全文获取类型
收费全文 | 1786篇 |
免费 | 61篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 33篇 |
大气科学 | 170篇 |
地球物理 | 390篇 |
地质学 | 640篇 |
海洋学 | 160篇 |
天文学 | 285篇 |
综合类 | 10篇 |
自然地理 | 172篇 |
出版年
2021年 | 20篇 |
2020年 | 20篇 |
2019年 | 24篇 |
2018年 | 28篇 |
2017年 | 33篇 |
2016年 | 40篇 |
2015年 | 31篇 |
2014年 | 41篇 |
2013年 | 98篇 |
2012年 | 51篇 |
2011年 | 83篇 |
2010年 | 64篇 |
2009年 | 103篇 |
2008年 | 53篇 |
2007年 | 57篇 |
2006年 | 73篇 |
2005年 | 50篇 |
2004年 | 66篇 |
2003年 | 64篇 |
2002年 | 44篇 |
2001年 | 42篇 |
2000年 | 36篇 |
1999年 | 45篇 |
1998年 | 45篇 |
1997年 | 22篇 |
1996年 | 23篇 |
1995年 | 31篇 |
1994年 | 21篇 |
1993年 | 26篇 |
1992年 | 26篇 |
1991年 | 19篇 |
1990年 | 20篇 |
1989年 | 17篇 |
1988年 | 23篇 |
1987年 | 26篇 |
1986年 | 14篇 |
1985年 | 24篇 |
1984年 | 36篇 |
1983年 | 27篇 |
1982年 | 23篇 |
1981年 | 33篇 |
1980年 | 22篇 |
1979年 | 17篇 |
1978年 | 28篇 |
1977年 | 21篇 |
1976年 | 19篇 |
1975年 | 31篇 |
1974年 | 18篇 |
1973年 | 18篇 |
1972年 | 13篇 |
排序方式: 共有1860条查询结果,搜索用时 15 毫秒
21.
Erika Sternberg Catherine Jeandel Juan-Carlos Miquel Beat Gasser Marc Souhaut Roseanna Arraes-Mescoff Roger Francois 《Marine Chemistry》2007,105(3-4):281-295
Biogenic barium, mostly in the barite (BaSO4) form, has been proposed as a tracer for export production in the ocean. Here we report on biogenic barium (Baxs) and particulate organic carbon (POC) fluxes from sediment traps deployed at the DYFAMED site in the Northwestern Mediterranean Sea. Baxs fluxes display average values of 37 ± 45 and 50 ± 58 μg/m2/d at 200 and 1000 m respectively, and are linearly correlated to POC fluxes (mean values of 7.9 ± 9.3 and 6.8 ± 6.8 mg C/m2/d at 200 and 1000 m). Export production estimates, calculated using published Baxs- or POC-based algorithms, all fall below or close to the lower limit of potential export values proposed in the literature. This work clearly demonstrates the usefulness of Baxs as a tracer of oceanic export production in the Northwestern Mediterranean Sea. However, development of a quantitative export production proxy requires a clear understanding of the underlying cause(s) for the observed spatial variations in the relationship between Baxs and POC fluxes. The present study confirms that the processes leading to barite formation differ between margin and open-ocean sites and probably account for much of the regional variability in the POC/Baxs ratio. 相似文献
22.
Myriam Sibuet Luc Floury Anne-Marie Alayse-Danet Andr Echardour Tanguy LeMoign Roger Perron 《Progress in Oceanography》1990,24(1-4)
In order to make progress in the study of processes in deep-sea ecosystems, we have developed a strategy for the use of submersibles in conjunction with free vehicles. The BIOCYAN programme, which was conducted by French oceanographers at a permanent station in the Bay of Biscay, involved the use of submersibles for benthic ecological studies. It lead to the development of a variety of equipment for sampling and experimental studies. The submersible Cyana has a limited equipment workload, so the initial step was to develop a free vehicle which could function as a shuttle to act as an independent transport for equipment to be used from the submersible. Two other devices have been developed for sampling with precision the surficial layers of the sediment, one of which can be used in conjunction with an injection module to measure rates of processes in situ by incubations, following injection with labelled substrates. These devices, the free-lift shuttle, tube corers, a blade box corer and the associated injection module are described. 相似文献
23.
