全文获取类型
收费全文 | 7194篇 |
免费 | 477篇 |
国内免费 | 1670篇 |
专业分类
测绘学 | 363篇 |
大气科学 | 703篇 |
地球物理 | 2894篇 |
地质学 | 3041篇 |
海洋学 | 968篇 |
天文学 | 293篇 |
综合类 | 484篇 |
自然地理 | 595篇 |
出版年
2024年 | 24篇 |
2023年 | 62篇 |
2022年 | 68篇 |
2021年 | 65篇 |
2020年 | 54篇 |
2019年 | 75篇 |
2018年 | 98篇 |
2017年 | 26篇 |
2016年 | 91篇 |
2015年 | 58篇 |
2014年 | 151篇 |
2013年 | 133篇 |
2012年 | 267篇 |
2011年 | 396篇 |
2010年 | 138篇 |
2009年 | 189篇 |
2008年 | 375篇 |
2007年 | 363篇 |
2006年 | 404篇 |
2005年 | 450篇 |
2004年 | 375篇 |
2003年 | 337篇 |
2002年 | 317篇 |
2001年 | 292篇 |
2000年 | 362篇 |
1999年 | 207篇 |
1998年 | 201篇 |
1997年 | 256篇 |
1996年 | 188篇 |
1995年 | 214篇 |
1994年 | 239篇 |
1993年 | 278篇 |
1992年 | 247篇 |
1991年 | 335篇 |
1990年 | 239篇 |
1989年 | 262篇 |
1988年 | 232篇 |
1987年 | 188篇 |
1986年 | 116篇 |
1985年 | 164篇 |
1984年 | 124篇 |
1983年 | 101篇 |
1982年 | 77篇 |
1981年 | 63篇 |
1980年 | 107篇 |
1979年 | 19篇 |
1964年 | 19篇 |
1959年 | 17篇 |
1958年 | 19篇 |
1957年 | 18篇 |
排序方式: 共有9341条查询结果,搜索用时 15 毫秒
141.
巴巴多斯和新几内亚的海平面变化表明:在过去的15000a中曾有过两次大规模的冰川融水流.同其它环境变化(如北大西洋深水环流)相比,这两次融水流形成的时间显然有助于我们进一步了解从未次冰期到问冰期的气候变迁及新仙女木事件. 相似文献
142.
激光烧蚀微探针等离子体质谱的地质应用。自1989年以来,我们实验室就积极开展了激光烧蚀微探针(LAM)试样引入系统的研究工作,这种技术适合于ICP—MS分析地质样品中小体积矿物(激光烧蚀孔穴直径低于10—100μm).对某些样品.采用Nd:YAG激光的基本IR波长(1064nm),作为多数用途,4倍频率(uv)的波长(266nm)能产生较小的直径,并较好地形成孔穴. 相似文献
143.
144.
Man-made defence structures (e.g., breakwaters, jetties) are becoming common features of marine coastal landscapes all around the world. The ecology of assemblages of species associated with such artificial structures is, however, poorly known. In this study, we evaluated the density and size of fish predators of echinoids (i.e., Diplodus sargus, Diplodus vulgaris, Sparus aurata), and the density of sea urchins (i.e., Paracentrotus lividus) at defence structures (i.e., breakwaters) inside and outside the marine protected area of Miramare (northern Adriatic Sea) in order to: (1) assess possible differences in fish predator density and size between protected and fished breakwaters; (2) assess whether fish predation may have the potential to affect sea urchin density in artificial rocky habitats. Surveys were carried out at four random times over a period of two years. Total density, and density of medium- and large-sized individuals of the three predatory fishes were generally greater at the protected than at the fished breakwaters, whereas no differences were detected in the density of small-sized individuals. Density of the sea urchin P. lividus did not show any difference between protected and fished breakwaters. The results of this study suggest that: (1) protection may significantly affect predatory fishes in artificial rocky habitats; (2) differences in predatory fish density, and size may be unrelated with the density of the sea urchin P. lividus; (3) protected artificial structures such as breakwaters, originally planned for other purposes, could represent a potential tool for fish population recovery and enhancement of local fisheries. 相似文献
145.
陆坡上水合物矿床中甲烷(CH4)的灾难性释放已显著干扰了古新世/始新世温度最高(PETM)期(约55.5Ma)的全球气候和碳循环。我们调查了所有PETM期古温度和古二氧化碳替代矿物是否与假设考虑大量增加的甲烷散发到大气圈的影响相一致。 相似文献
146.
A population of killifish (Fundulus heteroclitus) inhabiting a creosote-polluted inlet of the Elizabeth River demonstrates tolerance to the acute toxic effects exerted by contaminated sediments on reference site killifish. Previous data have suggested that upregulated antioxidant defenses contribute to short-term tolerance in killifish exposed to Elizabeth River sediments. This study investigated population differences in antioxidant defenses from wild caught Elizabeth River and reference population killifish in different seasons, and after being held in the laboratory. Parameters measured in the killifish were total glutathione concentrations (GSH(T)), activities of glutathione reductase (GR), glutathione peroxidase (GPx), and lipid peroxidation (LPO), all in adult hepatic tissues. The Elizabeth River population exhibited greater GSH(T), higher GPx activities, and increased LPO as compared to the reference population. Sex specific population differences were also observed in GSH(T) and GPx. Both populations displayed decreased GSH(T) and increased GR from early to late summer, as well as after being held in the laboratory. This study indicates that there are many factors that may contribute to differences in levels of antioxidant defenses in addition to exposure to contaminants, including reproductive status and environmental conditions. 相似文献
147.
不同厚度冰层下的冰下火山喷发产物:两个来自南极的实例J.L.Smellie等本文着重介绍南极半岛两个保存最好的冰下层序(亚历山大岛的芒特皮纳佛和格雷厄姆地北部的布朗布拉夫).通过对岩相和岩相组合的分析确定喷发时和喷发后作用过程,以及原始冰层的最小厚度... 相似文献
148.
149.
Jowett J 《Geography review》1989,3(2):26-31
The author presents a selection of cartograms designed to illustrate aspects of the demography of China. Topics covered include population growth, density, regional variations in fertility, and age distribution. A discussion of the advantages of using cartograms (maps drawn proportional to a given variable) as opposed to conventional, area-based maps, is included. 相似文献
150.
Trends in emigration from Ireland over time are reviewed. "During the eighteenth and nineteenth centuries overseas migration to the United States and seasonal harvest migration to Britain were the main types of movement, but over the past 100 years the Irish have developed a special affinity for settling in British towns. Although the outflow was halted for a time during the 1970s, when return migration took over, the 1980s have seen a renewal of the exodus. This time, however, the character of the flow has changed from predominantly low-skill construction and factory workers to embrace better-educated emigrants, including many graduates. This shift reflects Ireland's changing position in the international market for labour." 相似文献