全文获取类型
收费全文 | 10261篇 |
免费 | 375篇 |
国内免费 | 63篇 |
专业分类
测绘学 | 314篇 |
大气科学 | 744篇 |
地球物理 | 3687篇 |
地质学 | 3301篇 |
海洋学 | 641篇 |
天文学 | 1491篇 |
综合类 | 54篇 |
自然地理 | 467篇 |
出版年
2022年 | 75篇 |
2021年 | 190篇 |
2020年 | 188篇 |
2019年 | 157篇 |
2018年 | 411篇 |
2017年 | 400篇 |
2016年 | 583篇 |
2015年 | 429篇 |
2014年 | 488篇 |
2013年 | 631篇 |
2012年 | 534篇 |
2011年 | 526篇 |
2010年 | 487篇 |
2009年 | 509篇 |
2008年 | 421篇 |
2007年 | 357篇 |
2006年 | 302篇 |
2005年 | 268篇 |
2004年 | 271篇 |
2003年 | 241篇 |
2002年 | 176篇 |
2001年 | 187篇 |
2000年 | 163篇 |
1999年 | 118篇 |
1998年 | 151篇 |
1997年 | 132篇 |
1996年 | 84篇 |
1995年 | 105篇 |
1994年 | 120篇 |
1993年 | 88篇 |
1992年 | 77篇 |
1991年 | 75篇 |
1990年 | 98篇 |
1989年 | 70篇 |
1988年 | 60篇 |
1987年 | 60篇 |
1986年 | 65篇 |
1985年 | 68篇 |
1984年 | 66篇 |
1983年 | 73篇 |
1982年 | 74篇 |
1981年 | 59篇 |
1980年 | 61篇 |
1979年 | 58篇 |
1978年 | 70篇 |
1977年 | 56篇 |
1975年 | 64篇 |
1974年 | 56篇 |
1973年 | 69篇 |
1971年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Roberto Ventura Santos Edilton J. dos Santos João Adauto de Souza Neto Luis Christian Montreuil Carmona Alcides Nóbrega Sial Luis Henrique Mancini Lauro Cézar Montefalco de Lira Santos Gilzênia Henrique do Nascimento Lucas UpaDafubigin Santos Mendes Emerson Marcello Ferreira Anastácio 《Gondwana Research》2013,23(1):380-389
Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India.In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture.A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low 87Sr/86Sr ratios and highly positive δ13C values. Available geochemical data for the upper Paleoproterozoic indicate that the 87Sr/86Sr ratio of ocean water, varying between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks. 相似文献
92.
Olivier J. Hardy Céline Born Katarina Budde Kasso Daïnou Gilles Dauby Jérôme Duminil Eben-Ezer B.K. Ewédjé Céline Gomez Myriam Heuertz Guillaume K. Koffi Andrew J. Lowe Claire Micheneau Dyana Ndiade-Bourobou Rosalía Piñeiro Valérie Poncet 《Comptes Rendus Geoscience》2013,345(7-8):284-296
The biogeographic history of the African rain forests has been contentious. Phylogeography, the study of the geographic distribution of genetic lineages within species, can highlight the signatures of historical events affecting the demography and distribution of species (i.e. population fragmentation or size changes, range expansion/contraction) and, thereby, the ecosystems they belong to. The accumulation of recent data for African rain forests now enables a first biogeographic synthesis for the region. In this review, we explain which phylogeographic patterns are expected under different scenarios of past demographic change, and we give an overview of the patterns detected in African rain forest trees to discuss whether they support alternative hypotheses regarding the history of the African rain forest cover. The major genetic discontinuities in the region support the role of refugia during climatic oscillations, though not necessarily following the classically proposed scenarios. We identify in particular a genetic split between the North and the South of the Lower Guinean region. Finally we provide some perspectives for future study. 相似文献
93.
