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   118023篇
  免费   1912篇
  国内免费   1039篇
测绘学   2982篇
大气科学   8703篇
地球物理   24086篇
地质学   40893篇
海洋学   10131篇
天文学   26103篇
综合类   311篇
自然地理   7765篇
  2021年   944篇
  2020年   1121篇
  2019年   1183篇
  2018年   2524篇
  2017年   2377篇
  2016年   3128篇
  2015年   2001篇
  2014年   3099篇
  2013年   6128篇
  2012年   3265篇
  2011年   4602篇
  2010年   3940篇
  2009年   5380篇
  2008年   4947篇
  2007年   4570篇
  2006年   4481篇
  2005年   3691篇
  2004年   3726篇
  2003年   3477篇
  2002年   3266篇
  2001年   2944篇
  2000年   2859篇
  1999年   2389篇
  1998年   2447篇
  1997年   2347篇
  1996年   2006篇
  1995年   1959篇
  1994年   1765篇
  1993年   1604篇
  1992年   1526篇
  1991年   1393篇
  1990年   1596篇
  1989年   1372篇
  1988年   1228篇
  1987年   1506篇
  1986年   1310篇
  1985年   1646篇
  1984年   1841篇
  1983年   1750篇
  1982年   1628篇
  1981年   1509篇
  1980年   1345篇
  1979年   1254篇
  1978年   1315篇
  1977年   1193篇
  1976年   1139篇
  1975年   1069篇
  1974年   1070篇
  1973年   1065篇
  1972年   683篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
This work deals with the possible solution of the solar neutrino problem in the framework of the resonant neutrino spin-flavor precession scenario. The event rate results from the solar neutrino experiments as well as the recoil electron energy spectrum from SuperKamiokande are used to constrain the free parameters of the neutrino in this model (Δm2 and μν). We consider two kinds of magnetic profiles inside the sun. For both cases, a static and a twisting field are discussed.  相似文献   
62.
63.
The Cumberland Basin, a 118 km2 estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin.  相似文献   
64.
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale.  相似文献   
65.
Progress in the introduction of coulometry for the analysis of total carbon dioxide (TCO2) in marine waters is described. An extractor—stripper removes CO2 that is measured coulometrically by the quantity of electricity (coulombs) used to electrogenerate OH? ions for the titration of the acid formed by the reaction of CO2 and ethanolamine. The equivalence point is detected photometrically with thymolphthalein as the indicator, and Faraday's Law relates coulombs to equivalents of titrant generated and CO2 determined so that there are no standard curves needed or titrants to standardize or store. Accuracy was determined by adding gelatin capsules containing 100–1500 μg C of pure CaCO3 into the stripper, and accuracies of better than ± 1 μg C were achieved. The best precision for natural seawater (± 1 standard error) of ± 0.5 μmol l?1 was found for 17 samples of Bermuda coastal waters having a mean TCO2 of 2007.2 μmol l?1 (0.05% CV). Sources of error and precautions are discussed. This method, which has been used successfully at sea, can be used to study a variety of marine phenomena involving TCO2.  相似文献   
66.
67.
68.
Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent.  相似文献   
69.
Measurements of salinity perturbations in a partially mixed estuary have been used to evaluate the usefulness of an inductive salinometer and to determine some of the characteristics of the salinity perturbations. The salinometer performed satisfactorily under most conditions. Although internal wave like effects were present, the turbulence fluctuations were dominant. The salinity fluctuations and the turbulent fluxes sw and su were found to behave in a manner similar to the density fluctuations in a thermally stratified atmospheric boundary layer and a laboratory open channel flow. A quadrant analysis suggested that the contribution of each quadrant to the turbulent flux changed with Ri. The turbulence parameters ν and cγ were found to decrease and increase respectively as Ri increases.  相似文献   
70.
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