全文获取类型
收费全文 | 6309篇 |
免费 | 197篇 |
国内免费 | 94篇 |
专业分类
测绘学 | 177篇 |
大气科学 | 609篇 |
地球物理 | 1536篇 |
地质学 | 2220篇 |
海洋学 | 573篇 |
天文学 | 836篇 |
综合类 | 22篇 |
自然地理 | 627篇 |
出版年
2021年 | 66篇 |
2020年 | 74篇 |
2019年 | 90篇 |
2018年 | 140篇 |
2017年 | 126篇 |
2016年 | 168篇 |
2015年 | 158篇 |
2014年 | 218篇 |
2013年 | 340篇 |
2012年 | 266篇 |
2011年 | 282篇 |
2010年 | 208篇 |
2009年 | 335篇 |
2008年 | 313篇 |
2007年 | 285篇 |
2006年 | 237篇 |
2005年 | 207篇 |
2004年 | 207篇 |
2003年 | 201篇 |
2002年 | 193篇 |
2001年 | 143篇 |
2000年 | 149篇 |
1999年 | 124篇 |
1998年 | 117篇 |
1997年 | 97篇 |
1996年 | 91篇 |
1995年 | 95篇 |
1994年 | 81篇 |
1993年 | 70篇 |
1992年 | 67篇 |
1991年 | 73篇 |
1990年 | 70篇 |
1989年 | 64篇 |
1988年 | 58篇 |
1987年 | 72篇 |
1986年 | 46篇 |
1985年 | 75篇 |
1984年 | 87篇 |
1983年 | 74篇 |
1982年 | 69篇 |
1981年 | 80篇 |
1980年 | 65篇 |
1979年 | 62篇 |
1978年 | 39篇 |
1977年 | 58篇 |
1976年 | 65篇 |
1975年 | 46篇 |
1974年 | 58篇 |
1973年 | 48篇 |
1972年 | 28篇 |
排序方式: 共有6600条查询结果,搜索用时 15 毫秒
11.
Lance B. Morrissey Philippe Janvier Simon J. Braddy John P. Bennett Susan B. Marriott Peter R. Tarrant 《Geology Today》2006,22(2):66-67
Armoured jawless fish, or 'ostracoderms', lived 450–360 million years ago, and display unusual morphologies, unlike any modern fish group. Since they left no living descendants, their mode of swimming has, until recently, remained speculative, although this is a crucial question as the first true pectoral fins evolved within the 'ostracoderms'. The discovery of the oldest-known fish trails, from the Early Devonian (400 million year old) 'Lower Old Red Sandstone' of south-east Wales offers new insights into the swimming behaviour of these early fish, notably the osteostracan 'ostracoderms' (or cephalaspids), whose horseshoe-shaped head and paddle-shaped pectoral fins have remained a functional riddle. 相似文献
12.
Peter S. Fiske 《Meteoritics & planetary science》1996,31(1):42-45
Abstract— The size, shape, composition, and vesicle content of 6 kg of layered tektite fragments, excavated near the town of Huai Sai, Thailand, place some constraints on the formation of layered tektites. The mass, shape, and distribution of the fragments are not consistent with an origin as a “puddle” of impact melt but suggest that they were derived from a single equant block. The presence of vesicles up to 7 mm in mean diameter within the tektite fragments suggests that the material was too viscous to allow for significant gravity-driven flow. These results suggest that layered tektites may be analogous to lava bombs, which may have been stretched and deformed in flight but underwent little flow after landing. Rather than being a product of “unusual circumstances,” such as multiple impacts, layered tektites may differ from splash-form tektites only in initial temperature of formation, speed of ejection, and small differences in initial composition. 相似文献
13.
G. Caro V.C. Bennett B. Bourdon T.M. Harrison A. von Quadt S.J. Mojzsis J.W. Harris 《Chemical Geology》2008,247(1-2):253-265
146Sm–142Nd and 147Sm–143Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe 207Pb/206Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ?142Nd = ? 6 ± 12 ppm, ?143Nd = ? 32.5, and 147Sm/144Nd = 0.1150. These results do not confirm previous claims for a 30 ppm 142Nd excess in South African cratonic mantle. The lack of a 142Nd anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 × ?142Nd = 8 ± 10 ppm, ?143Nd = 45 ± 1, and 147Sm/144Nd = 0.5891. Back-calculation of this present-day ?143Nd yields an unrealistic estimate for the initial ?143Nd of ? 160 ?-units, clearly indicating post-crystallization disturbance of the 147Sm–143Nd system. Examination of 146,147Sm–142,143Nd data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U–Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5–10% of total sample, its high LREE content precludes a reliable evaluation of 146Sm–142Nd systematics in Jack Hills zircons. 相似文献
14.
F. Steinhauser W. Mörikofer J. Drimmel Peter Steinhauser 《Meteorology and Atmospheric Physics》1965,15(2):251-264
Ohne Zusammenfassung 相似文献
15.
16.
17.
Andrew R. Bowie Eric P. Achterberg Peter L. Croot Hein J.W. de Baar Patrick Laan James W. Moffett Simon Ussher Paul J. Worsfold 《Marine Chemistry》2006
The first large-scale international intercomparison of analytical methods for the determination of dissolved iron in seawater was carried out between October 2000 and December 2002. The exercise was conducted as a rigorously “blind” comparison of 7 analytical techniques by 24 international laboratories. The comparison was based on a large volume (700 L), filtered surface seawater sample collected from the South Atlantic Ocean (the “IRONAGES” sample), which was acidified, mixed and bottled at sea. Two 1-L sample bottles were sent to each participant. Integrity and blindness were achieved by having the experiment designed and carried out by a small team, and overseen by an independent data manager. Storage, homogeneity and time-series stability experiments conducted over 2.5 years showed that inter-bottle variability of the IRONAGES sample was good (< 7%), although there was a decrease in iron concentration in the bottles over time (0.8–0.5 nM) before a stable value was observed. This raises questions over the suitability of sample acidification and storage. 相似文献
18.
19.
20.