全文获取类型
收费全文 | 403篇 |
免费 | 13篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 25篇 |
地球物理 | 70篇 |
地质学 | 210篇 |
海洋学 | 66篇 |
综合类 | 4篇 |
自然地理 | 48篇 |
出版年
2024年 | 1篇 |
2023年 | 5篇 |
2021年 | 7篇 |
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 6篇 |
2017年 | 17篇 |
2016年 | 18篇 |
2015年 | 26篇 |
2014年 | 10篇 |
2013年 | 32篇 |
2012年 | 11篇 |
2011年 | 10篇 |
2010年 | 18篇 |
2009年 | 22篇 |
2008年 | 32篇 |
2007年 | 32篇 |
2006年 | 21篇 |
2005年 | 27篇 |
2004年 | 26篇 |
2003年 | 12篇 |
2002年 | 11篇 |
2001年 | 9篇 |
2000年 | 14篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有426条查询结果,搜索用时 15 毫秒
1.
Abstract. The major problem in coastal areas of developing countries is disturbance caused by anthropogenic influence. This disturbance can be quantified by analysing the distribution and composition of marine communities using uni- and multivariate techniques and the biotic index. A study of benthic macrofauna was carried out along the São Sebastião Channel, northern coast of São Paulo State, Brazil, in an area with a submarine outfall, a petroleum terminal and a commercial harbour. Sampling was undertaken seasonally, by means of a van Veen grab (0.1 m2 ) at 15 oceanographic stations, from November 1993 to August 1994. A total of 392 species were identified (129 Polychaeta, 127 Mollusca, 98 Crustacea, 28 Echinodermata and 10 other phyla). The sedimentary pattern recorded for the São Sebastião Channel is very heterogeneous, influenced mainly by strong wind-driven currents, and differs from nearby shelf areas. The study area could be divided into three regions based on sediment texture and fauna: one region dominated by medium and coarse sand with low values of organic carbon; a second area where pelitic fractions were dominant with high values of organic carbon and nitrogen; and a third region characterised by fine and very fine sand, presenting intermediate organic carbon values. No temporal significant variation in abundance and species composition was found, except in autumn. The use of the 'marine Biotic Coefficient' showed the same ecological trend as the faunal abundance and demonstrated that the central continental region of the channel is affected by human activities. 相似文献
2.
Miriam Borba-Roschel Anne Alexandre Anglica Fortes Drummond Chicarino Varajo Jean Dominique Meunier Csar Augusto Chicarino Varajo Fabrice Colin 《Journal of Geochemical Exploration》2006,88(1-3):172
This paper presents the preliminary results of phytolith analyses of a peat located in the cerrado of the Uberaba municipality, State of Minas Gerais, Brazil. The phytolith soil sequence is discussed by comparison with phytolith assemblages extracted from dominant plants (Cyperaceae and Poaceae). Increasing mean age of phytolith assemblages with depth is assumed. Poaceae Cuneiform bulliform cell, Parallelepiped bulliform cell and Elongate smooth long cell types dominate up to 80 cm, rapidly drop to 42% at 80 cm and regularly decrease from 42% to 2% upwards. Cyperaceae Rondel concave type shows the inverse trend, being dominant in the upper part of the profile. This pattern can be assigned to increasing selective dissolution of the Cyperaceae phytolith type with depth, or/and to a decrease of water stress suffered by the grasses, leading to a decrease of bulliform cell silicification. Soil processes and paleo-environmental changes hypotheses are discussed. 相似文献
3.
E.P. Oliveira S.F. Toteu M.N.C. Araújo M.J. Carvalho R.S. Nascimento J.F. Bueno N. McNaughton G. Basilici 《Journal of African Earth Sciences》2006,44(4-5):470
The Yaoundé belt (Cameroon) and the Sergipano belt (NE Brazil) belonged to a major and continuous Neoproterozoic orogen at the northern margin of the ancient Congo-São Francisco craton. The Yaoundé belt comprises schists, quartzites, gneisses and migmatitic gneisses grouped into three domains; the low-grade Mbalmayo Group in south and the medium- to high-grade Yaoundé and Bafia Group in north. The Sergipano belt is divided into six domains, the three southernmost of which are mostly made up of clastic and chemical metasedimentary rocks whereas the others are more diverse with a migmatite–gneiss complex, and two metavolcanicplutonic complexes. In general, the two belts show structural vergence and decrease of metamorphic grade towards the craton; three main deformation phases are recognized in the Sergipano belt in contrast with two described in the Yaoundé belt. The minimum age of Pan-African-Brasiliano collision in the Sergipano belt is constrained at 628 ± 12 Ma on syn-collision granites, whereas in the Yaoundé belt collision took place between 620 and 610 Ma, i.e. the age of granulite facies metamorphism. Sm–Nd isotope geochemistry and U–Pb age dating indicate that most clastic metasedimentary rocks in both belts were derived from sources to the north and, to a lesser degree, from the cratons to the south. 相似文献
4.
