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   177篇
  免费   8篇
  国内免费   4篇
测绘学   1篇
大气科学   22篇
地球物理   62篇
地质学   56篇
海洋学   20篇
天文学   18篇
自然地理   10篇
  2023年   1篇
  2022年   1篇
  2021年   6篇
  2020年   7篇
  2019年   2篇
  2018年   7篇
  2017年   8篇
  2016年   9篇
  2015年   8篇
  2014年   7篇
  2013年   9篇
  2012年   9篇
  2011年   15篇
  2010年   13篇
  2009年   9篇
  2008年   7篇
  2007年   6篇
  2006年   5篇
  2005年   5篇
  2004年   6篇
  2003年   7篇
  2002年   4篇
  2001年   3篇
  2000年   4篇
  1999年   1篇
  1998年   2篇
  1997年   8篇
  1995年   1篇
  1994年   2篇
  1992年   1篇
  1991年   2篇
  1990年   1篇
  1986年   2篇
  1984年   3篇
  1982年   2篇
  1981年   3篇
  1979年   2篇
  1978年   1篇
排序方式: 共有189条查询结果,搜索用时 15 毫秒
181.
The Foresta ichnosite is well known for preserving some of the oldest human fossil footprints recorded in Europe so far. This research aims to: i) describe new footprints that are larger than those already reported, some of which form a new trackway that moves in the opposite direction to all the others; ii) announce the discovery of some stone tools also in the surroundings of the Foresta ichnosite. The new results increase the total number of human fossil footprints to at least 81, specify the direction and the number of footprints of Trackway C, and identify three new directions of walking at the site. More compelling and complete estimates of the dimensional range of all ichnological evidence enables us, furthermore, to estimate the number of trackmakers walking on the trampled surface as a minimum of five, one of them likely being an adult male. The general shape of all the recorded footprints suggests that the Foresta trackmakers share some similarities with those at Sima de los Huesos, and belong to the same taxonomical group as the Ceprano skull. All the new evidence enables us to better understand the presence of hominin populations in the Roccamonfina volcano area during the Middle Pleistocene. © 2020 John Wiley & Sons, Ltd.  相似文献   
182.
Summary. The trans-neptunian objects (TNOs) constitute a new class of solar system object that was discovered only recently to exist beyond the orbit of Neptune. About 400 trans-neptunian objects have been detected over the past nine years and more than ten new objects are being discovered every month. All of the TNOs known to date fit into three dynamical classes: the classical, the resonant and the scattered objects. The total mass of the TNOs currently orbiting the Sun is estimated from the observed luminosity distribution to be of the order of 10–20% of the Earth's mass. However, theoretical investigations of the formation and evolution of the trans-neptunian belt into its currently observed shape suggest that it was much more massive in the past. The physical characterisation of TNOs starts to reveal some of the basic properties of these objects, such as size, shape and rotation and provides a first glance into the diversity of their surfaces. TNOs cover a very diverse range of colours, possibly reflecting different surface compositions. First evidence for the presence of water ice was found in a spectrum of one TNO while others do not show the characteristic absorption bands. The TNOs are now regarded as the likely source of some short-period comets. Owing to giant-planet and collisional perturbations, some TNOs may evolve into Centaurs, i.e. objects orbiting the Sun in the region between Jupiter and Neptune, which are further perturbed to become Jupiter-family short-period comets. Together with smaller debris generated by collisional shattering, the TNOs might represent a belt that has evolved from a more massive circumstellar disc into its present structure. Received 15 May 2001 / Published online 5 October 2001  相似文献   
183.
In the Boyal? area, northern Turkey, the tectonic units of the ?stanbul–Zonguldak Terrane and the IntraPontide suture zone are thrust over the deposits at the top of the Sakarya Terrane, known as Tarakl? Flysch. It consists of Early Maastrichtian–Middle Paleocene turbidite and mass-gravity deposits, whose source mainly corresponds to the ?stanbul–Zonguldak Terrane, and, with a lesser extent, to the IntraPontide suture zone. These deposits were sedimented in a foredeep basin developed during the convergence between Sakarya and Eurasian continental microplates. In the Late Paleocene–Early Eocene time span, the Tarakl? Flysch was deformed (D1 phase) during the closure of the foredeep basin. In the Miocene time, the strike-slip tectonics (D2 phase) related to the North-Anatolian fault produced further deformations of the Tarakl? Flysch.  相似文献   
184.
A review of the levels of polycyclic aromatic hydrocarbons (PAH) and tributyltin (TBT) in sediments from Mobile Bay, Alabama, shows that overall levels of PAH decrease from the upper bay (RI) to the lower bay (RIV). Analysis of PAH ratios indicate that pyrogenic sources such as fossil fuel combustion are dominant. There are so few data on TBT in sediments that spatial trends cannot be identified.Levels of PAHs in oyster tissues decrease from the upper bay (RI) to the lower bay (RIII). Although there is no significant temporal trend for PAH levels in bivalves for the US; in the bay, levels in Region II (RII) have decreased significantly and are currently below the national median. There is no trend in Region III (RIII), where levels remain below RII and the national median. RI and RIV had too few data for trend analysis.Mean levels of TBT in oyster tissues decrease from the upper bay (RI) to the lower bay (RIII). Temporal trend analysis indicates a significant decreasing trend for TBT in the US but no significant trend was detected for RII and RIII in Mobile Bay. TBT levels in RII are consistently higher than levels in the US and RIII, which are similar. Data from RI are too limited for analysis and there are no data for RIV.  相似文献   
185.
