全文获取类型
收费全文 | 3005篇 |
免费 | 532篇 |
国内免费 | 353篇 |
专业分类
测绘学 | 106篇 |
大气科学 | 65篇 |
地球物理 | 801篇 |
地质学 | 1662篇 |
海洋学 | 173篇 |
天文学 | 22篇 |
综合类 | 116篇 |
自然地理 | 945篇 |
出版年
2024年 | 17篇 |
2023年 | 49篇 |
2022年 | 96篇 |
2021年 | 116篇 |
2020年 | 114篇 |
2019年 | 130篇 |
2018年 | 87篇 |
2017年 | 103篇 |
2016年 | 124篇 |
2015年 | 109篇 |
2014年 | 135篇 |
2013年 | 168篇 |
2012年 | 134篇 |
2011年 | 144篇 |
2010年 | 131篇 |
2009年 | 160篇 |
2008年 | 195篇 |
2007年 | 167篇 |
2006年 | 163篇 |
2005年 | 144篇 |
2004年 | 161篇 |
2003年 | 127篇 |
2002年 | 126篇 |
2001年 | 121篇 |
2000年 | 139篇 |
1999年 | 106篇 |
1998年 | 95篇 |
1997年 | 99篇 |
1996年 | 68篇 |
1995年 | 60篇 |
1994年 | 63篇 |
1993年 | 52篇 |
1992年 | 55篇 |
1991年 | 40篇 |
1990年 | 35篇 |
1989年 | 23篇 |
1988年 | 14篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1981年 | 1篇 |
1975年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有3890条查询结果,搜索用时 250 毫秒
991.
晚冰期Younger Drayas环境灾变 总被引:3,自引:0,他引:3
新仙女木事件(Younger Dryas Event)是过去15000年里全球最为严重的环境灾变事件。它使得冰期气候急剧回返,陆地自然环境严重恶化,并且遣成更新世末期的生物界大绝灭。该事件在深海沉积、大陆冰盖和湖泊沉积物剖面都有多种十分显著的记录。对其成因和机制的研究,将会深入揭示太阳辐射—大气—海洋气—冰盖气—陆地气—生物圈相互作用系统中的非线性反馈现象,为探索人类面临的全球变化问题提供借鉴。 相似文献
992.
云南高原典型湖泊现代过程及环境演变研究进展 总被引:4,自引:1,他引:4
对云南省境内从南到北、由低至高分布的7个典型湖泊进行钻探取样,共钻取岩芯140m,同时对部分湖泊采集了表泥样品。对湖区古湖岸阶地、层状地貌和相关地层考察研究表明:主要存在着2~3次明显的高湖岸,高湖岸已受构造作用的强烈影响发生变形;杞麓湖、拉市海、文海等分布着高于落水洞时代很新的多层贝壳层,蛇山组沉积结束于末次冰期晚期及其它相关证据说明,该区现代水系可能是最近几万年才最终形成。 相似文献
993.
冰川,积雪及泥石流灾害研究的回顾与设想 总被引:5,自引:2,他引:3
文章总结了40年来中国科学院兰州冻川冻土研究所在冰川,积雪和泥石流灾害研究中取得的重要成果与进展,分析了冰雪和泥石流灾害的形势,提出了灾害研究的主要方向,其中有加强全球变化对灾害形成和发展的影响研究,以及国家经济建设重心西移后,人类活动可能造成的各类灾害的预研究等,最后提出了应采取的一些措施。 相似文献
994.
通过分析航空60MHz无线电回波数据,目前在南极已发现77个冰下湖。其中,最著名的是Vostok湖。这些湖泊主要是由于地热流,以及冰层底部滑动和冰内变形产生的热量,使冰盖底部冰温上升到压融点,并缓慢融化冰层形成的。冰下湖的发现具有冰川学、地质学,尤其是潜在的重要的生物学意义。 相似文献
995.
