全文获取类型
收费全文 | 78666篇 |
免费 | 11178篇 |
国内免费 | 14920篇 |
专业分类
测绘学 | 8288篇 |
大气科学 | 7780篇 |
地球物理 | 15632篇 |
地质学 | 41381篇 |
海洋学 | 10906篇 |
天文学 | 8963篇 |
综合类 | 3494篇 |
自然地理 | 8320篇 |
出版年
2023年 | 982篇 |
2022年 | 2657篇 |
2021年 | 3452篇 |
2020年 | 2777篇 |
2019年 | 3171篇 |
2018年 | 3460篇 |
2017年 | 3279篇 |
2016年 | 3437篇 |
2015年 | 3658篇 |
2014年 | 3961篇 |
2013年 | 5454篇 |
2012年 | 5146篇 |
2011年 | 5515篇 |
2010年 | 5224篇 |
2009年 | 5494篇 |
2008年 | 5191篇 |
2007年 | 5031篇 |
2006年 | 4867篇 |
2005年 | 3921篇 |
2004年 | 3184篇 |
2003年 | 2422篇 |
2002年 | 2499篇 |
2001年 | 2164篇 |
2000年 | 1878篇 |
1999年 | 1123篇 |
1998年 | 943篇 |
1997年 | 842篇 |
1996年 | 720篇 |
1995年 | 655篇 |
1994年 | 571篇 |
1993年 | 483篇 |
1992年 | 491篇 |
1991年 | 426篇 |
1990年 | 523篇 |
1989年 | 414篇 |
1988年 | 371篇 |
1987年 | 475篇 |
1986年 | 387篇 |
1985年 | 474篇 |
1984年 | 580篇 |
1983年 | 511篇 |
1982年 | 490篇 |
1981年 | 435篇 |
1980年 | 450篇 |
1979年 | 410篇 |
1978年 | 382篇 |
1977年 | 377篇 |
1975年 | 340篇 |
1974年 | 345篇 |
1973年 | 375篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
研究表明 :腐屑食性鱼类——梭鱼对环境中有机氮的摄入能力随体重的增加而增加 ,随温度的升高 ,体重系数显著减少。摄入氮 (CN,mg/ d)与体重 (W,g)和温度 (T,℃ )的数值关系可用如下模型表示 :CN=0 .6 370 W1 .0 642 ln(3.70 0 8T)。体重为 1 0 0 g的梭鱼每日可消耗环境中 332 mg氮有机质。梭鱼氮的吸收效率主要受温度的影响 ,而氮的转化效率与体重呈显著的正相关关系。随体重的增加 ,排泄氮所占比例降低 相似文献
993.
994.
给出了提取潮汐调和常数的一种新方法--正交方法,并应用1992~1997年的TOPEX/POSEIDON卫星高度计遥感资料,提取中国海M2分潮调和常数.同时,利用最小二乘法来提取中国海M2分潮调和常数,两种方法结果比较渤海、黄海、东海海域M2分潮振幅、迟角的均方差分别是3.3 cm,3.6°;南中国海海域M2分潮振幅、迟角均方差分别是1.1 cm,1.7°,结果表明正交方法是一种可信的具有实用性的方法. 相似文献
995.
996.
997.
U–Th decay series isotopes, δ18O and Si measurements in the river estuarine waters and sediments of the polluted Hooghly estuary as well as the surface waters of the Bay of Bengal, its high salinity end member, are reported. Dissolved Si indicates that there are probably two mixing regimes, dissolved U behaviour is nonconservative and δ18O behaves conservatively in the overall estuarine region. Isotopes of reactive elements, viz. 234Th and 210Po, are removed from the estuarine waters in <2 days and <1 month, respectively, which is due to high suspended matter (30–301 mg l−1). 228Ra and 226Ra are profusely released into the estuarine waters in the low to mid-salinity regions.As expected, the opposite trend is observed in the case of estuarine sediments and suspended matter. Reactive isotopes of Th, 210Pb and 210Po are enriched, whereas Ra isotopes are depleted with respect to their parent nuclides in the estuarine sediments and suspended matter. 232Th/Al ratio appears well suited to study the distribution and mixing of the bed load sediments of the Ganga–Brahmaputra (G–B) and the Hooghly rivers with those from other rivers on the Bay of Bengal floor. 相似文献
998.
闽南近海尖头斜齿鲨Scoliodon sorrakowah肝油的脂肪酸组成中,主要的饱和脂肪酸包括C14:0,C16:0,C18:0;单不饱和脂肪酸包括C16:1,C18:1ω7,C20:1;多不饱和脂肪酸包括C20:4ω6,C20:5ω3,C22:5ω3和C22:6ω3。尖头斜齿鲨肝油总饱和脂肪酸和总单不饱和脂肪酸含量在6月份降至最低值,8-9月升至最高值,总饱和脂肪酸的季节变化规律主要由C16:0决定,总单不饱和脂肪酸的变化主要由C16:1和C18:1ω9决定,总多不饱和脂肪酸含量在6月份达到最高值,8-9用降到最低值,11月份又上升至一定水平,这种变化主要由C22:6ω3决定。 相似文献
999.
1000.
The Bounty Channel and Fan system provides the basis for a model for deep-sea channel and fan development in a rifted continental margin setting. The sedimentary system results from an interplay between tectonics (fan location; sediment source), turbidity currents (sediment supply), geostrophic currents (sediment reworking and distribution) and climate (sea level, and hence sediment supply and type). Today, sediment is shed from the collisional Southern Alps, part of the Pacific/Indo-Australian plate margin, and passes east across the adjacent shelf and into the Otago Fan complex at the head of the Bounty Trough. Paths of sediment supply, and locations of sediment deposition, are controlled by the bathymetry of the Bounty Trough, with axial slopes as high as 37 m/km (2°) towards the trough head, diminishing to around 3.5 m/km (0.2°) along the trough axis. The Bounty Fan is located 800 km further east, where the Bounty Channel debouches onto abyssal oceanic crust at the mouth of the Bounty Trough. The Bounty Fan comprises a basement controlled fan-channel complex with high leveed banks exhibiting fields of mud waves, and a northward-elongated middle fan. Channel-axis gradients diminish from 6 m/km (0.35°) or more on the upper fan to less than 1 m/km (<0.06°) on the lower fan. Parts of the left bank levee and almost the entire middle fan are being eroded and re-entrained within a Deep Western Boundary Current (DWBC), which passes along the eastern New Zealand margin at depths below 2000 m. The DWBC is the prime source of deep, cold water flow into the Pacific Ocean, with a volume of ca. 20 Sv and velocities up to 4 cm/s or greater. The mouth of the Bounty Channel, at a depth of 4950 m at the south end of the middle fan, acts as a point source for an abyssal sediment drift entrained northward under the DWBC at depths below 4300 m. The Bounty Fan probably originated in the early to middle Neogene, but has mostly been built during the last 3 Myr (Plio-Pleistocene), predominantly as climate-controlled sedimentary couplets of terrigenous, micaceous mud (acoustically reflective; glacial) and biopelagic ooze (acoustically transparent; interglacial), deposited under the pervasive influence of the DWBC. 相似文献