Weak and strong ground motions were numerically predicted for three stations of the Ashigara Valley test site. The prediction was based on the records from a rock-outcrop station, one weak-motion record from a surface-sediments station, and the standard geotechnical model. The data were provided by the Japanese Working Group on the Effects of Surface Geology as a part of an international experiment. The finite-difference method for SH waves in a 2-D linear viscoelastic medium (a causalQ model) was employed.Comparison with the real records shows that at two stations the predictions fit better than at the third one. Strangely, the two better predictions were for stations situated at larger distances from the reference rock station (one station was on the surface, the other in a borehole). The strong ground motion (the peak acceleration of about 200 cm s–2) was not predicted qualitatively worse than the weak motion (8 cm s–2). A less sophisticated second prediction (not submitted during the experiment), in which we did not attempt to fit the available weak-motion record at the sedimentary station, agrees with the reality significantly better. 相似文献
94.
Dr. T. D. Peterson Dr. S. Esperança Dr. A. N. LeCheminant 《Mineralogy and Petrology》1994,51(2-4):251-276
Summary Early Proterozoic ultrapotassic dikes, lava flows, and pyroclastic rocks of the Christopher Island Formation (CIF) erupted throughout an area 600 × 300 km within the Churchill Province of the Canadian Shield at 1.84 Ga. The rocks range from mafic lamprophyres (mg # 60; SiO2 47–54%, mean K2O/Na2O > 4) with phenocrysts of phlogopite + diopside + apatite ± olivine ± magnetite, to phenocryst-poor felsic rocks and sanidine porphyries (SiO255–69%). All samples have high incompatible element contents and display large depletions of high field strength elements relative to K, Rb, Sr, Ba, and Th. The CIF has geochemical and petrographic characteristics of both minettes and lamproites, but overall most closely resembles young Mediterranean lamproites. Felsic rocks of the CIF were produced by crystal fractionation and crustal contamination of mafic ultrapotassic magma, and include both high-silica lamproites strongly enriched in Zr, U, and Th, and weakly potassic to sodic rocks of trachytic composition. Flows and feeder dikes have relatively homogeneous Nd, 1840 Ma (–6 to –11) but highly variable ES., 1840 Ma (–40 to + 100); samples classified as lamproites have higher average Sr. Dike samples have highly variable present-day Pb isotope compositions, ranging from moderately to strongly nonradiogenic. Geochemical and isotopic data are consistent with contributions from depleted Archean lithospheric mantle, and OIB-type convecting mantle, both metasomatized by subduction-related processes during the Early Proterozoic. The lithospheric mantle probably contained Archean enriched domains as well. Proterozoic enrichment may have accompanied shallow underplating of subducted oceanic lithosphere beneath the Churchill Province during amalgamation of the Laurentian supercontinent. There are strong analogies in isotopic composition, and interpreted source region history, between the CIF and lamproites and minettes of the Wyoming Province and western Greenland, which suggest the existence of a Laurentian ultrapotassic superprovince.