The Fulani of West Africa have increasingly migrated to humid West Africa since the Sahelian droughts of the 1970s, drawing opposition from indigenous groups that are concerned about the deprivation of their livelihood and monopolization of resources for herders’ livestock. This research, which involved farmers, shepherds, and Fulani herders, addressed three issues: the nature of indigenous grazing and watering regimes, shepherd–herder relations, and herders’ impact on resource access for indigenes’ livestock. The research indicates that in-community and off-community platforms for resource access exist. The former has not been impacted by the herders; the latter has experienced changes insufficient to constitute an overhaul of resource access at the expense of indigenes’ livestock. Instead, a “shepherd–herder” category of resource access has emerged that is characterized by negotiation for resource space, opening avenues for addressing broader resource access issues, and typically tense indigene–herder relationships.  相似文献   
186.
A better understanding of biological systems can only be gained if we understand what processes are important and how they operate to determine the distribution of organisms. Coastal orientation and depth can influence environmental conditions, including the degree of water motion and availability of light, which in turn may influence the horizontal and vertical patterns of organism distribution. Here, we used a mixed‐model design to examine the effects of coastal orientation and depth on the structure of benthic assemblages by comparing the abundance and distribution of macroalgae and invertebrates in shallow and deep waters on the opposing coasts of São Miguel. Generally, coastal orientation had little influence on the distribution of most taxa. In contrast, significant differences were generally associated with depth, although patterns were spatially variable at the scale of locations. This study suggests that depth, and processes operating at the scale of location, but not at the scale of the coast, have an important influence on these assemblages, and that failure to recognise such a scale of variability may hamper our ability to better understand the processes that structure these communities.  相似文献   
187.
The Southern region of Portugal is subjected to several forms of over-erosion. Most leached products, mainly composed of fine particles containing nutrients, metals or pesticides, are easily transported by river flows. When these are hindered by a physical barrier such as a dam, the particulate load accumulates on the bottom of the reservoirs, often leading to a pronounced decrease of water quality. Bottom sediments from three reservoirs were subjected to grain-size analysis and a study of clay minerals by X-ray diffraction. Most sediments contain a diverse set of clay minerals, mostly illites, smectites, chlorites and kaolinites. The nature of the clay minerals reflects the nature of the parent rocks. During the cycles of transport and temporary deposition, they may undergo significant chemical and physical transformations, which lead to an increase of expandable properties and therefore, to a higher cationic exchange capacity, determining its important role as vehicles of environmental pollutants.  相似文献   
188.
The Puigseslloses megalith tomb (Barcelona, Spain) is located on the summit of a hill in the Vic Plain where the sedimentary rocks that were used in its construction do not crop out. A sedimentological and petrological characterization of the monument's slabs and a multidisciplinary analysis of the outcropping materials in nearby areas has allowed the identification of three possible source areas associated with the Paleogene Banyoles Marls Formation and the Folgueroles Sandstone Formation and allows us to propose possible routes by which the slabs were transported from the catchment site to the place where the monument was built. In addition, the lithology and position of different slabs within the megalith denote the intention to separate the chamber from the corridor by using different types of slab. The position of the monument also reveals that tomb location was more of a priority than proximity to the slab sources. © 2010 Wiley Periodicals, Inc.  相似文献   
189.
This paper presents a morphological and hydrogeological reconstruction of the Murcia Valley at the location of the great Roman stadium Circus Maximus in Rome. We reconstruct a valley segment using ERT (electrical resistivity tomography) and geoarchaeological drilling data that identified three main layers. The basal layer, with high resistivity values and convex shapes, is correlated to alluvial gravel and lithified silt‐clay sediments. The middle layer shows low‐to‐medium resistivity values extending to concavities between the basal convex shapes. The very low resistivity values of this middle layer characterize elliptical to circular morphologies and have been ascribed to the presence of water‐saturated clay‐silt and peaty sediments. The surface layer is characterized by widespread lateral inhomogeneity interpreted as anthropogenic fill. The data indicate a pre‐Roman anastomosed alluvial plain subsequently modified by human intervention. In an effort to reclaim the valley for construction of the Circus, the Romans utilized the natural topography and created a central embankment, later becoming the Spina, by filling depressions with sand taken from adjacent bars. Our study contributes to (1) knowledge of the pre‐Roman landscape, (2) understanding anthropogenic modification of the Murcia Valley flood plain, and (3) archaeological interpretation of the monument.  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