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   552篇
  免费   32篇
  国内免费   6篇
测绘学   13篇
大气科学   37篇
地球物理   123篇
地质学   219篇
海洋学   59篇
天文学   76篇
综合类   1篇
自然地理   62篇
  2023年   1篇
  2022年   2篇
  2021年   5篇
  2020年   14篇
  2019年   14篇
  2018年   13篇
  2017年   19篇
  2016年   25篇
  2015年   26篇
  2014年   20篇
  2013年   24篇
  2012年   28篇
  2011年   38篇
  2010年   42篇
  2009年   39篇
  2008年   25篇
  2007年   32篇
  2006年   24篇
  2005年   26篇
  2004年   28篇
  2003年   20篇
  2002年   13篇
  2001年   8篇
  2000年   13篇
  1999年   6篇
  1998年   15篇
  1997年   8篇
  1996年   4篇
  1995年   4篇
  1994年   6篇
  1993年   8篇
  1992年   2篇
  1991年   4篇
  1990年   3篇
  1989年   4篇
  1988年   2篇
  1987年   4篇
  1985年   3篇
  1984年   4篇
  1983年   2篇
  1981年   2篇
  1980年   2篇
  1979年   2篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
排序方式: 共有590条查询结果,搜索用时 31 毫秒
121.
Paleomagnetic results obtained from over 2100 cores sampled at 132 early Pliocene to late Pleistocene sites in the Coastal Range of eastern Taiwan indicate that, since the late Pliocene, the margin of the Philippine Sea plate has undergone a clockwise rotation of about 30° as a direct consequence of the Plio-Pleistocene collision of this plate with the Chinese Continental plate. The rotation is diachronic and started between 3 and 4 Ma ago in the northern Coastal Range then propagated southward at an average speed of the order of 70 ± 10 km/Ma. This value is in very good agreement with estimates of the southward propagation of the collision between the Philippine Sea plate and the Eurasian margin in Taiwan. It is suggested that the rotation reflects the deformation of the Philippine Sea plate at depth and does not results from the shallow deformations involved in the building of the Coastal Range orogen.  相似文献   
122.
The eastern end of the Haylayn massif exposes a complex paleoridge structure interpreted as the tip of a northwestward propagating segment (Nicolas et al., this issue). The area, revisited from a petrostructural and geochemical viewpoint, offers the most documented exposures of the association of olivine gabbros and gabbronorites in Oman (Juteau et al., 1988). Gabbronorites were injected while the main gabbro unit was deforming in the magmatic state. Both units do not differ chemically, except for the SiO2 enrichment of the orthopyroxene-rich gabbros relative to olivine-gabbro. In addition, they display the same trace element signature, which implies the same parent magma for both units. The extension of the stability field of orthopyroxene is assigned to increase of oxygen fugacity due to hydration. The source of hydration is the ridge axis hydrothermal circulation, suggesting hydrothermal/magma interaction at temperatures above the gabbro solidus. The distribution of gabbronorites at the scale of the entire ophiolite suggests a relation with ridge tectonics where high-T conditions of hydrothermal-magmatic interaction are met. Such conditions are met when propagating segments rotate the structures of the dying magma chamber.  相似文献   
123.
The timing of diapause, or suppressed development, in the dominant California Current copepod Calanus pacificus was examined in two ways. Diapause timing was inferred from changes in the abundance of deep, diapausing C. pacificus over a full year at a station off southern California, USA. Jaw phase and ecdysteroid content, indicators of molt cycle status in C. pacificus, were also examined to determine if shifts toward early molt phases, as are observed in diapausing Calanus, were also found among surface C. pacificus during periods when the abundance of deep, diapausing C. pacificus was increasing. The abundance of diapausing C. pacificus in deep water increased from June to mid-October, then declined from October to March. The percentage of surface-living C. pacificus CV with postmolt jaws, the earliest jaw phase, was significantly higher from June to mid-October than at other times. The ecdysteroid content of surface-living C. pacificus CV was also significantly lower during June to mid-October than during late October to May. Both of these changes indicate a shift to earlier molt phases in surface-living C. pacificus CV during periods of onset of diapause. However, the molt status of surface CV was variable from June to mid-October, suggesting that preparation for diapause was spatially or temporally heterogeneous. Hypotheses about environmental cues that induce diapause were evaluated, although the effect of cues on possibly sensitive stages could not be considered. The abundance of diapausing C. pacificus increased during a period of warm upper water column temperature and generally high and declining photoperiod; however, evaluation of the cues inducing diapause was inconclusive.  相似文献   
124.
