全文获取类型
收费全文 | 922篇 |
免费 | 42篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 25篇 |
大气科学 | 84篇 |
地球物理 | 285篇 |
地质学 | 223篇 |
海洋学 | 89篇 |
天文学 | 161篇 |
自然地理 | 102篇 |
出版年
2021年 | 8篇 |
2020年 | 10篇 |
2019年 | 11篇 |
2018年 | 13篇 |
2017年 | 14篇 |
2016年 | 24篇 |
2015年 | 17篇 |
2014年 | 21篇 |
2013年 | 41篇 |
2012年 | 23篇 |
2011年 | 40篇 |
2010年 | 38篇 |
2009年 | 50篇 |
2008年 | 36篇 |
2007年 | 34篇 |
2006年 | 49篇 |
2005年 | 43篇 |
2004年 | 48篇 |
2003年 | 40篇 |
2002年 | 29篇 |
2001年 | 33篇 |
2000年 | 23篇 |
1999年 | 17篇 |
1998年 | 24篇 |
1997年 | 20篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 18篇 |
1993年 | 12篇 |
1992年 | 22篇 |
1991年 | 8篇 |
1990年 | 15篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 9篇 |
1983年 | 16篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1970年 | 2篇 |
排序方式: 共有969条查询结果,搜索用时 342 毫秒
251.
252.
Michael F Hochella Keith D Keefer Bernard H.W.S deJong 《Geochimica et cosmochimica acta》1983,47(11):2053-2058
The crystal chemistry of a naturally occurring magnesium hydroxide sulfate hydrate, (designated MHSH()), and a number of existing and predicted compounds which are structurally related to MHSH() are discussed. An x-ray diffraction powder pattern for MHSH() is calculated from single crystal data to aid in the identification of this phase, recently found associated with active hydrothermal vents along the East Pacific Rise. It is suggested that this MHSH-type crystal structure can have a variable composition due to a novel chemical substitution, Mg2+ → 2 H+, which this particular structure may be able to accommodate. In relation to this, a method for determining the stoichiometry of MHSH crystals via density measurements is also proposed. Finally, the leaching properties of heated seawater, instrumental in the genesis of metalliferous deposits on the sea-floor, may be due in part to the growth of MHSH-type phases in certain seawater-rock systems. 相似文献
253.
We have observed (66652) 1999 RZ253 with the Hubble Space Telescope at seven separate epochs and have fit an orbit to the observed relative positions of this binary. Two orbital solutions have been identified that differ primarily in the inclination of the orbit plane. The best fit corresponds to an orbital period, days, semimajor axis a=4660±170 km and orbital eccentricity e=0.460±0.013 corresponding to a system mass m=3.7±0.4×1018 kg. For a density of the albedo at 477 nm is p477=0.12±0.01, significantly higher than has been commonly assumed for objects in the Kuiper belt. Multicolor, multiepoch photometry shows this pair to have colors typical for the Kuiper belt with a spectral gradient of 0.35 per 100 nm in the range between 475 and 775 nm. Photometric variations at the four epochs we observed were as large as 12±3% but the sampling is insufficient to confirm the existence of a lightcurve. 相似文献
254.
255.
256.
Data use and information creation: challenges for marine scientists and for managers 总被引:1,自引:0,他引:1
In the coastal waters of European countries and in the offshore waters of the north-east Atlantic, there is an increasing need for scientists to meet challenging objectives, such as to identify meaningful measures of 'quality', and to recommend 'indicators' to underpin implementation of directives, conventions, statutes and other more informal national and international initiatives. Those indicators may relate to particular species or habitats, to changes in physical and chemical characteristics, and even to the use to which the system is put. The problems to be overcome are difficult, but new and developing approaches will make a significant contribution. The approaches include: criteria to identify 'sensitivity' and 'importance', structures to organise information and electronic information resources to access data.The real challenge is to make the results of the various scientific initiatives relevant to and understandable by a wide range of customers with similar overlapping requirements, and thus make a genuine contribution to protecting the marine environment. Above and beyond that is the need for scientists to drive the agenda to enable real and lasting progress to be made towards ecosystem-based management of our seas and a proper consideration of what 'sustainability' may mean in the marine environment and how we utilise its resources. 相似文献
257.
Fuel oil #6 is used for the production of electrical power in the United States. Orimulsion is being considered as an alternative fuel, but its value and risk compared to fuel oil #6 need to be assessed. Our study examined the relative impact of accidental spills of the two hydrocarbons on the meiofaunal community. To do so, we maintained microcosms of the shallow, sandy, subtidal environment for three months. Treatment microcosms received a single application of hydrocarbon-coated sand. As indicators of effect, we used copepod and nematode abundance and copepod species diversity, sex ratio, fecundity, age structure, and neutral-lipid content. A comparison of the hydrocarbon treatments showed no significant differences. The tests had adequate power to detect ecologically significant changes. Our results indicate that a spill of Orimulsion would have approximately the same impact as a spill of fuel oil #6 on the meiofauna. 相似文献
258.
The past decade has seen a revival of environmental determinism in palaeoenvironmental research, with palaeoclimatic shifts implicated in the collapse of many past civilizations. Implicit in these studies is a belief that the observed cultural transitions can be causally related to the magnitude of climatic change. However, examination of the processes of these declines suggests that many exhibit patterns characteristic of complexity cascading within self-organized systems. If so, the nonlinear nature of these systems' responses to external forcing means that the assumption of causality in many of these cases should be considered questionable. 相似文献
259.
Recent changes in the cultural and forest landscapes of the Meseta Purépecha in Michoacán, Mexico as a result of forest degradation underscore the complexity of forest change processes in the tropical highlands of Latin America. Differences in community perception and forest structure and composition between the furniture-making and lumber-producing towns of Pichátaro and Sevina, Purépechan indigenous communities located amidst pine forests on Michoacán's volcanic plateau illustrate the dynamics of this process. We base our comparisons on interviews and field measures of forest structure. Our results show dramatic changes in the forests and cultural landscapes of both communities during the past decade. Following high regional timber exports during the early 1990s, Sevina shifted from a self-sufficient to a timber importation community. By comparison, communal forests and individual parcels in Pichátaro continue to provide wood for approximately 300 wood shops. Field data and forest stand maps confirm the perception of forest degradation in both communities. While Pichátaro has maintained a larger and more diverse forest base to date, stand structure data indicate selective harvesting has led to a shift in dominance toward the less economically desirable pine species and oak. Deforestation and degradation of Sevina and Pichátaro's community forests are symptomatic of both the Meseta Purépecha and Mexico in general. Current forest conditions in both communities justify local, regional, and national concerns regarding declining biodiversity and sustainable economies. 相似文献
260.