全文获取类型
收费全文 | 1054篇 |
免费 | 53篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 15篇 |
大气科学 | 47篇 |
地球物理 | 243篇 |
地质学 | 300篇 |
海洋学 | 97篇 |
天文学 | 256篇 |
综合类 | 5篇 |
自然地理 | 145篇 |
出版年
2023年 | 6篇 |
2021年 | 16篇 |
2020年 | 17篇 |
2019年 | 17篇 |
2018年 | 28篇 |
2017年 | 26篇 |
2016年 | 40篇 |
2015年 | 30篇 |
2014年 | 26篇 |
2013年 | 54篇 |
2012年 | 39篇 |
2011年 | 37篇 |
2010年 | 47篇 |
2009年 | 59篇 |
2008年 | 41篇 |
2007年 | 43篇 |
2006年 | 42篇 |
2005年 | 37篇 |
2004年 | 54篇 |
2003年 | 36篇 |
2002年 | 34篇 |
2001年 | 35篇 |
2000年 | 25篇 |
1999年 | 13篇 |
1998年 | 21篇 |
1997年 | 18篇 |
1996年 | 9篇 |
1995年 | 13篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 11篇 |
1991年 | 6篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1987年 | 8篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 11篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 11篇 |
1979年 | 15篇 |
1978年 | 13篇 |
1977年 | 7篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1973年 | 7篇 |
1972年 | 8篇 |
1969年 | 5篇 |
1968年 | 7篇 |
排序方式: 共有1108条查询结果,搜索用时 46 毫秒
301.
Since the 1950’s the population structure of the Atlantic menhaden,Brevoortia tyrannus, has undergone dramatic changes. High rates of fishery exploitation during the 1960’s resulted in a drastic decline in the breadth of the age structure and numbers of spawning aged fish. The stock recovered under continued exploitation during the 1970’s in spite of these unfavorable conditions. Hence, the length and age at first spawning, the seasons and areas of spawning, and the potential number of ova produced as a function of length were reexamined during the 1978, 1979, and 1981 fishing seasons. No changes were detected in the reproductive biology of Atlantic menhaden. The size at first spawning, areas, and season of spawning were similar to results of previous studies. The minimum fork length of potential spawners was 180 mm. Few age-1 fish were mature but most females were mature at age-2. Analysis of the ovaries collected along the coast indicated some spawning occurred in the spring, a limited amount occurred in the summer, and most spawning occurred during the fall and winter. There was high intra-and interyear variability in the relationship between length and potential number of ova produced, and regression coefficients encompassed values from two previous studies, each based on a single fishing year. Data from this and the previous studies were combined and equally weighted to generate a representative predictive equation for the potential number of ova spawned as a function of length: eggs = 2563e0.0150(FL). 相似文献
302.
303.
Estuaries and Coasts - Effects of the herbicide, atrazine, on the submersed vascular plant,Potamogeton perfoliatus, were monitored for 4 wk in 700 l microcosms containing water, sediments and... 相似文献
304.
The satellites of the outer solar system planets are thought to be mixtures of ices and rocky material, in which decay of radioactive nuclides can lead to internal melting and solid-state convection. Time-dependent models indicate that melting will reach its maximum extent approximately 2.0 GYr after formation; bodies of radius <500 km will never melt, and those <750 km in radius will be totally refrozen by present. Surface water flows are not expected for bodies of <1500-km radius. However, even small (100 km) bodies may be unstable against solid-state convection, and their surfaces may show signs of tectonism. Other processes altering the surfaces include sublimation and photolysis of ices. Sublimation likely explains the absence of CH4 ices on any Saturnian satellite except Titan; photolysis explains the absence of NH3 ices on these bodies, and possibly the absence of water ice on the surface of Callisto. The photolysis rate of CH4 also implies a crustal reservoir of CH4 on Titan. 相似文献
305.
Helen Couclelis 《Transactions in GIS》2003,7(2):165-175
Considerable effort has been devoted over the years to fighting uncertainty in geographic information in its different manifestations. Thus far, research on handling inaccuracy, fuzziness, error and related issues has focused for the most part on problems with spatial data and their direct products, typically representations of spatial objects or fields. This paper seeks to broaden the discussion of uncertainty in the geospatial domain by shifting the focus from information to knowledge. It turns out that there is a surprising number of things that we cannot know (or questions we cannot answer) that are not the result of imperfect information. Forms of not knowing are pervasive in domains as diverse as mathematics, logic, physics, and linguistics, and are apparently irreducible. This being the case it may help to explore how these realms of ignorance may affect our efforts. The paper distinguishes three different modes or forms of geospatial knowledge production, and argues that each of them has built–in imperfections, for reasons of logical principle and not just empirical fact. While much can and needs to be done to manage and resolve uncertainties where possible, I argue for accepting that uncertainty is an intrinsic property of complex knowledge and not just a flaw that needs to be excised. 相似文献
306.
