首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   62918篇
  免费   1144篇
  国内免费   592篇
测绘学   1834篇
大气科学   4657篇
地球物理   13054篇
地质学   21552篇
海洋学   5345篇
天文学   14394篇
综合类   148篇
自然地理   3670篇
  2021年   428篇
  2020年   477篇
  2019年   560篇
  2018年   1220篇
  2017年   1123篇
  2016年   1535篇
  2015年   921篇
  2014年   1468篇
  2013年   3024篇
  2012年   1642篇
  2011年   2276篇
  2010年   2014篇
  2009年   2756篇
  2008年   2455篇
  2007年   2429篇
  2006年   2335篇
  2005年   1859篇
  2004年   1900篇
  2003年   1775篇
  2002年   1802篇
  2001年   1562篇
  2000年   1499篇
  1999年   1350篇
  1998年   1321篇
  1997年   1339篇
  1996年   1094篇
  1995年   1066篇
  1994年   998篇
  1993年   893篇
  1992年   806篇
  1991年   763篇
  1990年   789篇
  1989年   731篇
  1988年   735篇
  1987年   830篇
  1986年   737篇
  1985年   941篇
  1984年   1096篇
  1983年   1025篇
  1982年   937篇
  1981年   856篇
  1980年   816篇
  1979年   774篇
  1978年   788篇
  1977年   676篇
  1976年   673篇
  1975年   652篇
  1974年   655篇
  1973年   690篇
  1972年   467篇
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
951.
The first estimates of uptake kinetic parameters for NH4+, NO3, and urea in the Ross Sea, Antarctica were measured on three cruises during austral late winter–early spring 1996 (pre-bloom), late spring 1997 (bloom development), and summer 1997 (bloom decline). Nitrogen (N) uptake experiments were conducted with water collected at the 50% light penetration depth using trace-metal clean protocols and 15N tracer techniques. At all sites, ambient NO3 concentrations ranged from 5.8 to 30.5 μg-at N l−1 and silicic acid concentrations were greater than 62.0 μg-at Si l−1. The following trends were observed. First, based on maximum uptake rates (Vmax), apparent N utilization followed the order NO3>NH4+>urea during the pre-bloom and bloom development cruises. During the summer cruise, as the bloom was declining, the apparent order of utilization was NH4+>NO3>urea. Second, evidence for possible repression of NO3 uptake by elevated NH4+ concentrations was only observed at one site. Third, the kinetic parameters of NH4+ uptake rates corrected for isotope dilution were compared with the kinetic parameters determined from uncorrected rates. In this comparison, the measure of substrate affinity, α (α=Vmax/Ks) increased by an average of 4.6-fold when rates were corrected for isotope dilution, but values of Vmax remained unchanged. Fourth, using bacterial production data, the magnitude of bacterial N uptake was estimated. Assuming that all bacterial N demands were met with NH4+, the estimated bacterial portion of NH4+ uptake ranged from <1%, when the ratio of bacteria to autotrophic biomass was low, to 35%, when bacterial abundance and biomass were highest. Finally, dramatic changes in NH4+ uptake capacity were observed at one station (Stn. O), where kinetic parameters were measured during all three cruises. We hypothesize that a mutualistic relationship exists between phytoplankton and heterotrophic bacteria, and that the creation of microzones of high NH4+ concentrations contributed to the changes seen at this station.  相似文献   
952.
Gallo  D. G.  Kidd  W. S. F.  Fox  P. J.  Karson  J. A.  Macdonald  K.  Crane  K.  Choukroune  P.  Seguret  M.  Moody  R.  Kastens  K. 《Marine Geophysical Researches》1984,6(2):159-185
During the Fall of 1979, a manned submersible program, utilizing DSRV ALVIN, was carried out at the intersection of the East Pacific Rise (EPR) with the Tamayo Transform boundary. A total of seven dives were completed in the vicinity of the EPR/Tamayo intersection depression and documented the geologic relationships that characterize the juxtaposition of these types of plate boundaries. The young volcanic terrain of the EPR axis can be traced into and across the Tamayo Transform valley but becomes buried by sedimentary talus that is being shed from sediment scarps along the unstable sediment slope that defines the north side of the intersection depression. Within 4 km of the transform boundary, the dominant trend (000°) of the fissures and faults that disrupt the rise-generated volcanics is markedly oblique to the regional direction of sea floor spreading (120°). Since no evidence was found to suggest that these structures accommodate significant amounts of strike-slip displacement, they are taken to reflect a distortion of the EPR extensional tectonic regime by a transform generated shear couple. The floor of the Tamayo Transform valley in this area is inundated by mass-wasted sediment, and the principal transform displacement zone is characterized at the surface by a narrow (<1.5 km) interval of fault scarps in sediment that trends parallel with the transform valley. Extrapolated to the west, this zone links with zones of transform deformation investigated during earlier submersible studies (CYAMEX and Pastouret, 1981). Evidence of low-level hydrothermal discharge was seen at one locality on the EPR axis and at another 8 km west of the axis at the edge of the zone of transform deformation.  相似文献   
953.
