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   154篇
  免费   14篇
  2024年   1篇
  2023年   4篇
  2021年   3篇
  2020年   6篇
  2019年   7篇
  2018年   21篇
  2017年   18篇
  2016年   17篇
  2015年   12篇
  2014年   4篇
  2013年   31篇
  2012年   4篇
  2011年   5篇
  2010年   6篇
  2009年   3篇
  2008年   3篇
  2007年   3篇
  2005年   1篇
  2004年   1篇
  2003年   2篇
  2002年   3篇
  2001年   1篇
  2000年   3篇
  1999年   3篇
  1997年   2篇
  1989年   2篇
  1984年   1篇
  1983年   1篇
排序方式: 共有168条查询结果,搜索用时 140 毫秒
51.
How did traditional farming transform the natural environment in the Hawaiian Islands? This question is one that has largely been addressed for rainfed farming of crops like sweet potato (Ipomoea batatas), but evidence is lacking for irrigated farming of the critical staple crop taro (Colocasia esculenta). We describe the results of soil nutrient and sedimentological analyses of deeply-stratified pondfield deposits representing a 600-year-long record of irrigated taro farming in the North Kohala District, Hawai‘i Island. Soil is categorized by particle size to determine modes of transport and deposition, and concurrent soil nutrient analyses were conducted to infer shifts in the source of sediments and changes associated with taro harvesting. The advent of farming is clearly detectable in sedimentology, the presence of charcoal found within sediments, and soil chemistry. However, diminished nutrient concentrations can be attributed largely to deposition of a mixture of upstream sediments. Overall, there is no clear evidence for nutrient draw-down by taro harvesting, but we cannot yet rule it out as a factor. This study demonstrates the inherent difficulty of correlating changes in soil nutrients evident in irrigated pondfields with the long term history of soil nutrient cycling.  相似文献   
52.
Two equid species have been documented in the Pleistocene of the Iberian Peninsula, the horse Equus caballus, and the Eurasian hemione Equus hydruntinus. While the former survived the Holocene–Pleistocene until now, the timing for the extinction of the latter is unclear. Scarce, fragmented archaeological remains assess the presence of small equids living in the Holocene of Iberia. Those could possibly correspond to the Eurasian hemione although unambiguous morphological identification is often not possible. With the find of an equid tooth from Leceia, a Chalcolithic fortified site in Portugal, and using both morphological and mitochondrial genome analyses, we demonstrate for the first time the presence of a new equid species in Holocene Iberia, namely a donkey (Equus asinus). Radiocarbon dating of the tooth to Cal 2340–2130, and 2080–2060 BC with 95% probability, demonstrates that donkeys were present in Iberia well before the arrival of Phoenicians in the first quarter of the first millennium BC (900–750 years BC), which were considered so far as the first who introduced donkeys in the region.  相似文献   
53.
The Castro of Vila Nova de São Pedro (VNSP) is an emblematic settlement located at Azambuja, Portuguese Estremadura. It was occupied during the third and second millennia BC, predominantly during the Chalcolithic period. A diversified collection of 53 copper-based artefacts (most part in a fragmentary condition), belonging to an extensive metallic collection recovered during excavations carried out in VNSP, was studied using micro-EDXRF spectrometry, optical microscopy and SEM-EDS. Additionally, Vickers microhardness measurements were performed to establish the effectiveness of the thermomechanical treatment in the hardness of the artefacts. Results show that the Largo do Carmo, artefact collection is mainly composed of copper or arsenical copper, being 37% of the artefacts made of copper alloyed with arsenic (As > 2%). A statistically significant association was found between copper alloyed with arsenic and artefacts classified as tools/weapons (arrowheads, daggers and knives). In several cases, the presence of arsenic rich phases in the microstructure, resulting from an inverse segregation phenomenon, shows no evidence of chemical homogeneity control during the artefact manufacture. Microstructural analyses also show that the majority of this group (73%) was shaped with forging plus annealing operation cycles and 23% of the artefacts received a final cold hammering after the forging and annealing. An association between the presence of a final forging treatment and artefacts presenting higher arsenic contents was also identified. Nevertheless, no direct correlation was found between the arsenic content of the alloy and its hardness. Also no direct correlation was found between the hardness and a final forging operation. However, it was observed that a harder forging was applied to the cutting edge of the artefacts and consequently a high hardness in this area was obtained despite the arsenic content of the alloy. Concerning arsenical copper alloys, all evidences point out that the potential for obtaining a harder material was not recognized by the ancient metallurgists and the selection of the alloy was possibly made based on colour.  相似文献   
54.