Helen Fraser David Williams Ian Sims Anita Richards Jeremy Yates 《Astronomy& Geophysics》2003,44(4):4.29-4.33
Surface-catalysed chemical reactions, the molecules they form, and the implications for star formation were key themes in an international workshop on "Solid-state chemistry in star-forming regions" at the Lorentz Centre, Leiden University, the Netherlands from 14–17 April 2003. Helen Fraser, David Williams, Ian Sims, Anita Richards and Jeremy Yates report. 相似文献
307.
308.
Stuart G Wakeham Cynthia M Lewis Stefan Schouten 《Geochimica et cosmochimica acta》2003,67(7):1359-1374
We evaluate anaerobic oxidation of methane (AOM) in the Black Sea water column by determining distributions of archaea-specific glyceryl dialkyl glyceryl tetraethers (GDGTs) and 13C isotopic compositions of their constituent biphytanes in suspended particulate matter (SPM), sinking particulate matter collected in sediment traps, and surface sediments. We also determined isotopic compositions of fatty acids specific to sulfate-reducing bacteria to test for biomarker and isotopic evidence of a syntrophic relationship between archaea and sulfate-reducing bacteria in carrying out AOM. Bicyclic and tricyclic GDGTs and their constituent 13C-depleted monocyclic and bicyclic biphytanes (down to −67‰) indicative of archaea involved in AOM were present in SPM in the anoxic zone below 700 m depth. In contrast, GDGT-0 and crenarchaeol derived from planktonic crenarchaeota dominated the GDGT distributions in the oxic surface and shallow anoxic waters. Fatty acids indicative of sulfate-reducing bacteria (i.e., iso- and anteiso-C15) were not strongly isotopically depleted (e.g., −32 to −25‰), although anteiso-C15 was 5‰ more depleted in 13C than iso-C15. Our results suggest that either AOM is carried out by archaea independent of sulfate-reducing bacteria or those sulfate-reducing bacteria involved in a syntrophy with methane-oxidizing archaea constitute a small enough fraction of the total sulfate-reducing bacterial community that an isotope depletion in their fatty acids is not readily detected. Sinking particulate material collected in sediment traps and the underlying sediments in the anoxic zone contained the biomarker and isotope signature of upper-water column archaea. AOM-specific GDGTs and 13C-depleted biphytanes characteristic of the SPM in the deep anoxic zone are not incorporated into sinking particles and are not efficiently transported to the sediments. This observation suggests that sediments may not always record AOM in overlying euxinic water columns and helps explain the absence of AOM-derived biomarkers in sediments deposited during past periods of elevated levels of methane in the ocean. 相似文献
309.
The 750-Ma Seychelles granites have whole-rock '18O values that range from -1.2 to +7.5. Differences in '18O values between quartz, feldspar and whole-rock for samples that have a range of '18O values suggest that these minerals are in magmatic equilibrium, and that the whole-rock '18O values were little affected by post-crystallization interaction with fluids. Two suites of granites (the Mahé type and the Praslin type) have previously been recognized on the basis of chemical and radiogenic isotope composition. The former have a mean whole-rock '18O value of 5.25ǂ.65 (1C), whereas the latter are much more variable, with a mean whole-rock '18O value of 3.00Dž.27. Biotite and amphibole separates from both granite types range in 'D from -69 to -116, and show a positive correlation with whole-rock '18O values. Dolerite dykes that intruded the granites during or shortly after their crystallization also have whole-rock '18O values (mean 2.24ǃ.93) lower than that expected for mantle-derived basaltic rocks, but these values appear to be the result of fluid-rock interaction. We suggest that the Mahé-type granites are derived mainly from juvenile mafic to intermediate crust with 'normal' 'D and '18O values, whereas the Praslin-type granites are mixtures of this source and older crust that acquired its low 'D and '18O values by extensive interaction with meteoric water at high temperature. It is unlikely that meteoric water sufficiently depleted in D and 18O was available at 750 Ma because of the relatively low latitude of the region at that time. We suggest that alteration of the source took place significantly before production of the granite magmas. Depletion in 18O of the Seychelles granites does not necessarily require a regional extensional tectonic setting at 750 Ma, as has been proposed. 相似文献
310.