5.
Maria da Glria M. Garcia Mrio C. Campos Neto Anthony E. Fallick 《Journal of South American Earth Sciences》2003,15(8):871-883
Oxygen isotope studies were carried out across units of a Neoproterozoic nappe system, south of São Francisco Craton. A temperature decrease toward the base of the system is found, consistent with a previously recognized inverted metamorphic pattern. The tectonic contact of the basal unit and the reworked southern São Francisco craton show a steep temperature gradient, suggesting that low temperature thrusting acted as the dominant tectonic process. The contrasts between the δ18O values of the Três Pontas-Varginha and Carmo da Cachoeira nappes and the differences among the samples and minerals are consistent with the preservation of sedimentary isotopic composition during metamorphism. The small differences in the δ18O values between the undeformed and the deformed calc-silicate samples (1.6‰) suggest that the δ18O value of mylonitization fluids was close to that which equilibrated with the metamorphic assemblage. The distinct δ18O values of metapelitic and calc-silicate samples and the great temperature difference from one type to the other indicate that no large-scale fluid interaction processes occurred during metamorphism. Oxygen isotopic estimations of both Três Pontas-Varginha undeformed rocks and Carmo da Cachoeira unaltered equivalents indicate δ18O values of up to 18‰. Comparison between these values and those from the ‘basement’ orthogneisses (8.3–8.5‰) indicates the latter are not sources for the metapelites. 相似文献
6.
The use of GPR and VES in delineating a contamination plume in a landfill site: a case study in SE Brazil 总被引:3,自引:0,他引:3
Jorge L. Porsani Walter M. Filho Vagner R. Elis Fisseha Shimeles Joo C. Dourado Helyelson P. Moura 《Journal of Applied Geophysics》2004,55(3-4):199-209
This paper presents the results of the application of the Ground Penetrating Radar (GPR) method, or Georadar, in outlining a zone of contamination due to solid residues at the waste burial site of Rio Claro in the state of São Paulo, SE Brazil. A total of eight GPR profiles with 50- and 100-MHz antennae were surveyed. Six profiles were located within the landfill site and the remaining two were outside. The main objective of the GPR survey was to evaluate the side extension of contamination. A Vertical Electric Sounding (VES) survey was performed at four points within the site in order to investigate the vertical extent of the contamination plume and to define the bottom of the landfill. Two additional VESs were done outside the landfill with the purpose of determining the top of the ground water table and the geoelectric stratigraphy of the background. From the interpretation of the GPR profiles, it was possible to locate the top of the contamination plume and to infer that it was migrating laterally beyond the limits of the waste disposal site. This was observed along the profile situated close to the highway SP-127, which was about 20 m from the limit of the site. The signature of the contaminant appears as a discontinuous reflector that is believed to be a shallow ground water table. The discontinuity is marked by a shadow zone, which is characteristic of conductive contaminant residues. The contamination did not move far enough to reach a sugar cane plantation located at approximately 100 m from the border of the site. In the regions free from contamination, the ground water table was mapped at approximately 10 m of depth, and it was characterized by a strong and continuous reflector. The radar signal penetrated deep enough and enabled the identification of a second reflector at approximately 14 m deep, interpreted as the contact between the Rio Claro and the Corumbataí formations. The contact is marked by the presence of gravel characterized by ferruginous concretes, which cause the strong amplitude reflection in the GPR profile. Within the landfill site, the quantitative interpretation of the VES results showed the contamination zone. The base of the landfill varies between 11 and 15 m deep. Outside the landfill site, the VES results showed no indication of contamination and allowed the determination of the top of the ground water table and the contact between the Rio Claro and the Corumbataí formations. The results of GPR and VES showed a good agreement and the integrated interpretations were supported by local geology and information from several boreholes, about 17 m depth, on average. The bottom of the landfill reaches a maximum of 14.5 m depth. 相似文献
7.
