全文获取类型
收费全文 | 61803篇 |
免费 | 1110篇 |
国内免费 | 559篇 |
专业分类
测绘学 | 1495篇 |
大气科学 | 4500篇 |
地球物理 | 12549篇 |
地质学 | 21654篇 |
海洋学 | 5489篇 |
天文学 | 13750篇 |
综合类 | 108篇 |
自然地理 | 3927篇 |
出版年
2021年 | 492篇 |
2020年 | 514篇 |
2019年 | 560篇 |
2018年 | 1190篇 |
2017年 | 1064篇 |
2016年 | 1429篇 |
2015年 | 862篇 |
2014年 | 1345篇 |
2013年 | 3053篇 |
2012年 | 1542篇 |
2011年 | 2243篇 |
2010年 | 1992篇 |
2009年 | 2658篇 |
2008年 | 2302篇 |
2007年 | 2342篇 |
2006年 | 2240篇 |
2005年 | 1894篇 |
2004年 | 1868篇 |
2003年 | 1801篇 |
2002年 | 1763篇 |
2001年 | 1499篇 |
2000年 | 1447篇 |
1999年 | 1257篇 |
1998年 | 1237篇 |
1997年 | 1268篇 |
1996年 | 1038篇 |
1995年 | 1033篇 |
1994年 | 966篇 |
1993年 | 890篇 |
1992年 | 832篇 |
1991年 | 768篇 |
1990年 | 824篇 |
1989年 | 744篇 |
1988年 | 758篇 |
1987年 | 867篇 |
1986年 | 749篇 |
1985年 | 997篇 |
1984年 | 1082篇 |
1983年 | 1022篇 |
1982年 | 896篇 |
1981年 | 927篇 |
1980年 | 839篇 |
1979年 | 793篇 |
1978年 | 774篇 |
1977年 | 725篇 |
1976年 | 703篇 |
1975年 | 663篇 |
1974年 | 686篇 |
1973年 | 698篇 |
1972年 | 415篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
791.
Hydrodynamic and sediment transport measurements from instrumentation deployed during a 54-day winter period at two sites on the Louisiana inner shelf are presented. Strong extratropical storms, with wind speeds of 7.8 to 15.1 m s-1, were the dominant forcing mechanism during the study. These typically caused mean oscillatory flows and shear velocities about 33% higher than fair weather (averaging 12.3 and 3.2 cm s-1 at the landward site, and 11.4 and 2.7 cm s-1 at the seaward site, respectively). These responses were coupled with mean near-bottom currents more than twice as strong as during fair weather (10.3 and 7.5 cm s-1 at the landward and seaward sites, respectively). These flowed in approximately the same direction as the veering wind, causing a net offshore transport of fine sand. Weak storms were responsible for little sediment transport whereas during fair weather, onshore sand transport of approximately 25-75% of the storm values appears to have occurred. This contradicts previous predictions of negligible fair-weather sediment movement on this inner shelf. 相似文献
792.
Multibeam imagery of siliceous sponge reefs (Hexactinellida, Hexactinosida) reveals the setting, form, and organization of five reef complexes on the western Canadian continental shelf. The reefs are built by framework skeleton sponges which trap clay-rich sediments resulting in a distinctive pattern of low intensity backscatter from the reefs that colonize more reflective glacial sediments of higher backscatter intensity. Bathymetry and backscatter maps show the distribution and form of reefs in two large complexes in the Queen Charlotte Basin (QCB) covering hundreds of km2, and three smaller reef complexes in the Georgia Basin (GB). Ridges up to 7 km long and 21 m in height, together with diversely shaped, coalescing bioherms and biostromes form the principal reef shape in the QCB whereas chains of wave-form, streamlined mounds up to 14 m in height have developed in the GB. Reef initiation is dependent on the distribution of high backscatter-intensity relict glacial surfaces, and the variation in reef complex morphology is probably the result of tidally driven, near seabed currents. 相似文献
793.
The BIO Mark 8 thrust anemometer measures the drag of the wind on a perforated table tennis ball mounted on a vertical beam. The tri-axial displacement of the beam is sensed by eddy current proximity sensors. This anemometer has a flat frequency response from 0 to 10 Hz and can measure wind from 0 to 28 m s−1 at temperature from − 19 to + 28°C. It is designed for remote operation for extended periods of time such as on stable towers at sea. 相似文献
794.