The stable isotopic (2H/1H and 18O/16O) composition of precipitation has been used for a variety of hydrological and paleoclimate studies, a starting point for which is the behaviour of stable isotopes in modern precipitation. To this end, daily precipitation samples were collected over a 7‐year period (2008–2014) at a semi‐arid site located at the Macquarie Marshes, New South Wales (Australia). The samples were analysed for stable isotope composition, and factors affecting the isotopic variability were investigated. The best correlation between δ 18O of precipitation was with local surface relative humidity. The reduced major axis precipitation weighted local meteoric water line was δ 2H = 7.20 δ 18O + 9.1. The lower slope and intercept (when compared with the Global Meteoric Water Line) are typical for a warm dry climate, where subcloud evaporation of raindrops is experienced. A previously published model to estimate the degree of subcloud evaporation and the subsequent isotopic modification of raindrops was enhanced to include the vertical temperature and humidity profile. The modelled results for raindrops of 1.0 mm radius showed that on average, the measured D‐excess (=δ 2H ? 8 δ 18O) was 19.8‰ lower than that at the base of the cloud, and 18% of the moisture was evaporated before ground level (smaller effects were modelled for larger raindrops). After estimating the isotopic signature at the base of the cloud, a number of data points still plotted below the global meteoric water line, suggesting that some of the moisture was sourced from previously evaporated water. Back trajectory analysis estimated that 38% of the moisture was sourced over land. Precipitation samples for which a larger proportion of the moisture was sourced over land were 18O and 2H‐enriched in comparison to samples for which the majority of the moisture was sourced over the ocean. The most common weather systems resulting in precipitation were inland trough systems; however, only East Coast Lows contributed to a significant difference in the isotopic values. Copyright © 2016 Australian Nuclear Science and Technology Organisation. Hydrological Processes. © 2016 John Wiley & Sons, Ltd.  相似文献   
125.
Reducing greenhouse gas emissions from deforestation and forest degradation (REDD) is likely to be central to a post-Kyoto climate change mitigation agreement. As such, identifying conditions and factors that will shape the success or failure of a reduced deforestation scheme will provide important insights for policy planning. Given that protected areas (PAs) are a cornerstone in forest conservation, we draw on interviews and secondary data to analyze the effects of available PA resources, governance ability, the level of community involvement, and provincial deforestation rates on land-cover change in nine PAs in Panama. Our results illustrate that coupling surveillance measures with greater funding and strong governance are paramount to reducing deforestation. Alone, however, these factors are insufficient for forest protection. We argue that conservation approaches that complement effective surveillance with community participation and equitable benefit sharing will address the wider issues of leakage and permanence.  相似文献   
126.
127.
Two sediment cores (Southern Branch, PC-1, and Western Branch, WB-2) were taken from the highly industrialized Elizabeth River, Virginia. The concentrations of trace metals cadmium, cobalt, chromium, copper, nickel, lead and zinc, major elements iron, manganese and aluminum, organic carbon content and the specific surface area of the sediments were determined in each of the cores. Down-core variations in metals varied significantly in each core with maximum contamination events occurring at different times in different portions of the river. In PC-1, maximum metal concentrations were seen after the appearance of (137)Cs. In contrast, the highest levels in WB-2 occurred well before the appearance of (137)Cs. Although stricter environmental regulations have caused a decrease in metal concentrations since the 1980s, the concentrations in the surface sediments of many trace metals were elevated to levels 2-5 times higher than the levels at the bottom of the cores in both the Southern and Western Branches of the river.  相似文献   
128.
