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   262264篇
  免费   4328篇
  国内免费   3416篇
测绘学   6996篇
大气科学   19281篇
地球物理   54866篇
地质学   91338篇
海洋学   21741篇
天文学   56717篇
综合类   1026篇
自然地理   18043篇
  2021年   2246篇
  2020年   2619篇
  2019年   2859篇
  2018年   4017篇
  2017年   3647篇
  2016年   5999篇
  2015年   4232篇
  2014年   6947篇
  2013年   14246篇
  2012年   6661篇
  2011年   8173篇
  2010年   7184篇
  2009年   9785篇
  2008年   8658篇
  2007年   8133篇
  2006年   9697篇
  2005年   7838篇
  2004年   7740篇
  2003年   7218篇
  2002年   6791篇
  2001年   6053篇
  2000年   5975篇
  1999年   5206篇
  1998年   5245篇
  1997年   5046篇
  1996年   4684篇
  1995年   4429篇
  1994年   4118篇
  1993年   3858篇
  1992年   3644篇
  1991年   3600篇
  1990年   3763篇
  1989年   3520篇
  1988年   3303篇
  1987年   3858篇
  1986年   3422篇
  1985年   4236篇
  1984年   4742篇
  1983年   4409篇
  1982年   4319篇
  1981年   3921篇
  1980年   3646篇
  1979年   3508篇
  1978年   3487篇
  1977年   3285篇
  1976年   3055篇
  1975年   2960篇
  1974年   2917篇
  1973年   3079篇
  1972年   2024篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Normal and imposex-affected female Buccinum undatum were sampled from the open North Sea at three locations, one with low, and two with high shipping densities. Cytochrome P450 components and P450 aromatase activity were determined in the microsomal fractions isolated from pooled digestive gland/gonads. Cytochrome P450 aromatase activity was significantly higher (P < 0.05) in normal females collected in the low shipping density area (1,325 +/- 295 fmol/h/mg protein) than levels from imposex animals from a high shipping density area (620 +/- 287 fmol/h/mg protein). A negative correlation was found between aromatase activity and organotin body burden (r = -0.99). Levels of CYP450, cytochrome b5 and NADPH cytochrome c reductase activity did not show differences among groups. This is the first field evidence of depressed aromatase activity in imposex affected females, although additional research under laboratory controlled conditions is required to fully understand the mechanisms underlying the development of imposex in this species.  相似文献   
993.
This paper deals with the solution of the problem of the dynamic response of the leg of a tension leg platform subjected to an axial, suddenly applied load at one end, considering a highly simplified model but taking into account several complicating factors. The problem is solved by means of a non-harmonic Fourier expansion in terms of eigenfunctions obtained from a non-regular Sturm-Liouville system.  相似文献   
994.
A densely populated soft-sediment community was experimentally analysed for interspecific competition for space in laboratory experiments. No interspecific competitive interactions leading to a decrease in survivorship could be documented over a five-week period. The four most abundant species all coexisted in laboratory microcosms. The two tube-buiding species, the tanaid Leptochelia dubia and the polychaete Rhynchospio arenincola, could not exclude the mobile bivalve Transennella tantilla or the burrowing amphipod Paraphoxus spinosus. None of the species increases its emigration frequency when other species are present. Paraphoxus is found to be a browsing predator on Phynchospio but inflicts no detectable mortality. The coexistence of these species is permitted by virtue of the fact that none of the species can alter the soft-sediment habitat sufficiently to make the habitat unsuitable for the other species.  相似文献   
995.
The interstitial water composition ( , alkalinity, Ca2+, Mg2+, Sr2+, Na+, K+) and the cation exchange capacity (CEC) were determined for the muddy sediments of the continental shelf off the Gironde Estuary (France), in the area where the sediment represents the deposit of the muddy suspension of the river. In comparison with seawater concentrations, the pore waters below 10 cm depth, show depletions of and Ca2+ and below a 30 cm depth show depletions of Mg2+. Inversely, the upper 10 cm an enrichment of Ca2+ concentration, and an increase of K+ concentration to a 40 cm depth. High values of are observed at the top 4 cm. Alkalinity enrichment is observed along the length of the core. Applying the alkalinity models for the sediment below a 10 cm depth demonstrates generally that calculated alkalinities are higher than the measured ones. Ca2+ dissolution occurs at the first 10 cm and authigenic carbonate precipitation starts beneath that level. Mg2+ depletion is accompanied by bicarbonate loss. This proves that Mg2+ depletion is due to a Mg-silicate reaction. The result of the CEC does not confirm the Mg2+ uptake by clay minerals in exchangeable site, under reducing conditions. Diffusion and bioturbation play an important role in the pore water concentration at the top of the core.  相似文献   
996.
997.
998.
999.
 Real-time trackline plots of surficial sediment acoustic impedance delineate several sedimentary facies off Garden Key in the Dry Tortugas. The sea floor within a 6×6 km surveyed area consists of carbonate muds (silts), sands and shell, rock, and live corals. The 4-kHz acoustic data supports this finding by providing a pictographic representation of the distribution and structure of several sediment facies types. Plotting the gridded acoustic data with commercial mapping software (Surfer) provides a three-dimensional (3D) perspective of the bottom topography with a color contour map of surficial sediment impedance (upper 0.4 m) draped over the 3D surface.  相似文献   
1000.
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