全文获取类型
收费全文 | 1769篇 |
免费 | 58篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 38篇 |
大气科学 | 159篇 |
地球物理 | 363篇 |
地质学 | 593篇 |
海洋学 | 144篇 |
天文学 | 337篇 |
综合类 | 9篇 |
自然地理 | 193篇 |
出版年
2021年 | 17篇 |
2020年 | 21篇 |
2019年 | 21篇 |
2018年 | 21篇 |
2017年 | 39篇 |
2016年 | 33篇 |
2015年 | 23篇 |
2014年 | 50篇 |
2013年 | 72篇 |
2012年 | 39篇 |
2011年 | 64篇 |
2010年 | 61篇 |
2009年 | 88篇 |
2008年 | 64篇 |
2007年 | 59篇 |
2006年 | 68篇 |
2005年 | 53篇 |
2004年 | 75篇 |
2003年 | 80篇 |
2002年 | 56篇 |
2001年 | 45篇 |
2000年 | 43篇 |
1999年 | 49篇 |
1998年 | 49篇 |
1997年 | 26篇 |
1996年 | 33篇 |
1995年 | 28篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 27篇 |
1991年 | 18篇 |
1990年 | 21篇 |
1989年 | 21篇 |
1988年 | 18篇 |
1987年 | 21篇 |
1986年 | 14篇 |
1985年 | 21篇 |
1984年 | 37篇 |
1983年 | 22篇 |
1982年 | 25篇 |
1981年 | 29篇 |
1980年 | 17篇 |
1979年 | 25篇 |
1978年 | 24篇 |
1977年 | 25篇 |
1976年 | 17篇 |
1975年 | 29篇 |
1974年 | 19篇 |
1973年 | 15篇 |
1972年 | 12篇 |
排序方式: 共有1836条查询结果,搜索用时 15 毫秒
11.
Roger T. Trindell 《The Professional geographer》1969,21(5):328-332
12.
13.
14.
Scaling analysis of deposition from turbidity currents 总被引:3,自引:0,他引:3
Many oil-bearing sedimentary deposits are formed by the settling of particles from turbidity currents. Modeling sedimentary processes that form these turbidites enables the calculation of properties such as extent, depth, porosity and permeability of hydrocarbon-bearing reservoirs. This paper estimates the extent and thickness of turbidites from the initial conditions of the turbidity flow. This is achieved by the application of scaling analysis of the partial differential equations that govern the dynamics of and deposition from turbidity currents. We apply the results of scaling analysis to five modern submarine fans. The predicted and actual values of the dimensions of the fan deposits match well. We then compare the derived results against tabulated sizes of ancient turbidites. The comparisons are good as long as we correctly identify the flow regimes in which the deposition took place. The good agreements observed in the two cases show that the estimates obtained using scaling analysis can provide useful first-guess values for the dimensions of the deposits. 相似文献
15.
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
16.
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
17.
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
18.
The measurement of enzymic activity in plankton communities is useful in the study of marine ecosystems. Such measurements can lead to a clearer understanding of the biological transformations in plankton communities at a particular time. However, the assays are somewhat time-consuming. To facilitate analysis of large numbers of samples, we have developed a method of quick-freezing of whole cells and of cell-free extracts of the diatom, Skeletonema costatum, followed by storage at ?60°C for up to one week. No loss in either the electron transport system (ETS) or the glutamate dehydrogenase (GDH) activity occurs. Similar conditions of storage can be used for the preservation of ETS activity in the marine copepod Calanus pacificus. Also, no measurable loss in either the GDH or the ETS activity is detectable after the quick-frozen whole cells of S. costatum have been kept frozen for over a year. 相似文献
19.
20.
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