper climate sensitivity is estimated from the global temperature record by assuming models for greenhouse forcing, climate response to forcing, and climate variability. We find optimal estimates of climate sensitivity are remarkably insensitive to assumptions, at least for forcing excluding the effect of aerosols, and these values are considerably less than most predictions arising from General Circulation Models (GCM's). It is, however, the statistical significance of these estimates that is sensitive to assumptions about climate variability. Assuming climate variability with a time scale of a decade or less, climate sensitivity is estimated to be significantly greater than zero, but also significantly lower than that predicted by GCM's. Climate variability with a century time scale is consistent with both the recent temperature record and the pre-instrumental record for the last millenium; if this type of variability is assumed, the estimate of climate sensitivity has a confidence band wide enough to encompass both zero and typical values obtained by GCM's. With century time-scale variability it will be several decades before confident estimates can be made.  相似文献   
997.
Deng  Xuejie  Liu  Hao  Feng  Jianye  Li  Yu  de Wit  Benjamin  Yang  Yinchao 《Geotechnical and Geological Engineering》2021,39(8):5983-5992
Geotechnical and Geological Engineering - This paper presents a mining technology that enables the continuous conversion from top coal caving (TCC) to multi-slice mining (MSM). The technology helps...  相似文献   
998.
In both nature and synthetic experiments, the common iron oxide haematite (α‐Fe2O3) can incorporate significant amounts of U into its crystal structure and retain radiogenic Pb over geological time. Haematite is a ubiquitous component of many ore deposit types and, therefore, represents a valuable hydrothermal mineral geochronometer, allowing direct constraints to be placed on the timing of ore formation and upgrading. However, to date, no suitable natural haematite reference material has been identified. Here, a synthetic haematite U‐Pb reference material (MR‐HFO) is characterised using LA‐ICP‐MS and ID‐TIMS. Centimetre‐scale ‘chips’ of synthesised α‐Fe2O3 were randomly microsampled via laser ablation‐extraction and analysed using ID‐TIMS. Reproducible U/Pb and Pb/Pb measurements were obtained across four separate chips (n = 13). Subsequently, an evaluation of the suitability MR‐HFO in constraining U‐Pb data via LA‐ICP‐MS is presented using a selection of natural samples ranging from Cenozoic to Proterozoic in age. The MR‐HFO normalised U‐Pb ratios are more concordant and ages more accurate versus the same LA‐ICP‐MS spot analyses normalised to zircon reference material, when compared with independently acquired ID‐TIMS data from the same natural haematite grains. Results establish MR‐HFO as a suitable reference material for LA‐ICP‐MS haematite U‐Pb geochronology.  相似文献   
999.
The evolution of melt segregation in deforming partially moltenolivine-rich rocks has been studied in a series of laboratoryexperiments. During deformation, melt segregates into networksof anastamosing channels (or ‘bands’) surroundinglenses of melt-depleted material. We quantify the nature ofthe melt distribution in the samples, including thickness, angle,spacing, volume fraction, and melt fraction of melt-rich bands,to understand the dynamics of melt-network organization. Twoseries of experiments were designed to isolate the effects of(1) increasing shear strain (at similar stress levels), and(2) varying stress levels (deformed to similar shear strains).Melt-rich bands develop by a shear strain of unity. In samplesdeformed at varying stress levels, higher stress produces smallercharacteristic band spacings. We relate these variations tothe compaction length, c, which varies only as a result of thereduction of matrix viscosity with increasing stress. Simpleapproaches to scaling from experimental to mantle conditionssuggest that stress-driven melt segregation can occur in theasthenosphere; if so, it will significantly affect rheological,transport and seismic properties, with enticing consequencesfor our understanding of plate–mantle interactions. KEY WORDS: melt segregation; rock rheology; magma transport; self-organization; mid-ocean ridges  相似文献   
1000.
Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO2 calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO2. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO2-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号