Martinelli, A.G., Bogan, S., Agnolin, F.L., Ribeiro, L.C.B., Cavellani, C.L., Ferraz, M.L.F. & Teixeira, V.P.A., iFirst article. First fossil record of amiid fishes (Halecomorphi, Amiiformes, Amiidae) from the Late Cretaceous of Uberaba, Minas Gerais State, Brazil. Alcheringa, 1–9. ISSN 0311-5518.

The first fossil amiid fishes (Halecomorphi, Amiiformes) from the Late Cretaceous Marília Formation (Bauru Group) at Uberaba County, Triângulo Mineiro region (Minas Gerais State, Brazil), are described. The material includes some partial maxillae, a dermopterotic, a cleithrum, several vertebral centra and teeth. Features such as the absence of a supramaxillary notch on the dorsal edge of the maxilla, a wide and deep pit on the maxilla for the articulation of the premaxilla, anterior portion of the maxilla with a sub-circular cross-section, teeth with acrodine cup with strong mesial and distal keels, among others, permit confident referral of the material to the Subfamily Vidalamiine (Amiidae), previously recognized in Lower Cretaceous strata of northeasthern Brazil. These specimens constitute the first Late Cretaceous record of this group in Brazil and one of the few in South America.

Agustín G. Martinelli [agustín_martinelli@yahoo.com.ar], Centro de Pesquisas Paleontológicas Llewellyn Ivor Price, Complexo Cultural e Científico Peirópolis (CCCP/UFTM), BR-262, Km 784, Bairro Peirópolis, Uberaba, Minas Gerais, Brazil; Sergio Bogan [sergiobogan@yahoo.com.ar], Fundación de Historia Natural ‘Félix de Azara’, Departamento de Ciencias Naturales y Antropología, CEBBAD—Universidad Maimónides, Hidalgo 775 piso 7 (1405BDB), Buenos Aires, Argentina. Federico Agnolín* [fedeagnolin@yahoo.com.ar], Sección Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’, Av. Ángel Gallardo 470 (C1405BDB), Buenos Aires, Argentina. Luiz Carlos Borges Ribeiro [lcbrmg@terra.com.br], Camila Lourencini Cavellani [camila@patge.uftm.edu.br], Mara Lúcia da Fonseca Ferraz [mara@patge.uftm.edu.br] and Vicente de Paula Antunes Teixeira [vicente@patge.uftm.edu.br], Centro de Pesquisas Paleontológicas Llewellyn Ivor Price, Complexo Cultural e Científico Peirópolis (CCCP/UFTM), BR-262, Km 784, Bairro Peirópolis, Uberaba, Minas Gerais, Brazil. *Also affiliated with: Fundación de Historia Natural ‘Félix de Azara’, Departamento de Ciencias Naturales y Antropología, CEBBAD—Universidad Maimónides, Valentín Virasoro 732 (C1405BDB), Buenos Aires, Argentina. Received 15.4.2012; revised 11.6.2012; accepted 20.6.2012.  相似文献   
55.
56.
