全文获取类型
收费全文 | 78096篇 |
免费 | 14027篇 |
国内免费 | 21218篇 |
专业分类
测绘学 | 6373篇 |
大气科学 | 10508篇 |
地球物理 | 15322篇 |
地质学 | 48251篇 |
海洋学 | 10528篇 |
天文学 | 3756篇 |
综合类 | 5744篇 |
自然地理 | 12859篇 |
出版年
2024年 | 333篇 |
2023年 | 934篇 |
2022年 | 2476篇 |
2021年 | 2758篇 |
2020年 | 3024篇 |
2019年 | 3705篇 |
2018年 | 3007篇 |
2017年 | 3298篇 |
2016年 | 3448篇 |
2015年 | 3845篇 |
2014年 | 4514篇 |
2013年 | 4558篇 |
2012年 | 4953篇 |
2011年 | 5258篇 |
2010年 | 4342篇 |
2009年 | 5168篇 |
2008年 | 5105篇 |
2007年 | 5763篇 |
2006年 | 5593篇 |
2005年 | 4856篇 |
2004年 | 4449篇 |
2003年 | 4155篇 |
2002年 | 3633篇 |
2001年 | 3132篇 |
2000年 | 3109篇 |
1999年 | 2941篇 |
1998年 | 2410篇 |
1997年 | 2124篇 |
1996年 | 1887篇 |
1995年 | 1580篇 |
1994年 | 1578篇 |
1993年 | 1320篇 |
1992年 | 1071篇 |
1991年 | 731篇 |
1990年 | 586篇 |
1989年 | 505篇 |
1988年 | 357篇 |
1987年 | 239篇 |
1986年 | 161篇 |
1985年 | 105篇 |
1984年 | 48篇 |
1983年 | 41篇 |
1982年 | 39篇 |
1981年 | 29篇 |
1980年 | 27篇 |
1979年 | 35篇 |
1978年 | 37篇 |
1977年 | 26篇 |
1975年 | 5篇 |
1954年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
571.
572.
Salt-water inflows into the Baltic Sea are important events for renewing the deep and bottom waters of the deep basins of the Baltic Sea. These events occur only at irregular intervals. The last strong event was in January 1993 followed by minor inflows in winter 1993/1994. As a result of these inflows, the deep water of the central Baltic basins was completely renewed.Based on extensive observations of polycyclic aromatic hydrocarbons (PAHs) in water, fluffy layer material and surface sediments between 1992 and 1998, the transformation of PAHs and the modification of their distribution in the Baltic deep water is discussed in connection with the spreading of the inflowing highly saline and oxygen-rich water along its pathway from the sills into the central basins. In the course of the inflows in 1993/1994, the PAH concentration in the deep water of the different basins increased significantly. The concentrations were elevated, at least by a factor of 2 and as much as seven to eight times (for the four-ring PAHs) compared to the previous and the following years. Two hypotheses for the causes were discussed: the inflowing salt water may have entrained more highly polluted surface water in the western Baltic Sea, or it may have entrained contaminated fluffy layer material or sediment particles along the route of transport. 相似文献
573.
574.
575.
Sea surface temperature variations in the southwestern South China Sea over the past 160 ka 总被引:1,自引:0,他引:1
Sea surface temperatures (SSTs) in the southwestern South China Sea have been reconstructed for the past 160 ka using the Uk37 paleothermometer from the core MD01-2392. The temperature differences between glacial times (MISs 6 and 2) and interglacial times (MISs 5.5 and 1) are 2.2~2.5 ℃. Younger Dryas event during the last deglaciation was documented in both the planktonic foraminiferal δ18O and SST records. After MIS 5.5, SSTs displayed a progressive cooling from 28.6 to 24.5 ℃, culminating at the LGM. During this gradual cooling period, warm events such as MISs 5.3, 5.1 and 3 were also clearly documented. By comparison of SST between the study core and Core 17954, a pattern of low or no meridional SST gradients during the interglacial periods and high meridional SST gradients during the glacial periods was exhibited. This pattern indicates the much stronger East Asian winter monsoon at the glacial than at the interglacial periods. Spectral analysis gives two prominent cycles: 41 and 23 ka, with the former more pronounced, suggesting that SSTs in the southern SCS varied in concert with high-latitude processes through the connection of East Asian winter monsoon. 相似文献
576.
Pyruvate kinase (PK) activity measurements are used to assess the role of carbohydrates in global feeding of mesozooplankton communities inhabiting an estuary plume. As a consequence of a remarkably low freshwater discharge rate, the sea surface layers of the area under estuarine influence showed a very moderate salinity fall and a nearly total depletion in nitrates, whereas higher levels of these nutrients were found in deeper, more saline, layers. Small-scale PK activity variations in mesozooplankton appear to be closely correlated to nitrate integration values within the water column. The results were analysed in comparison with literature reports. The study produced a coherent overall interpretation, which strongly supports the reliability of this new biochemical tool in detecting assimilation of trace carbohydrates in the diet of mesozooplankton. 相似文献
577.
578.
As a fundamental study to evaluate the contribution of the Kuroshio to primary production in the East China Sea (ECS), we
investigated the seasonal pattern of the intrusion from the Kuroshio onto the continental shelf of the ECS and the behavior
of the intruded Kuroshio water, using the RIAM Ocean Model (RIAMOM). The total intruded volume transport across the 200m isobath
line was evaluated as 2.74 Sv in winter and 2.47 Sv in summer, while the intruded transport below 80m was estimated to be
1.32 Sv in winter and 1.64 Sv in summer. Passive tracer experiments revealed that the main intrusion from the Kuroshio to
the shelf area of the ECS, shallower than 80m, takes place through the lower layer northeast of Taiwan in summer, with a volume
transport of 0.19 Sv. Comparative studies show several components affecting the intrusion of the Kuroshio across the 200 m
isobath line. The Kuroshio water intruded less onto the shelf compared with a case without consideration of tide-induced bottom
friction, especially northeast of Taiwan. The variations of the transport from the Taiwan Strait and the east of Taiwan have
considerable effects on the intrusion of the Kuroshio onto the shelf. 相似文献
579.
ABSTRACT Understanding temporal patterns in restored environments is important for identifying potential barriers to recovery and improved management of degraded habitats. In this paper, we use temporal beta diversity analyses to compare invertebrate community recovery trajectories in three restored agricultural stream sites under different integrated catchment plans, a native forest reference site, and two unmodified pasture control stream sites over 24 years. The restored sites diverged from their initial community composition over time and became more similar to the reference site community, which was relatively stable over time. Variation partitioning showed that prior to restoration beta diversity was primarily associated with environmental and spatial drivers, whereas post-restoration beta diversity was more influenced by temporal and environmental drivers, including changes in substrate size, fine sediments, water clarity, and nutrients, as well as temperature and flow regime. Species’ contributions to beta diversity varied between sites and years, with sensitive EPT taxa contributing more in reference and control sites. However, contributions of some EPT species, particularly mayflies, increased in restored sites post-ICM. In summary, after nearly two decades of ICM, restored stream sites show recovery towards reference conditions, yet differences persist, indicating that rehabilitation may take longer, depending on the restoration goals. 相似文献
580.
In recent years many fuller ship hull forms have been designed and constructed in various shipbuilding countries, but the data available for the development of the fuller forms are inadequate from the point of view of preliminary ship design. In this paper the authors describe how they have systematically tested vessel forms of block coefficients ranging from 0.80 to 0.90. The analysis and presentation of the test results have been made in such a manner that designers can produce hull forms like those of tankers and other bulk carriers quickly and reliably. 相似文献