People living in the northeast of Thailand interchangeably label themselves and are labelled by others as Isan, Thai Isan, Lao Isan, Thai or Lao, depending on the ethnic, political, social or familial nuances of any given situation. I use the term Lao Isan to refer specifically to Isan people of Lao origin or ethnicity. Lao Isan are subject to complex and often competing notions of Isan-ness, Lao-ness and Thai-ness by insiders and outsiders. Using data derived from an ethnographic study of popular Thai television and Lao Isan youth (aged 17–25) living in the city of Khon Kaen and the town of Mahasarakham in northeast Thailand in 2002, this paper explores contemporary and co-existing interpretations of Isan identity or Khwampenisan among Lao Isan youth in relation to self-image and connections with national understandings of physical beauty as they are perpetuated in Thai-produced television programs.
Catherine Hesse-SwainEmail:
  相似文献   
129.
We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multicluster system at   z ∼ 0.165  has been the subject of an 80-orbit F606W Hubble Space Telescope (HST) /Advanced Camera for Surveys (ACS) mosaic covering the full     span of the supercluster. Extensive multiwavelength observations with XMM–Newton , GALEX, Spitzer , 2dF, Giant Metrewave Radio Telescope and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star formation rate, nuclear activity and stellar mass. In addition, with the multiwavelength data set and new high-resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of an environment we will be able to evaluate the relative importance of the dark matter haloes, the local galaxy density and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction and creation of a master catalogue. We perform the Sérsic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star formation rates for this field. We define galaxy and cluster sample selection criteria, which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues.  相似文献   
130.
Abstract— We describe the results of a variety of model calculations for predictions of observable results of the LCROSS mission to be launched in 2009. Several models covering different aspects of the event are described along with their results. Our aim is to bracket the range of expected results and produce a useful guide for mission planning. In this paper, we focus on several different questions, which are modeled by different methods. The questions include the size of impact crater, the mass, velocity, and visibility of impact ejecta, and the mass and temperature of the initial vapor plume. The mass and velocity profiles of the ejecta are of primary interest, as the ejecta will be the main target of the S‐S/C observations. In particular, we focus on such quantities as the amount of mass that reaches various heights. A height of 2 km above the target is of special interest, as we expect that the EDUS impact will take place on the floor of a moderate‐sized crater ?30 km in diameter, with a rim height of 1–2 km. The impact ejecta must rise above the crater rim at the target site in order to scatter sunlight and become visible to the detectors aboard the S‐S/C. We start with a brief discussion of crater scaling relationships as applied to the impact of the EDUS second stage and resulting estimated crater diameter and ejecta mass. Next we describe results from the RAGE hydrocode as applied to modeling the short time scale (t 0.1 s) thermal plume that is expected to occur immediately after the impact. We present results from several large‐scale smooth‐particle hydrodynamics (SPH) calculations, along with results from a ZEUS‐MP hydrocode model of the crater formation and ejecta mass‐velocity distribution. We finish with two semi‐analytic models, the first being a Monte Carlo model of the distribution of expected ejecta, based on scaling models using a plausible range of crater and ejecta parameters, and the second being a simple model of observational predictions for the shepherding spacecraft (S‐S/C) that will follow the impact for several minutes until its own impact into the lunar surface. For the initial thermal plume, we predict an initial expansion velocity of ?7 km s?1, and a maximum temperature of ?1200 K. Scaling relations for crater formation and the SPH calculation predict a crater with a diameter of ?15 m, a total ejecta mass of ?106kg, with ?104kg reaching an altitude of 2 km above the target. Both the SPH and ZEUS‐MP calculations predict a maximum ejecta velocity of ?1 km s?1. The semi‐analytic Monte Carlo calculations produce more conservative estimates (by a factor of ?5) for ejecta at 2 km, but with a large dispersion in possible results.  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