This article describes the dynamical behavior of free-standing block structures under seismic loading. A comprehensive experimental investigation has been carried out to study the rocking response of four single blocks of different geometry and associations of two and three blocks. The blocks, which are large stones of high strength blue granite, were subjected to free vibration, and harmonic and random motions of the base. In total, 379 tests on a shaking table were carried out in order to address the issues of repeatability of the results and stability of the rocking motion response. Significant understanding of the rocking motion mechanism is possible from the high quality experimental data. Extensive experimental measurements allowed to discuss the impulsive forces acting in the blocks and the three-dimensional effects presented in the response.  相似文献   
57.
Performance-based earthquake engineering methodologies often require a probabilistic model of structural demand. Since observations masking the probability distribution of the majority of the data are frequently found, robust estimation methods are proposed to estimate the probabilistic model parameters (i.e., central value and dispersion). The performance of thirty-three robust dispersion estimators is evaluated, for different sample sizes, using the chord rotation, curvature, shear force, and inter-story drift demands obtained after analyzing five reinforced concrete structures under real earthquake records scaled to several intensities. Based on the results, combinations involving dispersion and central value (defined in a companion article) estimators are proposed.  相似文献   
58.
Over the course of the 1960s, Portugal, Rhodesia, and South Africa, hard-pressed by African nationalist movements and international opinion, grew closer together, realising that their common enemies could only be defeated by a common stance. The most important attempt to meet the threat to white political domination in Southern Africa was Exercise ALCORA, a military understanding negotiated, in secret, in October 1970. From then until the Portuguese Revolution in 1974, regular meetings of the representatives of the three countries’ armed forces pooled intelligence and defined new strategies for the on-going conflicts in Rhodesia, Angola, and Mozambique, and put in train plans for future, larger-scale clashes. This article examines the origins of ALCORA, charting the process by which Lisbon, Salisbury, and Pretoria came together despite considerable obstacles. It highlights the importance of domestic factors, notably in South Africa, ALCORA's senior partner. There, the murder in 1966 of Prime Minister Hendrik Verwoerd led to a circumstance wherein leading figures of the regime enjoyed much greater freedom in the definition of policy than before. One beneficiary was Defence Minister P. W. Botha who, with the army's backing, would develop a total strategy against what was perceived as a total threat. Exercise ALCORA was a key component of this strategy.  相似文献   
59.
In the last few decades many studies have underlined the role of philhellenism in shaping the Risorgimento as a transnational movement. But philhellenism also had a significant impact in embodying the pre-unification Italian state in a new imaginative framework, marked by the binary oppositions of civilized/barbarian, liberal/despotic and north/south. This article analyses the influence of philhellenism in shaping the imagery of the Kingdom of Sardinia in the eighteenth and nineteenth centuries and the role played by this local imagery in promoting opposing patriotisms within the national political sphere in the years leading up to the unification of Italy. Cavour’s moderate party stressed the positive impact of the Piedmontese domination over the island of Sardinia in order to underline Piedmont's image as a force of modernization of a land strongly marked by feudal despotism and pastoral violence. By contrast, Mazzini and the democrats also deployed a philhellenic narrative scheme to explain the backwardness of Sardinia with reference to the despotic and ‘oriental’ character of the Piedmontese domination in an attempt to encourage opposition to unification under the leadership of the subalpine government.  相似文献   
60.
Landscapes of the Ghost Dance: A Cartography of Numic Ritual   总被引:1,自引:0,他引:1  
Recent historical and ethnographic research indicates that the study of ritual behavior could be greatly enhanced by combining parameters of place and landscape use with interpretation of material culture. This strategy is especially useful for identifying the archaeological record of ritual among societies that incorporated topographic features and natural resources into their liturgical order. In this article we apply a behavioral framework to the study of Numic ritual technologies. By intergrating ethnographic and historic data on the geography, practice, and material culture of the nineteenth-century Nevada Ghost Dance, we demonstrate how this framework helps to determine the configuration of a ritual place and its position relative to other ritual and nonritual places.  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