全文获取类型
收费全文 | 721篇 |
免费 | 12篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 25篇 |
大气科学 | 22篇 |
地球物理 | 144篇 |
地质学 | 240篇 |
海洋学 | 78篇 |
天文学 | 148篇 |
自然地理 | 80篇 |
出版年
2020年 | 9篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 9篇 |
2016年 | 10篇 |
2015年 | 15篇 |
2014年 | 12篇 |
2013年 | 34篇 |
2012年 | 20篇 |
2011年 | 27篇 |
2010年 | 23篇 |
2009年 | 22篇 |
2008年 | 27篇 |
2007年 | 21篇 |
2006年 | 27篇 |
2005年 | 20篇 |
2004年 | 24篇 |
2003年 | 16篇 |
2002年 | 12篇 |
2001年 | 13篇 |
2000年 | 19篇 |
1999年 | 13篇 |
1998年 | 19篇 |
1997年 | 21篇 |
1996年 | 11篇 |
1995年 | 16篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 13篇 |
1983年 | 16篇 |
1982年 | 10篇 |
1981年 | 18篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1977年 | 9篇 |
1975年 | 12篇 |
1974年 | 10篇 |
1973年 | 11篇 |
1972年 | 6篇 |
1971年 | 7篇 |
1966年 | 4篇 |
排序方式: 共有737条查询结果,搜索用时 437 毫秒
181.
182.
Biogeochemical climatologies in the Ross Sea, Antarctica: seasonal patterns of nutrients and biomass
Walker O. Smith Jr. Michael S. Dinniman John M. Klinck Eileen Hofmann 《Deep Sea Research Part II: Topical Studies in Oceanography》2003,50(22-26):3083
The seasonal patterns of nutrient (nitrate and silicic acid) and chlorophyll distributions in the Ross Sea are formulated by two independent methods. The first procedure compiles all available data from cruises from 1970 to the present and generates a three-dimensional grid for the months from November through February using an iterative difference-correction scheme. The second method uses a three-dimensional circulation model and the phytoplankton standing stock climatology to investigate the effects of currents and phytoplankton uptake on nutrient distributions. The two approaches produced similar results, although the circulation model produced distributions that were more variable in space due to its finer resolution. The nutrient distributions were characterized by elevated concentrations in early spring and gradual reductions to ca. 15 and 40 μM (nitrate and silicic acid, respectively) in summer. Nutrient depletion did not occur despite the favorable growth conditions (elevated macronutrient concentrations, strong vertical stratification) in summer, suggesting that an alternative limitation (such as by dissolved iron concentrations) occurs. Chlorophyll concentrations reached ca. 6 μg l−1 in December and declined thereafter. Seasonal primary production calculated from the nitrate deficits and the circulation model suggested that production was ca. 73 g C m−2, slightly lower but similar to other estimates using independent methods. Using the nutrient climatology, losses (vertical flux plus respiration) through Feb. 15 from the upper 100 m were ca. 50% of the seasonal production, and the rest of the organic production was removed after that date. Results also suggest that carbon export from the surface layer may vary significantly in space and time. 相似文献
183.
184.
185.
186.
We discuss the altitude and intensity of the nocturnal green line emission resulting from O2+ recombination. The Doppler profile of this emission is markedly non-thermal, the width being characteristic of the energy at which the O(1S) atoms are produced. A simple theory is formulated which predicts the profile of this line. Experimental evidence on airglow line profiles is reviewed and it is found that the effect predicted here has not yet been observed. However a deliberate search for this effect should yield immediate results. 相似文献
187.
Thomas P OConnor Henry A Walker John F Paul Victor J Bierman 《Marine environmental research》1985,16(2):127-150
It has been proposed that future ocean disposal of sewage sludge from the US east coast be done at a site beyond the edge of the continental shelf. In anticipation of that, a monitoring strategy has been developed to determine the average spatial distribution of contamination. The strategy is an iterative series of measurements developed from models of sludge dispersion and settling which are based on characteristics of the disposal site and sewage sludge. Once disposal is initiated at the site, the strategy requires sampling the upper mixed layer at 36 stations, mostly within 100 km of the site, and deployment of near-bottom sediment traps along a line extending 300 km away from the site. Based on initial results, subsequent sampling locations will be selected to refine estimates of the detectable extent of sludge-derived contamination. The sludge constituents which can be used to detect sludge in water at a dilution of 106 and sludge in sediment traps when diluted by 100 with natural material include zinc, PCBs, coprostanol and spores of the bacterium Clostridium perfringens. Other synthetic organic compounds, besides PCB, may prove to be useful tracers. 相似文献
188.
We frequently forget that there is more to science than the making of a more precise measurement or a more elaborate calculation. It is even more than applying to new problems the methods that worked on old problems. These activities, which keep most of us busy most of the time, are important, but the new and unexpected discoveries are more important. And many radically new discoveries arise from scientific puzzles, the "anomalies." We believe that studies of past climates have exposed an anomaly that deserves attention and that may result in a fundamentally new understanding of the climate system. The poles have been much warmer throughout much of Earth's history than they are now. Ice-age episodes with durations of millions of years have been separated by periods of hundreds of millions of years that have left little or no evidence of polar ice sheets. The data are best for the most recent of these ice-free episodes so we will concentrate on the Eocene. 相似文献
189.
It has been said that concrete is a ‘man‐made rock’ and although the comparison is not perfect, we support this general statement. It is broadly similar in texture to breccias or conglomerates, has a cementitious matrix, has broadly similar physical properties to some sedimentary and igneous rocks and performs somewhat similarly to masonry during its working life in a building or engineering structure. The investigation and testing of concrete have many similarities to those used for the evaluation of building stone. 相似文献
190.
Susan M. Waugh Dominique P. Filippi David S. Kirby Edward Abraham Nathan Walker 《Marine Policy》2012,36(4):933-946
The risk of seabird–fishery interactions in the Western and Central Pacific Ocean (WCPO) was examined by analysing the overlap of seabird distributions with tuna and swordfish pelagic longline fisheries managed by the Western and Central Pacific Fisheries Commission (WCPFC) and its constituent members. The study used spatially-explicit Productivity–Susceptibility Analysis (PSA). Key data inputs were species productivity, fishing effort, likelihood of capture and species density by region. The outputs tailored results to the needs of fisheries- and wildlife-managers, indicating areas of greatest risk of species interactions, species of greatest concern for population impacts, and the flags or fisheries most likely to contribute to the risk. Large albatross species were found to be most likely to suffer population effects when exposed to longline fishing activity, followed by the larger petrels from the genuses Procellaria, Macronectes and Pterodroma. A mixture of coastal states with nesting seabird populations in their Exclusive Economic Zones (New Zealand, Australia and United States of America), distant water fishing nations (Japan, Taiwan) and flags of convenience (Vanuatu) contributed 90% of the risk to seabird populations. Recommendations include enhancing the level of fisheries observer monitoring in areas indicated as high to medium risk for seabird interactions, and consideration of spatial management tools, such as more intensive or more stringent seabird bycatch mitigation requirements in high- to medium-risk areas. The methods used, and similar studies conducted in the Atlantic Ocean could lead to improved targeting of monitoring resources, and greater specificity in the needs for seabird-mitigation measures. This will assist in reducing seabird mortality in longline fishing operations and with more effective use of resources for fishery managers in both domestic fisheries and RFMOs. 相似文献