J.F. Boyle A.W. Mackay N.L. Rose R.J. Flower P.G. Appleby 《Journal of Paleolimnology》1998,20(2):135-150
Geochemical analysis of six radiometrically dated short cores of recent sediment from Lake Baikal shows clear evidence of enhanced Pb supply. However, the sediment concentration increases are very small; the average Pb concentration rises from a baseline value of 10.9 g g-1 to a peak value of only 14.8 g g-1. In contrast to the more polluted lakes commonly studied in Europe and North America, variation in Pb concentration is far more strongly influenced by natural variation than by pollution. In sediment deposited over the last 150–200 years 73% of the variance in the sediment Pb concentration can be accounted for by variation in bulk composition of the sediment, and by atmospheric pollution. Factors influencing Pb concentrations over this time period are, in order of decreasing average importance (fraction of total variance explained), catchment supply (indicated by 226Ra activity variation) (43%), anthropogenic Pb emissions (24%), and dilution by ferromanganese hydroxides (6%). On longer (1000s of years) time scales dilution by biogenic silica is probably more important.The recent enhanced supply of catchment Pb correlates with accelerating accumulation rates, indicating a link with enhanced erosion. Anthropogenic sources dominate only in the southern basin, where local fossil-fuel burning industry is situated. The evidence for a local industrial source for the Pb pollution is strengthened by the high correlation between the inventories for Pb and for spheroidal carbonaceous particles. The absence of detectable anthropogenic Pb enrichment in the northern part of the lake suggests that long-distance Pb pollution is small compared with the local natural supply. 相似文献
996.
997.
ThebackgroundfeaturesofnaturalenvironmentandtheharnesingpaternforthespreadofsalinelandintheSongnenPlainZHANGBaiChangchunInsti... 相似文献
998.
Stable isotopes and sediments from Pickerel Lake, South Dakota, USA: a 12ky record of environmental changes 总被引:2,自引:0,他引:2
Sedimentological parameters and stable O- and C-isotopic composition of marl and ostracode calcite selected from a 17.7-m-long core from the 8-m-deep center of Pickerel Lake, northeastern South Dakota, provide one of the longest (ca. 12ky) paleoenvironmental records from the northern Great Plains. The late Glacial to early Holocene climate in the northern Great Plains was characterized by changes from cold and wet to cold and dry, and back to cold and wet conditions. These climatic changes were controlled by fluctuations in the positions of the Laurentide ice sheet and the extent of glacial Lake Agassiz. We speculate that the cold and dry phase may correspond to the Younger Dryas event. A salinity maximum was reached between 10.3 and 9.5 ka, after which Pickerel Lake shifted from a system controlled by atmospheric changes to a system controlled by groundwater seepage that might have been initiated by the final withdrawal of Glacial Lake Agassiz. A prairie lake was established at approximately 8.7 ka, and lasted until about 2.2 ka. During this mid-Holocene prairie period, drier conditions than today prevailed, interrupted by periods of increased moisture at about 8, 4, and 2.2 ka. Prairie conditions were more likely dry and cool rather than dry and warm. The last 2.2 ka are characterized by higher climatic variability with 400-yr aridity cycles including the Medieval Warm Period and the Little Ice Age.Although the signal of changing atmospheric circulation is overprinted by fluctuations in the positions of the ice sheet and glacial Lake Agassiz during the late Glacial-Holocene transition, a combination of strong zonal circulation and strong monsoons induced by the presence of the ice sheet and high insolation may have provided mechanisms for increased precipitation. Zonal flow introducing dry Pacific air became more important during the prairie period but seems to have been interrupted by short periods of stronger meridional circulation with intrusions of moist air from the Gulf of Mexico. More frequent switching between periods of zonal and meridional circulation seem to be responsible for increased climatic variability during the last 2.2 ka. 相似文献
999.
Potential seasonal variation in the U-series radioisotopic signatures of lake and input waters was investigated in Hawes Water, north-west England, and the sedimentary record was examined to establish whether any observed seasonality is preserved in the sediment U and Th radioisotopic activity ratios. U-series disequilibrium in the waters is indicative of both surface and shallow sub-surface origins for the dissolved U, and that this supply is influenced by changes in the prevailing hydrological regime. However, the lake is buffered from this seasonality in that the 234U/238U activity ratio remains constant during the year, and the U content undergoes only a marginal decrease during summer. The change in the mechanism of lake sedimentation from allochthonous organic input during winter to autochthonous carbonate formation during summer has a more significant influence on the U-series radioisotopic characteristics of sedimentary record in that the proportion of organic and carbonate material both exhibit a strong control on the234U/238U activity ratio and U content. In addition, significant proportions of sediment U and Th are associated with ferromanganese oxyhydroxides. Given this seasonality, it is of some significance that the 234U/238U activity ratios of the bottom sediments preserve a reasonable record of lake U-series disequilibrium and are, therefore, potential indicators of long-term changes in catchment hydrology. 相似文献
1000.