全文获取类型
收费全文 | 82708篇 |
免费 | 22631篇 |
国内免费 | 46405篇 |
专业分类
测绘学 | 4591篇 |
大气科学 | 27102篇 |
地球物理 | 23377篇 |
地质学 | 55377篇 |
海洋学 | 20088篇 |
天文学 | 7397篇 |
综合类 | 7046篇 |
自然地理 | 6766篇 |
出版年
2023年 | 705篇 |
2022年 | 1650篇 |
2021年 | 2158篇 |
2020年 | 3117篇 |
2019年 | 6468篇 |
2018年 | 7118篇 |
2017年 | 6717篇 |
2016年 | 7120篇 |
2015年 | 6047篇 |
2014年 | 6004篇 |
2013年 | 7244篇 |
2012年 | 6224篇 |
2011年 | 6289篇 |
2010年 | 6052篇 |
2009年 | 5745篇 |
2008年 | 4801篇 |
2007年 | 4756篇 |
2006年 | 4351篇 |
2005年 | 4014篇 |
2004年 | 4109篇 |
2003年 | 3825篇 |
2002年 | 3602篇 |
2001年 | 3319篇 |
2000年 | 3061篇 |
1999年 | 3406篇 |
1998年 | 3147篇 |
1997年 | 3269篇 |
1996年 | 2614篇 |
1995年 | 2526篇 |
1994年 | 2282篇 |
1993年 | 2123篇 |
1992年 | 1802篇 |
1991年 | 1378篇 |
1990年 | 1277篇 |
1989年 | 1098篇 |
1988年 | 1033篇 |
1987年 | 911篇 |
1986年 | 758篇 |
1985年 | 795篇 |
1984年 | 857篇 |
1983年 | 738篇 |
1982年 | 672篇 |
1981年 | 591篇 |
1980年 | 569篇 |
1979年 | 494篇 |
1978年 | 453篇 |
1977年 | 394篇 |
1976年 | 411篇 |
1974年 | 396篇 |
1973年 | 386篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
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
992.
Sand dabs, an important commercial flatfish in the Bohai Sea, has recently decreased gradually in biomass, resulting from overfishing and marine pollution. Artificial culture and multiplication of sand dabs are essential and practical in China. The present paper examines the food habitof this fish to provide detailed information for its culture and multiplication. 相似文献
993.
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
994.
995.
Effectsoflight,temperatureandnutrientsonphotosynthesisofBiddulphiaregiaINTRODUCTIONLight,temperatureandnutrientsareimportante... 相似文献
996.
Wen Shengchang 《海洋学报(英文版)》1988,7(1):1-16
In this part ot the paper theoretical wind-wave spectra nave been derived oy (I) expressing the spectrum in series composed of exponential terms; (2) assuming that the spectrum satisfies a high order linear ordinary differential equation; (3) introducing proper parameters in the spectrum; and (4) making use of some known charateristics of wind-wave spectrum, for instance, the law governing the equilibrium range. The spectrum obtained contains the zero order moment of the spectrum m0, the peak frequency ω0 and the ratio R =ω/ω0 (ω being the mean zero-crossing frequency) as parameters. The shape of the nondimensional spectrum S(ω) = ω0S(ω)/m0(ω=ω/ω0) changes with R and theoretically reduces to a Dirac delta function δ(ω-1) when R = 1. A spectrum of simplified form is given for practical uses, in which R is replaced by a peakness factor P=S(1). 相似文献
997.
998.
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
999.
By utilizing the denser upper-air observations from the Okinawa region and the Japanese islands during August 17-23, 1975, the structure of Typhoon 7507 formed in this area has been analysed. It has been indicated that the cyclonic tangential wind maximum occurred in the eastern sector of the typhoon and the strongest inflow occurred in the southern portion of the typhoon about 150 km away from the typhoon centre. There was a deep inflow layer below the 300 hPa level. The anticyclonic outflow was the strongest at 150 hPa and was located in the NE quadrant of the typhoon. The convergence mainly took place in the southern part of the typhoon, while the divergence occurred at the lower and middle levels in the northern sector of the typhoon. The pattern of vorticity and vertical motion field were characterized by the similar asymmetry. 相似文献
1000.
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