全文获取类型
收费全文 | 58088篇 |
免费 | 5410篇 |
国内免费 | 6900篇 |
专业分类
测绘学 | 1969篇 |
大气科学 | 8265篇 |
地球物理 | 13870篇 |
地质学 | 24411篇 |
海洋学 | 5476篇 |
天文学 | 8196篇 |
综合类 | 3096篇 |
自然地理 | 5115篇 |
出版年
2023年 | 569篇 |
2022年 | 1233篇 |
2021年 | 1501篇 |
2020年 | 1293篇 |
2019年 | 1391篇 |
2018年 | 1983篇 |
2017年 | 1751篇 |
2016年 | 2198篇 |
2015年 | 1663篇 |
2014年 | 2236篇 |
2013年 | 2966篇 |
2012年 | 2032篇 |
2011年 | 2488篇 |
2010年 | 2358篇 |
2009年 | 2697篇 |
2008年 | 2408篇 |
2007年 | 2298篇 |
2006年 | 2073篇 |
2005年 | 1838篇 |
2004年 | 1735篇 |
2003年 | 1720篇 |
2002年 | 1638篇 |
2001年 | 1453篇 |
2000年 | 1689篇 |
1999年 | 2099篇 |
1998年 | 1806篇 |
1997年 | 1804篇 |
1996年 | 1597篇 |
1995年 | 1452篇 |
1994年 | 1359篇 |
1993年 | 1182篇 |
1992年 | 993篇 |
1991年 | 843篇 |
1990年 | 769篇 |
1989年 | 675篇 |
1988年 | 657篇 |
1987年 | 635篇 |
1986年 | 528篇 |
1985年 | 656篇 |
1984年 | 652篇 |
1983年 | 652篇 |
1982年 | 555篇 |
1981年 | 535篇 |
1980年 | 515篇 |
1979年 | 458篇 |
1978年 | 434篇 |
1977年 | 397篇 |
1976年 | 427篇 |
1974年 | 410篇 |
1973年 | 404篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
103.
Yao Chun-xial Li Bao-sheng Jin He-ling David Dian Zhang Yan Man-cun Zhu Yi-zhi Li Hou-xin Zhang Yu-hong Luo Kai-li 《地理学报(英文版)》2002,12(1):65-71
The paper makes some analyses on 11 trace elements in the Milanggouwan stratigraphical section in the Salawusu River valley,
which is regarded as a prototype geology-palaeoclimate record since 150 ka BP. The results show that the content and variation
of trace elements has experienced remarkably regular changes in the pace with coarse and fine sedimentary cycles of palaeo-aeolian
sands to its overlying fluvio-lacustrine facies or/and palaeosols. The trace elements with chemical properties of relatively
active (V, Sr, Cu, Ni, As) and relatively stable (P, Pb, Rb, Mn, Nb, Zr) are a manifestation of the corresponding 27 changeable
cycles between peak and valley values, appearing a multi-fiuctuational process line of relative gathering and migration since
then. The low numerical value distribution of these two types of trace elements in the aeolian sand facies represents erosion
and accumulation under wind force during the cold-dry climate. Whereas their enrichments in both fluvio-lacustrine facies
and palaeosols are related to the valley’s special low-lying physiognomic position between the Ordos Plateau and the Loess
Plateau under the warm and humid climate conditions. The above relatively migrated and gathered change of the trace elements
is the result of 27 climatic cycles of cold-dry and warm-humid, which is probably caused by repeated alternations of winter
monsoon and summer monsoon in the Mu Us Sandy Land influenced by the climate vicissitudes in northern hemisphere during glacial
and interglacial periods since 150 ka BP. 相似文献
104.
105.
Samples of dune sands, surveys of the morphology and field measurements of wind velocity and direction of a simple linear dune in Taklimakan Sand Sea show that the airflow and sand flux vary with the change of wind direction on the dune surface. Decrease of the airflow stress on the lee flank does not result in much decrease of the sand flux because of the low threshold shear velocities and the airflow conditions. There are no significant relations between the sand flux on the lee flank and the angle of incidence of the airflow. The low threshold shear velocities and the maintenance of the sand flux at the lee flank are the main mechanisms keeping the linear shape of the dunes. Measurements of the sand flux shows that it reaches a maximum on the crest of the dune. The grain size of the transported sands has some differences compared to that of the dune surface. The sands transported are finer than that on the dune surface, but better sorted under the influence of the medium to low wind activity. The field experiment results exhibit that it is possible for the dunes to be shaped as linear dunes during the processes of accumulation and elongation. 相似文献
106.
107.
108.
109.
110.
A generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0–5 Ma has been constructed for use with a relational database management system. The database includes all data whose virtual geomagnetic poles (VGP) lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling. Because many of these data were collected and processed more than 15–20 years ago, each result has been characterized according to the demagnetization procedures carried out. Analysis of these data in terms of the latitude variation of the angular dispersion of VGPs (palaeosecular variation from lavas) strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods. Differences between the angular dispersions for separate normal- and reverse-polarity data sets confirm that many older studies have not been adequately cleaned magnetically. Therefore, the use of the database for geomagnetic field modelling should be carried out with some caution. Using a VGP cut-off angle that varies with latitude, the best data set consists of 2636 results that show a smooth increase of VGP angular dispersion with latitude. Model G for palaeosecular variation, which is based on modelling of the antisymmetric (dipole) and symmetric (quadrupole) dynamo families, provides a good fit to these results. 相似文献