954.
The wavemaking capability and energy absorbing characteristics of a single submerged body, constrained to move horizontally in a surge mode, are examined. The body possesses both a vertical axis of symmetry and a horizontal plane of symmetry and is ‘thin’ in the sense that the body draft is small compared to the length. The behaviour of a single body subject to a constraint being placed upon the amplitude of its displacement and the properties of an array of interacting identical devices are also discussed.  相似文献   
955.
Three methods for the determination of dissolved organic carbon in seawater were compared. Samples were analysed using persulphate oxidation, high-temperature combustion, and ultraviolet photo-oxidation. The dissolved organic carbon content of the seawater samples ranged from 0.6 to 1.6 mg C/I. This study shows that results of high-temperature oxidation and photo-oxidation procedures differ by less than 5%, whereas results with persulphate oxidation are about 15% less than those obtained with the high-temperature oxidation. The relative merits of each of the oxidation techniques for the determination of organic matter in seawater are discussed.  相似文献   
956.
957.
958.
The chemical and mineralogical composition of suspended particle suites has been used to assess off-shelf transport on the West African shelves of Liberia and Sierra Leone. Using the ratios of Si/Al, Fe/Al, Mg/Al and Mn/Al as tracers, it was possible to detect shelf-derived materials in slope waters. In the majority of cases, these inputs could not have been detected using particle mass or light scattering measurements and could only be measured by using the chemical signatures of the particulate matter. At the time of sampling, the suspensate which had been moved seaward over the slope was detected adjacent to submarine canyons and highly turbid areas on the outer shelf. The chemical and mineralogical composition of the suspensates, and sediments in the adjacent eastern Atlantic basin are similar to those found in the water column seaward of the West African continental shelves and yet distinct from the Sahelian dust which is considered the major source of sedimentary material for the tropical and semi-tropical areas of the deep eastern basin. These findings suggest that materials from this shelf area could be a more important input to the deep sea than was previously realised.  相似文献   
959.
The drainage pattern in the northeast and central Tufts Abyssal Plain is described in detail. Satellite navigation on the systematic survey has allowed precise location of the major channel systems of the northeast Pacific Ocean. Two hundred channel profiles were collected from the echograms showing the Moresby-Scott, Mukluk, and Horizon Channel Systems trending in either a southwestward or westward direction across this section of the Tufts Plain. The channel profiles illustrate the prominence of the higher and wider right-hand levee (facing downstream). The Moresby-Scott Channel System disperses in the form of several distributaries throughout the area studied, and is probably responsible for much of the sediment deposits. Unlike the Moresby-Scott, the Mukluk extends throughout the survey area as a solitary channel with one minor branch. The Horizon Channel crosses the Sedna Fracture Zone east of the Sedna Seamount and terminates in distributary fashion in the central portion of the Tufts Plain. The Moresby-Scott, Mukluk, and Horizon Channels form one major system which encompasses the entire northeast and central Tufts Abyssal Plain.  相似文献   
960.
Existing methods for the determination of the various forms of iodine present in seawater are laborious or unwieldy in use. This paper describes modified forms of the iodate and total iodine methods described by Barkley and Thompson (1960), a spectrophotometric procedure for iodate determination derived from Johannesson's (1958) work, and an automatic method for total iodine determination. Procedures for iodate and total-iodine determination which are suitable for use aboard ship, are recommended. Both procedures for iodate determination returned a standard deviation close to 0.5 μg/l when several replicate samples of a seawater containing approximately 30 μg/l of iodate-iodine were analysed. The automatic method for total iodine determination yielded a standard deviation of 0.8 μg/l by the repeated analysis of a seawater containing a total of approximately 50 μg/l of iodine. Suitable methods for the filtration and storage of seawater are also described.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号