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   5905篇
  免费   226篇
  国内免费   90篇
测绘学   179篇
大气科学   555篇
地球物理   1469篇
地质学   2050篇
海洋学   566篇
天文学   759篇
综合类   22篇
自然地理   621篇
  2021年   59篇
  2020年   67篇
  2019年   88篇
  2018年   127篇
  2017年   117篇
  2016年   160篇
  2015年   145篇
  2014年   204篇
  2013年   327篇
  2012年   250篇
  2011年   267篇
  2010年   194篇
  2009年   308篇
  2008年   292篇
  2007年   269篇
  2006年   227篇
  2005年   190篇
  2004年   201篇
  2003年   192篇
  2002年   186篇
  2001年   135篇
  2000年   141篇
  1999年   114篇
  1998年   113篇
  1997年   95篇
  1996年   85篇
  1995年   87篇
  1994年   75篇
  1993年   64篇
  1992年   65篇
  1991年   69篇
  1990年   67篇
  1989年   61篇
  1988年   58篇
  1987年   65篇
  1986年   44篇
  1985年   74篇
  1984年   84篇
  1983年   73篇
  1982年   66篇
  1981年   77篇
  1980年   64篇
  1979年   57篇
  1978年   38篇
  1977年   57篇
  1976年   64篇
  1975年   48篇
  1974年   56篇
  1973年   49篇
  1972年   27篇
排序方式: 共有6221条查询结果,搜索用时 125 毫秒
41.
42.
43.
44.
45.
A sonic well log was obtained within the basement complex of the Walvis Ridge during Deep Sea Drilling Project Leg 74. The top of the basement complex is characterized by smooth acoustic reflectors. The rocks recovered within the basement complex consist of basalts with intercalated sediments. According to the log ~-50% of the upper 75 m of basement are igneous rocks and the other 50% sedimentary. Sonobuoy results indicate that the ratio of sediments to basalt increases with depth for an additional 225 m until a typical oceanic velocity structure is observed. Paleontological results suggest that the processes forming this upper 300 m of the basement complex was accomplished within a short time interval.  相似文献   
46.
The plot of210Pb activity against depth in carbonate sands on the Virgin Island Bank is a negative asymmetric hyperbolic curve. As depth increases, an initial rapid decrease in210Pb activity caused by the decay of unsupported210Pb and226Ra is followed by increasing activity as a result of210Pb achieving equilibrium with ingrowing230Th. As this curve is time dependent, an estimate of the relative ages in carbonate sequences and the rates of net carbonate accumulation can be made. The ease of210Pb activity determinations makes this procedure an attractive method in obtaining carbonate sand accumulation rates.  相似文献   
47.
Unusual large-scale phytoplankton blooms in the equatorial Pacific   总被引:1,自引:0,他引:1  
Unusual large-scale accumulations of phytoplankton occurred across 10,000 km of the equatorial Pacific during the 1998 transition from El Niño to La Niña. The forcing and dynamics of these phytoplankton blooms were studied using satellite-based observations of sea surface height, temperature and chlorophyll, and mooring-based observations of winds, hydrography and ocean currents. During the bloom period, the thermocline (nutricline) was anomalously shallow across the equatorial Pacific. The relative importance of processes that enhanced nutrient flux into the euphotic zone differed between the western and eastern regions of the blooms. In the western bloom region, the important vertical processes were turbulent vertical mixing and wind-driven upwelling. In contrast, the important processes in the eastern bloom region were wave-forced shoaling of nutrient source waters directly into the euphotic zone, along-isopycnal upwelling, and wind-driven upwelling. Advection by the Equatorial Undercurrent spread the largest bloom 4500 km east of where it began, and advection by meridional currents of tropical instability waves transported the bloom hundreds of kilometers north and south of the equator. Many processes influenced the intricate development of these massive biological events. Diverse observations and novel analysis methods of this work advance the conceptual framework for understanding the complex dynamics and ecology of the equatorial Pacific.  相似文献   
48.
The largest known submarine slope instabilities occur on gently inclined slopes or in the deep sea. The sedimentation rates are mostly too low to induce an excess pore-water pressure sufficient to create failure. A possible triggering mechanism for these instabilities is additional horizontal ground acceleration caused by earthquakes. Old zones of weakness, represented by fracture zones, can be reactivated by isostatic movements and induce seismic activity. The distribution of some major slope instabilities and the trend of fracture zones in the Eastern Atlantic are compared and Fracture activity is suggested as the main triggering mechanism for these slope instabilities.  相似文献   
49.
Abstract. The biology of the thalassinidean shrimp Upogebia pusilla was studied on a tidal flat in the Lagoon of Grado (Northern Adriatic). Burrows were investigated using in situ resin casting and with additional in situ and laboratory observations. Burrows show a basic pattern consisting of a U or a double U with turning chambers and a vertical shaft. Mean burrow diameter depends on animal size, it is smaller than the rigid carapace of the animal. Dimensions, distance between openings, depth of U, total depth, volume and surface are size dependent. The burrow wall is smooth and oxidized. Burrows are always inhabited by a single shrimp. Although they overlap, they are never interconnected. They are mainly constructed by compression of the sediment and are relatively permanent structures. A comparison of the burrows of Thalassinidea with regard to shape, number and appearance of openings, dimensions, properties of the burrow wall and dynamics is given.  相似文献   
50.
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