With 7 Figures 相似文献
Geochemie und Entstehung der Proterozoischen ultrapotassischen Gesteine der Churchill Provinz, Kanada
Zusammenfassung Altproterozoische, ultrapotassische Gänge, Lavaströme und pyroklastische Gesteine der Christopher Island Formation (CIF), eruptierten in einem Gebiet von 600 × 300 km in der Churchill Provinz des Kanadischen Schildes vor 1.84 Ga. Die Zusammensetzung dieser Gesteine variiert von mafischen Lamprophyren (mg > 60; SiO2 = 47–54%, durchschnittliches K2O/Na2O > 4) mit Phänokristallent von Phlogopit + Diopsid + Apatit + Olivin + Magnetit, bis zu phänokristallarmen felsischen Gesteinen und Sanidinporphyren (SiO2 = 55–69%). Alle Proben zeigen hohe Gehalte an inkompatiblen Elementen und zeigen beträchtliche Verarmung an high field strength Elementen relativ zu K, Rb, Sr, Ba und Th. Die CIF hat geochemische und petrographische Eigenschaften sowohl von Minetten wie von Lamproiten, aber im allgemeinen ähnelt sie am stärksten jungen mediterranen Lamproiten. Felsische Gesteine der CIF wurden durch Fraktionierung und Krustenkontamination aus mafischen ultrapotassischen Magmen gebildet. Letztere umfassen sowohl siliziumreiche Lamproite, die deutlich an Zr, U und Th angereichert sind und schwach potassische bis sodische Gesteine von trachytischer Zusammensetzung. Lavenergüsse und zufuhrgänge zeigent ein relativ homogenes Nd, 1840 Ma (–6 bis –11) aber ein sehr variables Sr, 1840 Ma (-40 bis + 100); Proben die als Lamproite klassifiziert wurden, zeigent höhere durchschnittliche Sr-Werte. Proben von Gängen haben sehr variable Bleiisotopen-Zusammensetzungen, die von mäßig bis stark nichtradiogen variieren. Geochemische und Isotopendaten weisen auf Beiträge aus verarmtem archaischen lithosphärischen Mantel und aus konvektierendem OIB-Typ Mantel hin, die beide während des Alproterozoikums durch Subduktions-Vorgänge metasomatisiert wurden. Der lithosphärische Mantel enthielt wahrscheinlich auch angereicherte archaische Domänen. Proterozoische Anreicherungsvorgänge dürften seichtes Underplating subduzierter ozeanischer Lithosphäre unter der Churchill Provinz während der Amalgamation des laurentischen Superkontinentes begleitet haben. Es gibt starke Analogien in der Isotopenzusammensetzung und in der interpretierten Geschichte der Ursprungsregion, zwischen den CIF und Lamproiten und Minetten der Wyoming Provinz, und des westlichen Grönland. Diese weisen auf die Existenz einer laurentischen ultrapotassischen Superprovinz hin.
With 7 Figures 相似文献
95.
塔里木盆地是我国最大的含油气盆地,拥有巨大的油气资源潜力。但塔中地区奥陶系岩溶缝洞型储层埋藏深、类型复杂、储层非均质性强,所表现出的岩石地球物理特性不清楚。为了分析典型缝洞型储层模型含不同流体情形下的AVO特征,建立了过玉北1井的缝洞型储层地震地质模型,模型的3个储层段对应的孔隙度分别为10%,6%和8%,分别对应Ⅰ类储层、Ⅲ类储层和Ⅱ类储层,设计了3个储层段都充填气和3个储层段分别充填气、油、水3种不同流体时的2个模型,基于正演模拟共成像点道集记录(CMP),提取截距、梯度、纵横波反射系数及流体因子,采用AVO属性交汇分析方法,寻找油气识别的最佳属性组合,为叠前反演解释提供理论基础。 相似文献
96.
97.
The precipitation/replacement of Ca‐phosphate is a complex process that commonly takes place during the early diagenesis in marine sediments. The unusual occurrence of shallow marine, early diagenetic phosphatic deposits associated with glassy tuffs in the Neogene Gaiman Formation, in the Chubut Province, Patagonia, Argentina, constitutes a good case example for the study of replacement and precipitation of Ca‐phosphate on an unstable substrate. Isocon diagrams illustrate that chemical changes during glass diagenesis include gains in loss on ignition and Ca, and losses of K. These changes are the result of glass hydration during sea water–glass interaction, together with adsorption and diffusion of ions into the bulk shard; combined, these represent an incipient process of volcanic glass replacement by Ca‐phosphate. Subsequent early diagenetic P enrichment in the pore solutions led to phosphate precipitation, associated with pitting on the glass shards and pumice. The associated development of a reactive surface promoted the incorporation of P and Ca into their margins. Lastly, precipitation of calcium phosphate filled the vesicles and other open cavities, inhibiting further glass dissolution. The high porosity and reactivity of the volcanic glass provided an appropriate substrate for phosphate precipitation, leading to the development of authigenic apatite concretions in the volcanic‐glass bearing strata of the Gaiman Formation. This research is of significance for those concerned with marine phosphatic deposits and sheds light on the processes of early diagenetic phosphate precipitation by replacement of an atypical, unstable substrate like hydrated volcanic glass. 相似文献
98.
