全文获取类型
收费全文 | 1805篇 |
免费 | 288篇 |
国内免费 | 374篇 |
专业分类
测绘学 | 199篇 |
大气科学 | 351篇 |
地球物理 | 356篇 |
地质学 | 787篇 |
海洋学 | 241篇 |
天文学 | 69篇 |
综合类 | 121篇 |
自然地理 | 343篇 |
出版年
2024年 | 8篇 |
2023年 | 33篇 |
2022年 | 91篇 |
2021年 | 130篇 |
2020年 | 96篇 |
2019年 | 118篇 |
2018年 | 112篇 |
2017年 | 105篇 |
2016年 | 134篇 |
2015年 | 143篇 |
2014年 | 130篇 |
2013年 | 146篇 |
2012年 | 147篇 |
2011年 | 150篇 |
2010年 | 117篇 |
2009年 | 106篇 |
2008年 | 117篇 |
2007年 | 104篇 |
2006年 | 77篇 |
2005年 | 69篇 |
2004年 | 35篇 |
2003年 | 34篇 |
2002年 | 35篇 |
2001年 | 31篇 |
2000年 | 26篇 |
1999年 | 22篇 |
1998年 | 9篇 |
1997年 | 16篇 |
1996年 | 18篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1988年 | 7篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1972年 | 4篇 |
1971年 | 4篇 |
1969年 | 3篇 |
排序方式: 共有2467条查询结果,搜索用时 265 毫秒
981.
982.
983.
984.
I Introduction Phytoplankton play an important role in the primary production of ocean (Ning et al., 1995). They are impor-tant biological mediators of carbon turnover in seawater ecosystems (Zhu et al., 1993). Phytoplankton in Jiaozhou Bay have been preliminarily studied on the subjects of community structure, primary productivity and carbon budget (Qian et al., 1983; Guo et al., 1992; Jiao et al., 1994). It has been found that seasonal variation of phytoplankton cell abundance presents w… 相似文献
985.
By using field survey data from the sixth forest inventory of Jiangxi Province in 2003, the biomass and carbon storage for three studied species (Pinus massoniana, Cunninghamia lanceolata, and Pinus elliottii) were estimated in Taihe and Xingguo counties of Boyang Lake Basin, Jiangxi Province, China. The relationship between carbon density and forest age was analyzed by logistic equations. Spatio-temporal dynamics of forest biomass and carbon storage in 1985-2003 were also described. The results show that total stand area of the three forest species was 3.10 × 10^5 ha, total biomass 22.20 Tg, vegetation carbon storage 13.07 Tg C, and average carbon density 42.36 Mg C/ha in the study area in 2003. Carbon storage by forest type in descending order was: P. massoniana, C. lanceolata and P. elliottii. Carbon storage by forest age group in descending order was: middle stand, young stand, near-mature stand and mature stand. Carbon storage by plantation forests was 1.89 times higher than that by natural forests. Carbon density of the three species increased 8.58 Mg C/ha during the study period. The carbon density of Taihe County was higher in the east and west, and lower in the middle. The carbon density of Xingguo County was higher in the northeast and lower in the middle. In general, the carbon density increased with altitude and gradient. Afforestation projects contribute significantly to increasing stand area and carbon storage. Appropriate forest management may improve the carbon sequestration capacity of forest ecosystems. 相似文献
986.
987.
“层间氧化矿”是个旧锡矿的主要矿石类型,大部分具有特征的层控性,传统的观点认为“层间氧化矿”是原生硫化矿石在后期被雨水氧化而形成,已有部分地质研究者注意到了“层间氧化矿”的特殊性并提出它们是沉积成因或热水沉积成因。根据个旧锡矿的矿床特征与红海型热水沉积的对比,结合稀土元素地球化学研究,本文认为个旧锡矿存在的“层间氧化矿”的原生矿化类型是特殊的红海型热水沉积,以大量铁锰氧化物为主要成矿标志。个旧锡矿可能是目前红海型热水沉积作用登陆的唯一实例,其成矿模式可概括为三叠纪拉张形成多层原生红海型热水沉积矿化,燕山期花岗岩侵入改造;进而提出通过系统岩相古地理研究以确定古扩张中心和古卤水盆地中心的找矿模式。 相似文献
988.
989.
A quantitative procedure for analyzing and comparing distortion in mental maps is presented. Using a microcomputer and digitizer, and by developing a distortion index, comparisons are made by direct analysis of mental maps. A case study of the mental maps of Israel of high school and university students showed a tendency to generalize shape to simple geometric forms, with foreshortening of north-south lines and exaggeration of east-west distances. 相似文献
990.
At present the Nordic Seas are a key region of North Atlantic Deep Water (NADW) formation. Two alternative scenarios have been suggested by some authors for the Last Glacial Maximum: (i) the Nordic Seas were permanently covered by sea ice, preventing the formation of NADW, or (ii) that they were seasonally free of ice and that deep water formation did occur. A modified scenario is presented here based on parallel ocean circulation modelling results from the GFDL primitive equation model and a planetary geostrophic model. It is suggested that the glacial Nordic Seas were at least seasonally ice free, but it is observed that there was never deep water formation from the surface; rather it occurred only in the North Atlantic south of 40°–50°N. North of 40°N, the weaker LGM northward flowing thermohaline conveyor is subducted below a reverse conveyor which occurred to a depth of over 1000 m. Various modelling experiments presented here indicate that the reversed conveyor was primarily caused by the colder conditions of the glacial North Atlantic that led to far stronger zonality of glacial analogue of the North Atlantic Current. 相似文献