D. Cossa 《Marine environmental research》1988,26(4)
A worldwide literature survey of data on cadmium concentration in the soft tissue of the mussel, Mytilus spp., from 591 stations is presented. These stations are from 13 regions. Geometric means for the regions vary from 0·6 to 3·3 μg g−1 (dry weight) for the Barents Sea and the Northeastern Pacific coast, respectively.The averages of seven of these regions, for which reliable cadmium concentrations in seawater were available, were used to calculate a relationship between cadmium concentrations in seawater and mussel soft tissue. The relationship was highly significant: (Cd) mussel (μg g−1, dry weight) = 0·074 (Cd) water (ng litre−1) + 0·39 (P ≤ 0·0005).This model has been successfully applied in the context of the contamination of the Gironde estuary (France). It can also be used to define a water quality criterion for mussel maturing parks consistent with the quality criterion defined for shellfish for human consumption. 相似文献
795.
Chemical and isotopic analyses of pore waters from Jamaican reef sediment suggest the importance of microbial sulfate reduction as a major control upon the origin, distribution, and composition of submarine cements in this fringing reef setting. Fore-reef sediment pore waters exhibit active sulfate reduction and enrichment in 18O which is consistent with associated active magnesian calcite cementation, alkalinity consumption, and cement enrichment in 18O. Conversely, lack of widespread submarine cementation in the back-reef corresponds to the diminished resupply of sulfate coupled with input of CO2-charged meteoric water from a nearby unconfined aquifer into the more stagnant pore waters which lower pore-water magnesian calcite saturation states and preclude active submarine cementation. 相似文献
796.
Theoretical and laboratory results indicate that bottom velocities within shoaling internal gravity waves intensify upslope approximately inversely proportional to the water depth. The elevated velocities (and bottom stresses) caused by shoaling and, possibly, breaking internal waves might explain the generation and maintenance of near-bottom nepheloid zones and attached turbid plumes that have been observed over certain continental shelves and slopes. This process is proposed as an explanation of zones of relatively low transmissibility that emanate from the upper continental slope near Newport submarine canyon off southern California. 相似文献
797.
Brent L. Lewis Pamela D. Holt Steven W. Taylor Steven W. Wilhelm Charles G. Trick Alison Butler George W. Luther III 《Marine Chemistry》1995,50(1-4)
Thermodynamic stability constants have been estimated for the complexation of iron(III) with catecholate-type siderophores isolated from the marine bacterium Alteromonas luteoviolacea and from the marine cyanobacterium Synechococcus sp. PCC 7002. Stability constants were determined utilizing the “chelate scale” of Taylor et al. (1994). The scale is based upon a linear relationship between the reduction potentials and the pH-independent thermodynamic stability constants for known iron(III) complexes. Log K values for the alterobactin B ferric iron complex are 43.6 ± 1.5 at pH 8.2 and 37.6 ± 1.2 at pH 6, consistent with a shift from bis-catecholate to monosalicylate/monocatecholate iron coordination with decreasing pH. Synechococcus isolates PCC 7002 Nos. 1 and 3 formed iron(III) complexes with stability constants of approximately 38.1 ± 1.2 and 42.3 ± 1.5, respectively. The binding strengths of the iron(III) complexes examined in this study are quite high, suggesting that catecholate siderophores may play a role in the solubilization and biological uptake of iron in the marine environment. 相似文献
798.
Growth rate of Raia clavata in the Northeast Irish Sea 总被引:1,自引:0,他引:1
799.
Abstract. The distribution of Ibla cumingi DARWIN on different types of hard substrata along 30 km of coast in the Gulf of Elat was studied. I. cumingi was found among and underneath Tetraclita squamosa rufotincta , in oyster beds, underneath slabs of beachrock, and in fissures in igneous rocks. In the Tetraclita belt, I. cumingi is most frequent in empty shells of dead Tetraclita while in the oyster bed it is common inside the meshwork made by the oysters. The distribution is regarded as clustered distribution. The density and size of the clusters of I. cumingi is determined by the available sheltered space within the midlittoral zone. The form of the habitat determines the shape of the animals. Those found in the oyster bed and fissures are usually longer than those found in the Tetraclita belt due to the depth of the fissures or to the bigger spaces within the oyster bed. I. cumingi is orientated so that the capitulum with the cirri is projecting towards the open water, and the water current. It is concluded that I. cumingi can withstand the physical stress encountered in a tropical intertidal zone, but that the distribution pattern is caused by predation. 相似文献
800.
A survey of the aerobic heterotrophic bacteria present in sediments at Sawyers Bay, New Zealand, receiving tannery effluent high in Cr, and a control site, indicates that the populations present are different and show seasonal variation. The bacterial population present at the polluted site appears more able to tolerate CrIII at concentrations less than 0·2 μmol ml?1. 相似文献