Problems of dissolution of the so-called secondary copper sulphides are very important for dynamic as well as percolation leaching of ores. This paper deals with dissolution kinetics of sulphides in an alkaline medium, using complex-forming agents in connection with possibility to leach basic, partially oxidized ores. Natural minerals and their polydispersions were investigated and the so-called geometrical model was used for evaluation. Basic dependences of the dissolution rate on the reagent concentration and pH value of the leaching solution were found and an attempt at their mathematical expression and physical interpretation was made. The first part of this work refers to the systems of covellite-ethylene diamine tetra-acetic acid (EDTA) and covellite-ethylene diamine (EDA). The established dependences are not only of theoretical but also of practical importance. 相似文献
99.
Margarita Jambrina-Enríquez Clemente Recio Ildefonso Armenteros 《Environmental Earth Sciences》2018,77(2):49
The present multi-isotopic study (δ18O–δDwater, δ34S–δ18Odissolved-sulphate, δ13Cdissolved-inorganic-carbon, δ13C–δ18Oshells-modern-gastropods, δ13Cplants, and δ13Csedimentary-organic-matter) is aimed at assessing the hydrogeochemical changes and biogeochemical dynamics in a Mediterranean shallow lake fed by a Quaternary–Tertiary aquifer, the “Laguna del Cristo” (NW Iberian Peninsula), a system sensitive to climate fluctuations, between 2010 and 2011. Lake water is of the bicarbonate type, and there are no major pollutants. δ18O-δDwater values plot on a local evaporation line (δD=5.29δ18O–12.29) indicating that evaporative enrichment had a significant impact on lake water isotopic features. Periods of high water levels are characterized by lower δ34S–δ18Odissolved-sulphate and δ13Cdissolved-inorganic-carbon values and suggest sulphate derived from weathering of sulphides in the catchment area, delivered to the lake by surface run-off or via groundwater, and in situ decay of organic matter. During lower water levels, sulphate reduction and enhanced primary productivity lead to higher δ34Sdissolved-sulphate and δ13Cdissolved-inorganic-carbon values. Evaporation induced enrichment in 18Osulphate, 13Cdissolved-inorganic-carbon and 13C–18Oshells-Galba-Gyraulus. δ13Cplant confirms the C3 photosynthetic pathway. Enrichment in 13C submerged aquatic plants indicates that HCO3? is the main carbon source, except for 13C-depleted Potamogeton. The TOC, δ13Corg values, and TC/TN ratios in sediments all confirm the autochthonous character of organic matter contribution. This study provides a baseline for isotopic research into shallow, flow-through lakes fed by siliciclastic aquifers, and stresses the importance of evaporation and refilling (direct precipitation and groundwater discharge) in controlling the solute chemistry and stable isotopic composition in temperate regions with contrasting seasonal climates. The results also provide a snapshot of modern lake isotope variability that can be applied to paleoenvironmental reconstructions. 相似文献
100.
Methylene blue and rhodamine 6G were used as molecular sensors for the spectrophotometric titrations of the aqueous colloids
of clay minerals (montmorillonite, illite and kaolinite). The dyes adsorbed on colloid particles form molecular aggregates,
which exhibit spectral properties significantly different from those of dye solutions. Spectrophotometric titrations provide
the most sensitive detection of smectites in aqueous colloids (sub-ppm concentrations); and the sensitivity further increases
using second derivative spectroscopy. The endpoint of spectrophotometric titrations can be used for the determination of exchange
capacity of the mineral in colloids and in this way to estimate its amount. The method is selective only to expandable clays,
which was proven by experiments with kaolinite and illite. Spectrophotometric titrations have promising future in the analysis
of clays and can be applied in many fields of geology, mineralogy, chemistry, material sciences or in industry. Its application
may expand to the analysis of other nanomaterials built from charged particles and exhibiting metachromasy in the systems
with organic dyes. 相似文献