全文获取类型
收费全文 | 69889篇 |
免费 | 11844篇 |
国内免费 | 17287篇 |
专业分类
测绘学 | 5123篇 |
大气科学 | 9247篇 |
地球物理 | 12941篇 |
地质学 | 39042篇 |
海洋学 | 12016篇 |
天文学 | 2340篇 |
综合类 | 4946篇 |
自然地理 | 13365篇 |
出版年
2024年 | 234篇 |
2023年 | 687篇 |
2022年 | 2020篇 |
2021年 | 2367篇 |
2020年 | 2444篇 |
2019年 | 2991篇 |
2018年 | 2432篇 |
2017年 | 2677篇 |
2016年 | 2850篇 |
2015年 | 3100篇 |
2014年 | 4007篇 |
2013年 | 3912篇 |
2012年 | 4212篇 |
2011年 | 4568篇 |
2010年 | 3773篇 |
2009年 | 4714篇 |
2008年 | 4548篇 |
2007年 | 5025篇 |
2006年 | 4847篇 |
2005年 | 4321篇 |
2004年 | 4062篇 |
2003年 | 3807篇 |
2002年 | 3393篇 |
2001年 | 2912篇 |
2000年 | 2707篇 |
1999年 | 2516篇 |
1998年 | 2118篇 |
1997年 | 1946篇 |
1996年 | 1766篇 |
1995年 | 1462篇 |
1994年 | 1457篇 |
1993年 | 1210篇 |
1992年 | 964篇 |
1991年 | 708篇 |
1990年 | 571篇 |
1989年 | 501篇 |
1988年 | 342篇 |
1987年 | 238篇 |
1986年 | 150篇 |
1985年 | 111篇 |
1984年 | 53篇 |
1983年 | 43篇 |
1982年 | 50篇 |
1981年 | 39篇 |
1980年 | 24篇 |
1979年 | 34篇 |
1978年 | 43篇 |
1977年 | 24篇 |
1975年 | 4篇 |
1954年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
141.
142.
The sand–loess transition zone in north China is sensitive to climate change, and is an ideal place to investigate past environmental changes. However, past climate change at millennial–centennial timescales in this region has not been well reconstructed because of limited numerical dating. Alternations of sandy loam soils with aeolian sand layers in the Mu Us and Otindag sand fields, which lie along the sand–loess transition zone, indicate multiple intervals of dune activity and stability. This change is probably a response to variations of the East Asian monsoon climate during the late Quaternary. The single aliquot regeneration (SAR) optically stimulated luminescence (OSL) dating protocol, which has been successfully applied to aeolian deposits worldwide, is applied to these two sand fields in this study. The OSL ages provide reliable constraints for reconstruction of past climate changes at suborbital timescale. Sections in both sand fields contain aeolian sand beds recording millennial‐scale episodes of dry climate and widespread dune activation, including episodes at about the same time as Heinrich Event 5 and the Younger Dryas in the North Atlantic region. These results demonstrate the potential of aeolian sediments in semi‐arid north China to record millennial‐scale climatic events, and also suggest that dry–wet climate variation at the desert margin in China may be linked to climatic change elsewhere in the Northern Hemisphere, through atmospheric circulation. This article was published online on 27 November 2008. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected (16 December 2008). Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
143.
144.
145.
从胶东所处的板块构造位置和区域控热构造入手,根据地热传导理论,对胶东温泉的地热属性进行分析研究,认为胶东半岛温泉地热属性为毗邻板块边缘高温地热带的板内中低温地热系统,它以偏高的大地热流值为地热背景,以局部控热构造聚敛的大地热流为热源,以地形高差影响和相应的水力压差作用为动力,入渗的大气降水被岩石中的热量加热,并沿断裂出露形成温泉地热水。根据山东半岛温泉地热属性的特点和地热水开发中所面临的问题,提出保持胶东温泉可持续开发的相关对策。 相似文献
146.
The equilibrium of a self gravitating cylindrical polytrope with a general magnetic field and rotation has been discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
147.
Dr M. J. Wooller D. L. Swain K. J. Ficken A. D. Q. Agnew F. A. Street-Perrott G. Eglinton 《第四纪科学杂志》2003,18(1):3-15
Woody, subalpine shrubs and grasses currently surround Lake Rutundu, Mount Kenya. Multiple proxies, including carbon isotopes, pollen and grass cuticles, from a 755‐cm‐long core were used to reconstruct the vegetation over the past 38 300 calendar years. Stable carbon‐isotope ratios of total organic carbon and terrestrial biomarkers from the lake sediments imply that the proportion of terrestrial plants using the C4 photosynthetic pathway was greater during the Late Pleistocene than in the Holocene. Pollen data show that grasses were a major constituent of the vegetation throughout the Late Pleistocene and Holocene. The proportion of grass pollen relative to the pollen from other plants was greatest at the last glacial maximum (LGM). Grass cuticles confirm evidence that C4 grass taxa were present at the LGM and that the majority followed the cold‐tolerant NADP‐MEC4 subpathway. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
148.
149.
150.
Lightning discharge generated in the protoplanetary nebula is viewed as a temporally isolated surge in the flow of electrically charged particles, similar to that of terrestrial lightning. If the current is intense enough, a powerful circular impulse magnetic field is generated around the instantaneous virtual electric conductor. Such magnetic field is capable of magnetizing dust grains containing ferromagnetic components present in its vicinity to their saturation levels. As a result, dust grains attract one another, forming the aggregates. This magnetically driven attraction suggests an important process possibly operational at an early stage of the planetary accretion. Based on both a classical model for electric conductor, and the theory of Lienard–Wiechert electromagnetic potentials, our calculations show that the magnetic impulse due to a discharge channel of a few cm in diameter transferring a charge of about 104 electrons reaches as high as 10 T. At these magnetic fields, the ferromagnetic dust grains, and possibly the already-formed larger aggregates as well, are easily magnetized to the saturation levels, producing compact clusters exhibiting permanent magnetic moments. 相似文献