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   164篇
  免费   14篇
  2023年   2篇
  2022年   2篇
  2021年   4篇
  2020年   4篇
  2019年   6篇
  2018年   3篇
  2017年   5篇
  2016年   10篇
  2015年   7篇
  2014年   12篇
  2013年   64篇
  2012年   8篇
  2011年   4篇
  2010年   6篇
  2009年   7篇
  2008年   3篇
  2007年   3篇
  2006年   5篇
  2005年   2篇
  2004年   2篇
  2003年   4篇
  2002年   2篇
  2001年   2篇
  2000年   1篇
  1999年   2篇
  1998年   1篇
  1997年   2篇
  1996年   1篇
  1993年   2篇
  1983年   1篇
  1980年   1篇
排序方式: 共有178条查询结果,搜索用时 15 毫秒
111.
鲁顺元 《攀登》2007,26(2):144-148
柴达木盆地自然生态环境十分脆弱,这种状况不但由其禀赋所决定,更与近50多年来人类在盆地开展的大规模生产活动不无关系。该时段的大部分时间,盆地内人与自然的关系基本上是对立的,从而在很大程度上导致了其自然生态环境更加脆弱并向恶化的方向持续变迁,已经严重威胁到该区域人类正常的生产生活。因此,在统筹柴达木盆地各种关系、构建和谐社会时,要更加重视其人与自然关系的和谐,以确保我国生态安全和资源安全。  相似文献   
112.
Mortoniceratid ammonites of the Eromanga Basin of Queensland, although uncommon, are reviewed and reassessed utilising all known collections. Representatives of this group are restricted to the Allaru Formation and almost all specimens are Goodhallites goodhalli, a well-known species from the English Gault. The Allaru Formation overlies the Toolebuc Formation, widely considered to be an essentially isochronous unit because of its unusual sedimentary and geochemical character. Using G. goodhalli, the middle and upper Allaru Formation can be directly correlated with the early late Albian orbignyi and auritus Subzones of the Mortoniceras inflatum Zone as recognized in the reference ammonite zonation embedded in the standard Cretaceous time scale. Overlapping ranges of G. goodhalli and Labeceras and Myloceras allow these common Austral heteromorph genera to be also confidently assigned a late Albian age in Australia, matching their biostratigraphic occurrence in South Africa.  相似文献   
113.
Archaeology has always faced the problem of making informed inferences based on an incomplete record. Zooarchaeological studies of prehistoric hunting and diet offer a clear case in point, where a range of behavioral and taphonomic factors can produce a substantial disconnect between what people actually captured and ate and what archaeologists recover and interpret. We explore this disconnect by presenting stable C and N data for wild faunas, archaeological maize, and three human burials from Fremont-period sites in southeastern Utah, the United States. We use these data to estimate faunal contributions to prehistoric diets and compare the results with previous zooarchaeological analyses of faunas from the same sites. Results for the two approaches differ sharply, with isotopic estimates showing much higher contributions of small and lowland game. We discuss these results in terms of both local prehistory and wider issues of taphonomy and dietary analysis.  相似文献   
114.
本文分别从个人主义的泛滥与新道德和民主政治构建的难产、理想主义的滥觞与激进主义者实践道路的迷惘、唯科学主义思潮的横行与国人人生观的困惑等视角发掘出误读进化论所产生的近代社会转型问题,希望唤起学界对进化论展开深刻反思,审视它在近代所带来的负面效应.  相似文献   
115.
Dettmann, M.E., Clifford, H.T., Peters, M., June 2012. Emwadea microcarpa gen. et sp. nov.—anatomically preserved araucarian seed cones from the Winton Formation (late Albian), western Queensland, Australia. Alcheringa, 217–237. ISSN 0311-5518.

A new genus and species, Emwadea microcarpa Dettmann, Clifford & Peters, is established for ovulate/seed cones with helically arranged cone scales bearing a centrally positioned, inverted ovule from the basal Winton Formation (late Albian), Eromanga Basin, Queensland. The cones are small, prolate ellipsoidal (9.5–14 mm vertical axis, 6.3–8.7 mm transverse axis) with wedge-shaped cone scales bearing winged seeds attached adaxially to the scale only by tissues surrounding the vasculature entering the ovule. Ovuliferous tissue that is free from the cone scale extends distally from the chalaza; the seeds' lateral wings are derived from the integument. Foliage attached to the cones is spirally arranged, imbricate and with spreading and incurved bifacial blades with acute tips; stomata are arranged in longitudinal files and are confined to the adaxial surface. The cone organization testifies to placement within the Araucariaceae, and is morphologically more similar to Wollemia and Agathis than to Araucaria.