Chemical analyses suggest that the metavolcanic rocks of the Almas Greenstone Belt (AGB), Tocantins State, Brazil have a continental affinity, possibly related to a continental rift environment. They were metamorphosed to amphibolite facies during a regional tectono-metamorphic event (Dn), retrogressed to greenschist facies assemblages and then hydrothermally altered within dextral strike–slip shear zones (Dn+1). Fracture sets related to Dn+2 intersect Sn+1.The Paiol Gold Mine is one of several mineralised zones within metabasic and meta-intermediate rocks of the AGB. It exploits shoots of sulphide–Au–quartz mineralisation that occupy dilational zones approximately perpendicular to an elongation lineation (Ln+1) within mylonitic foliation Sn+1 (Sn+1=S within the S–C fabric). The dilational zones probably formed due to dextral displacement on sinistrally en echelon C surfaces. Minor amounts of gold may have been introduced or remobilised during Dn+2.Coexisting primary and pseudosecondary fluid inclusions in mineralised quartz veins from ore shoots comprise a high-salinity three-phase type (Type II) and a lower salinity two-phase type (Type I). Homogenisation temperatures for Type II inclusions range from 200 to 410 °C and Type I from 90 to 320 °C. The inclusions and their temperature ranges are believed to reflect heat exchange and some mixing between the two fluid types under relatively constant ambient temperatures, but variable (though broadly declining) fluid temperatures. This took place late in Dn+1 in conjunction with greenschist facies retrogression and localised hydrothermally induced metasomatism. 相似文献
8.
Bruce J. MacFadden 《Quaternary Research》2005,64(2):113-124
The toxodont megaherbivores Toxodon and Mixotoxodon were endemic to South and Central America during the late Quaternary. Isotopic signatures of 47 toxodont teeth were analyzed to reconstruct diet and ancient habitat. Tooth enamel carbon isotope data from six regions of South and Central America indicate significant differences in toxodont diet and local vegetation during the late Quaternary. Toxodonts ranged ecologically from C3 forest browsers in the Amazon (mean δ13C = −13.4‰), to mixed C3 grazers and/or browsers living either in C3 grasslands, or mixed C3 forested and grassland habitats in Honduras (mean δ13C = −9.3‰), Buenos Aires province, Argentina (δ13C = −8.7‰), and Bahia, Brazil (mean δ13C = −8.6‰), to predominantly C4 grazers in northern Argentina (δ13C = −4.4‰), to specialized C4 grazers in the Chaco of Bolivia (δ13C = −0.1‰). Although these toxodonts had very high-crowned teeth classically interpreted for grazing, the isotopic data indicate that these megaherbivores had the evolutionary capacity to feed on a variety of dominant local vegetation. In the ancient Amazon region, carbon isotope data for the toxodonts indicate a C3-based tropical rainforest habitat with no evidence for grasslands as would be predicted from the Neotropical forest refugia hypothesis. 相似文献
9.
Christian Brannstrom 《Geoforum》2005,36(2):257-271
The expansion of modern agriculture in developing countries presents numerous challenges for environmental policy makers. Environmental policies for agriculture in north-eastern Brazil’s soybean belt are analyzed, with emphasis on the role of a non-state actor in leading policy reforms. An organization representing large farmers is leading policy reforms to reduce the environmental impacts of modern agriculture. By contrast, state agencies are relatively weak and ineffective. The analysis situates this question conceptually in literatures stressing the political and structural causes of corporate environmentalism and literatures explaining the increasingly strong role of non-state actors in environmental governance. The case study focuses on the content of reforms, reasons why the non-state actor is so prominent, and the implications of policy reforms. State agencies face major challenges in environment-agriculture policy debates in places where environmental subsidies are unlikely, environmental information is poor, and organized private interests are influential. 相似文献
10.
The chemical composition and Rb–Sr ages of mica, feldspar, and whole rock samples from the emerald mineralisation of Capoeirana and Belmont, from emerald-barren pegmatites and of the Borrachudos granitic gneiss, Monlevade banded and granitic gneisses from the area of Nova Era–Itabira–Ferros (Minas Gerais, Brazil) as well as from the Guanhães gneiss (Minas Gerais, Brazil) have been determined. The Borrachudos granitic and Monlevade banded gneiss with connected pegmatitic schlieren and pegmatite veins, as well as the Guanhães gneiss, got their actual textures and mineralogical composition at about 1.9 Ga in the context of the Transamazonic tectonothermal event.The Monlevade banded gneiss belongs to a volcano-metasedimentary greenstone belt sequence, which is the typical country rock of the emerald occurrences. The main emerald-forming event at Belmont and Capoeirana was a metasomatic reaction of Be-rich anatectic pegmatites with Cr-rich ultrabasic rocks during the Transamazonic event. At Capoeirana, K–feldspar-bearing Be-rich pegmatites were transformed during the emerald-forming process into plagioclase–quartz rocks. Rb–Sr ages on biotite of about 480 Ma from the emerald mineralisation result from the rejuvenation of Transamazonic biotite by the Brasiliano event.The widespread macroscopically unmetamorphosed pegmatites of the study area formed in the Brasiliano event at 477±14 Ma. These pegmatites resulted to be emerald-barren although the differentiation degree, as given by diagrams such as Cs vs. K/Rb for muscovite and K–feldspar, starts from ceramic and ends with Be pegmatites. Some pegmatites display a marked internal differentiation.