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   98247篇
  免费   1616篇
  国内免费   804篇
测绘学   2310篇
大气科学   6393篇
地球物理   19571篇
地质学   35962篇
海洋学   8668篇
天文学   22472篇
综合类   297篇
自然地理   4994篇
  2022年   692篇
  2021年   1149篇
  2020年   1263篇
  2019年   1357篇
  2018年   3120篇
  2017年   2947篇
  2016年   3373篇
  2015年   1760篇
  2014年   3186篇
  2013年   5169篇
  2012年   3446篇
  2011年   4457篇
  2010年   3809篇
  2009年   4843篇
  2008年   4462篇
  2007年   4445篇
  2006年   4074篇
  2005年   3304篇
  2004年   3296篇
  2003年   2956篇
  2002年   2483篇
  2001年   2332篇
  2000年   2143篇
  1999年   1662篇
  1998年   1750篇
  1997年   1718篇
  1996年   1357篇
  1995年   1403篇
  1994年   1206篇
  1993年   1081篇
  1992年   1056篇
  1991年   983篇
  1990年   1102篇
  1989年   945篇
  1988年   860篇
  1987年   998篇
  1986年   795篇
  1985年   1074篇
  1984年   1153篇
  1983年   1075篇
  1982年   1023篇
  1981年   894篇
  1980年   853篇
  1979年   748篇
  1978年   786篇
  1977年   708篇
  1976年   657篇
  1975年   643篇
  1974年   642篇
  1973年   639篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Abstract. The stomach contents of poor cod, Trisopterus minutus capelanus (Lacepède), were taken at monthly intervals off the eastern coast of the Gulf of Valencia (Spain). A total of 1276 were analyzed to determine diet according to fish size and season. The basic food consists of crustaceans (Mysidacea and Decapoda) and teleosts. Feeding habits varied with size: decapods and fishes were more abundant in the stomachs of larger specimens. Little seasonal variation in food habits was recorded.  相似文献   
994.
Stable carbon isotope ratios have been used to study the sources of particulate organic carbon (POC) and total dissolved inorganic carbon in the Orinoco Basin. The isotopic composition of total dissolved inorganic carbon shows a range of from -8·1 to -23·0 ppt, an indication of dominance of biological processes. The isotopic composition of POC exhibits a range of from -24·1 to -34·6 ppt with little seasonal variation. The isotopic evidence indicates that the POC is predominantly of terrestrial origin rather than a result of in situ planktonic production. The similarity of isotopic composition of POC and coastal sediments suggests that riverine organic detritus has been transported 30-50 km offshore in a direction parallel to the Orinoco river channel.  相似文献   
995.
Geophysical data from 900 km of the Southwest Indian Ridge are used todescribe the pattern of evolution of the plate boundary between 61° Eand 70° E over the past 20 million years. The SWIR is anobliquely-opening, ultra slow-spreading axis, and east of61° E comprises a series of ridge sections, each about 100–120 kmin length. The orientation of these sections varies fromsub-orthogonal to oblique to the approximately N–S spreadingdirection. In general, the suborthogonal sections are shallower, commonlysubdivided into an array of discrete axial segments, and carry recognisablecentral magnetic anomalies. The majority of the oblique sections are single,continuous rifts without continuous axial magnetic signatures.Morphotectonics of the Southwest Indian Ridge crust have not previously beenwell constrained off-axis, and we here present sidescan sonar andswath bathymetric data up to 100 km from the ridge to demonstrate the complexities of its spatial and temporal evolution.A model is proposed that the segmentation style correlates with analong-axis variation between: (a) relatively thick crustal sections which overlie mantle sections with higher magmatic supply created in orthogonally-spreading segments and (b) those oblique sections associated with cooler, magmatically-starved mantle and thinner crust. These latter sections are formed at broad offset zones in theplate boundary, more precisely defined on faster-spreading ridges asnontransform discontinuities. The nonsystematic pattern of crustalconstruction, extensional basin formation and the absence of extension-parallel traces of discontinuities off-axis suggest that the oblique spreading sections are not fixed in space or time.  相似文献   
996.
 Sediment cores from the upper continental slope of the eastern Arabian Sea have high organic carbon (OC), CaCO3, and sand content at the top. The values decrease with increasing depth in the Holocene and Upper Pleistocene. Topographic highs show highest OC and lower CaCO3 in the Holocene clayey sediments and vice versa in the Pleistocene sandy sediments. The OC is immature and marine or a mixture of both marine and terrestrial in the Holocene sediments and is mostly terrestrial and/or reworked marine in the Pleistocene sediments. Productivity is the main controlling factor for the organic carbon enrichment. Texture and reworking also influence the organic carbon variations. Received: 29 May 1996/Revision received: 10 January 1997  相似文献   
997.
998.
The distribution of dissolved and participate concentrations of As, Cr and V was investigated in the mixing area of the Po river in five surveys over the period March 1992-March 1995. The data obtained indicate noticeable contamination of coastal waters of the northern Adriatic with Cr but not As or V. Total dissolved concentrations for As and V were comparable with oceanic levels but were higher by a factor of 4 for Cr. These results suggest either low anthropogenic inputs of As and V, from terrigenous and atmospheric sources, or efficient removal mechanisms. The investigated metals had low reactivity during freshwater-seawater mixing and the distribution of concentrations in the dissolved and solid phases suggests that the dilution process is the main factor controlling the transport of these metals to the sea in low-to-medium solid transport conditions. Further investigations are required to elucidate the behavior of these metals during high solid transport conditions. Significant changes in the speciation of As occurred during the transition between the two end-member matrices. In the coastal waters of the northern Adriatic, the arsenite fraction in winter, and the arsenite and dimethylarsinate fractions in summer increased with respect to riverine waters. Changes in the speciation of chromium were minor, but also showed a small increase in reduced species in marine waters.  相似文献   
999.
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