Mary Dettmann [mary.dettmann@qm.qld.gov.au] and Trevor Clifford, Queensland Museum, PO Box 3300, South Brisbane, Q 4101, Australia; Mark Peters, PO Box 366 Gumeracha, SA 5233, Australia. Received 31.3.2011; revised 23.8.2011; accepted 5.9.2011.

  相似文献   
116.
The design of a projectile delivery system often plays a critical role in the durability and breakage patterns associated with spent projectile points. This paper presents the results of an experimental project designed to examine projectile point durability and breakage patterns between three different hafting methods. Specifically, we compare two asphaltum hafting techniques drawn from archaeological and ethnohistoric accounts from the Central Valley of California with a more stereotypical cross-hatched sinew hafting system. Our results suggest a small yet statistically significant increase in durability among asphaltum hafted points, opening the door to future research on these ethnohistorically documented hafting techniques.  相似文献   
117.
Zheng, D., Wang, H., Nel, A., Dou, L., Dai, Z., Wang, B. & Zhang, H. 27 June 2019. A new damsel-dragonfly (Odonata: Anisozygoptera: Campterophlebiidae) from the earliest Jurassic of the Junggar Basin, northwestern China. Alcheringa XX, X–X. ISSN 0311-5518.

A new genus and species of campterophlebiid damsel-dragonfly, Jurassophlebia xinjiangensis gen. et sp. nov., is described from the Lower Jurassic Badaowan Formation in the Junggar Basin, northwestern China. Jurassophlebia differs from all other campterophlebiid genera in having PsA in the same orientation as the distal branch of AA, and in its uniquely open subdiscoidal cell with very acute apical angle in the hind wing. The new discovery adds to the Asian diversity of damsel-dragonflies in the earliest Jurassic.

Daran Zheng* [], He Wang [], Bo Wang [], and Haichun Zhang [], State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, PR China; André Nel [], Institut de Systématique, Évolution, Biodiversité, ISYEB-UMR 7205-CNRS, MNHN, UPMC, EPHE, Muséum national d’Histoire naturelle, Sorbonne Universités, 57 rue Cuvier, CP 50, Entomologie, F-75005, Paris, France; Longhui Dou [], Comprehensive Geology Exploration Team, Xinjiang Coalfield Geology Bureau, West Mountain Road, Ürümqi 830000, PR China; Zhenlong Dai [], No.9 Geological Team, Xinjiang Bureau of Geology and Mineral Resources, Ürümqi 830011, PR China; Daran Zheng also affiliated with Department of Earth Sciences, The University of Hong Kong, Hong Kong Special Administrative Region, PR China.  相似文献   
118.
The effects of the spatial variability of ground motion (loss of coherence, wave passage, and local site conditions) on the response of isolated bridges are investigated.

Therefore, a statistical approach is adopted to represent uncertainties in both the bridge configuration and the ground motion variability. The response of isolated bridges, designed for a standard input motion, under a spatially varying ground motion, is evaluated by nonlinear time-history analyses; the system performance is measured by the displacement demand on isolators.

Results show that the phenomenon affects the structural response considerably; the demand increases for the majority of isolators, irrespective of bridge configuration.  相似文献   
119.
This is a reconnaissance report on the damage to bridges during the 2008 Wenchuan, China, earthquake. Site investigation was conducted by the authors on August 10–14, 2008. Presented is a detailed discussion of the damage to 12 bridges as well as possible damage mechanisms. Characteristics of two near-field ground accelerations and Chinese seismic bridge design practices are also presented. An investigation of the damage finds insufficient intensity of seismic design force, inadequate structural detailing for enhancing the ductility capacity, and an absence of unseating prevention devices.  相似文献   
120.
ABSTRACT

We investigate the inherent characteristics of 2D basins subjected to vertically propagating P-SV waves using the finite difference approach. The ratio of fundamental frequencies between an inhomogeneous basin (f0) and its equivalent flat layers (fh) is found to be a function of both its outer overall geometry and inner geometric formation. 2D resonance can occur in inhomogeneous basins under certain conditions, and the f0/fh can be used to demarcate deep from shallow basin behaviors. With other conditions remaining the same, changing a basin from homogeneity to inhomogeneity results in a lower f0/fh and makes 2D resonance less likely to occur.  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