24.
A 120 m thick section of the Cozy Dell Formation (middle Eocene), southem California, consists of a graylaminated mudstone and a tanungraded mudstone; sandstone beds are associated with the laminated mudstone. Sedimentary structures, stratification sequences, foraminiferal
distributions, and composition indicate that the ungraded mudstone is an upper slope hemipelagic deposit, while the laminated
mudstone is an overbank deposit associated with shallow channels or gullies in which the sandstone beds were deposited. This
depositional setting may be analogous to that of the modern Mississippi River delta front. 相似文献
25.
Eurgain H. John Sonia D. Batten Roger P. Harris Graeme C. Hays 《Journal of Sea Research》2001,46(3-4)
Zooplankton sampling has been carried out by the Continuous Plankton Recorder (CPR) survey since the 1930s enabling the study of long-term changes in plankton populations, the elucidation of seasonal patterns of abundance, and more recently providing zooplankton biomass estimates for ecosystem models. Data for zooplankton abundance collected by CPR tows in the Western English Channel (between 1988 and 1998) were compared to vertically integrated samples collected from station L4 off Plymouth, UK. Comparisons were made for locally abundant copepods (including Acartia, Calanus, Para/Pseudocalanus, Centropages, Oithona and Temora) collected by CPR and WP-2 nets. All dominant species recorded at L4 were also common to the CPR data. However, the position of the taxa in the two datasets was not equivalent. Seasonal cycles revealed by CPR data were significantly similar to those recorded throughout the water column at L4 for most taxa. However, absolute levels of abundance differed for the two datasets: abundances were underestimated by CPR samples when compared to those of vertically integrated samples by a factor of between 2 and 35, with the exception of Centropages. The differing mesh sizes (200 and 270 μm) of the WP-2 net and CPR mesh could only partially explain these differences in abundance, implying that the behaviour of individual taxa and their depth in the water column also influenced the abundance recorded. 相似文献
26.
During their early history the Tongue of the Ocean and the Providence Channels were broad, relatively shallow basins flanked by growing carbonate banks. As the Blake-Bahama platform subsided, sedimentation kept pace with subsidence on the banks, but not in these flat-bottomed troughs, thus increasing the relief. At the outer end of the troughs the Blake-Bahama escarpment, bounding the platform on the east, dropped steeply to the abyssal plain. Sediment gravity flows coursing down this escarpment began to erode a valley headward into the flat-bottom ancestral Northeast Providence Channel. As the relief between the banks and the troughs increased, the flows increased in vigor, and some of them were able to move down the troughs and into the headwardly eroding central valley. The rate of headward erosion thus increased, with the result that still more flows found their way into the valley. The head of this valley is now off central Andros Island, about 225 km from its point of origin, and headward erosion is continuing.Study of bathymetric charts, observations made during sixteen dives in the Tongue of the Ocean using the submersible DSRV “Alvin”, and analogies with subaerial geomorphic processes and their products contributed to the development of this model. The model is consistent with available stratigraphic information.It is emphasized that the morphology of the Tongue of the Ocean and the Providence Channels cannot be explained as the result of a single unidirectional process, such as upbuilding alone or erosion alone. Both have occurred and both are still occurring. Upbuilding predominated early in the history of the Bahamas; it is responsible for the high walls. Erosion began later and has been accelerating through time. It is responsible for the central valley. 相似文献
27.
28.
29.
An extremely simple CFD tool is used to compare the calm-water drags of a series of hull forms and to define ‘optimized’ monohull ships for which the total (friction+wave) calm-water drag is minimized. The friction drag is estimated using the classical ITTC formula. The wave drag is predicted using the zeroth-order slender-ship approximation. Comparisons of theoretical predictions and experimental measurements for a series of eight hull forms show that—despite the extreme simplicity of the method that is used here to estimate the friction drag and the wave drag—the method is able to rank the drags of a series of hull forms roughly in accordance with experimental measurements. Thus, the method may be used, with appropriate caution, as a practical hull form design and optimization tool. For purposes of illustration, optimized hull forms that have the same displacement and waterplane transverse moment of inertia as the classical Wigley hull, taken as initial hull in the optimization process, are determined for three speeds and for a speed range. 相似文献