全文获取类型
收费全文 | 1385篇 |
免费 | 253篇 |
国内免费 | 243篇 |
专业分类
测绘学 | 58篇 |
大气科学 | 231篇 |
地球物理 | 390篇 |
地质学 | 744篇 |
海洋学 | 129篇 |
天文学 | 52篇 |
综合类 | 128篇 |
自然地理 | 149篇 |
出版年
2024年 | 6篇 |
2023年 | 18篇 |
2022年 | 43篇 |
2021年 | 49篇 |
2020年 | 49篇 |
2019年 | 54篇 |
2018年 | 61篇 |
2017年 | 59篇 |
2016年 | 82篇 |
2015年 | 71篇 |
2014年 | 83篇 |
2013年 | 79篇 |
2012年 | 77篇 |
2011年 | 74篇 |
2010年 | 77篇 |
2009年 | 71篇 |
2008年 | 61篇 |
2007年 | 50篇 |
2006年 | 56篇 |
2005年 | 39篇 |
2004年 | 40篇 |
2003年 | 57篇 |
2002年 | 86篇 |
2001年 | 72篇 |
2000年 | 69篇 |
1999年 | 61篇 |
1998年 | 46篇 |
1997年 | 38篇 |
1996年 | 49篇 |
1995年 | 47篇 |
1994年 | 35篇 |
1993年 | 24篇 |
1992年 | 22篇 |
1991年 | 18篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 14篇 |
1987年 | 10篇 |
1986年 | 3篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1981年 | 3篇 |
1978年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有1881条查询结果,搜索用时 179 毫秒
991.
大别山区司空山两类花岗岩体的岩石学和地球化学特征及其成因 总被引:3,自引:0,他引:3
对司空山片麻状花岗岩及中生代花岗闪长岩和白岗岩的岩石学、地球化学、微量元素以及稀土组成模式的研究,说明司空山两种类型花岗岩体物质来源不同,片麻状花岗岩是由上部陆壳重熔产生的;花岗闪长岩则由深部低Rb/Sr比的岩石同熔作用形成的。 相似文献
992.
活动断裂的遥感影像研究 总被引:1,自引:0,他引:1
本文以遥感信息作为研究活动断裂的依据,探讨了利用不同种类、不同比例尺遥感图像分析活动断裂的方法、程序和效果。 相似文献
993.
膨胀土的矿物组成与膨胀特性关系的试验研究 总被引:15,自引:1,他引:15
介绍了用χ射线衍射法、热分析和次甲基兰-氯化亚锡定量蒙脱石等方法,对采自我国十省的膨胀土进行了矿物组成特性方面的试验研究。并讨论了它们与膨胀土的膨胀势等的关系 相似文献
994.
995.
Few studies of land use change were particularly considered the hierarchical data structure originating from different scales
and levels. Using interviewing data collected from 107 villages, 1,050 households and 4,780 fields between November 2003 and
August 2005, the objective of this paper is to predict the occurrence of land use from field to village level in mountainous
area, China, and to improve our understanding of the causes of land use. Household’s behavior in the choice of land use type
is guided by multiple, often confiding, household objectives, subject to the available resources, possible productive activities,
and external economic and biophysical constraints. For rice model, the household level variables cannot be substituted by
village level aggregates. Aggregated variables at village level do not capture any of the variability at the household level.
Village level variables can virtually be explained in virtue of the variables of field and household level. The households
and the villages show significant clustering of the occurrence of rice, and they explain the 11.3 and 4.5% variance, respectively.
For corn model, corn as dependent variable does not show any significant variance component. The variables of household and
village level have lower effects on the occurrence of corn. There is a significant relation between slope of a field and the
choice to cultivate corn and a significant random effect at the village level. However, cropland size, input–output, transportation
cost, even family income in household level and road density and food market development in village level, at some extent,
are controlled by slope. These variables do not influence corn cultivation significantly and that those are predominantly
determined by slope. In a word, the household level can be crucial in explaining land use at the field level. Multilevel analysis
can be applied to statistically model the occurrence of land use, and to explore a number of cross-scale propositions.
An erratum to this article can be found at 相似文献
996.
新疆塔里木板块西北缘早二叠世深水遗迹化石的发现及意义 总被引:2,自引:0,他引:2
在新疆塔里木板块西北缘下二叠统比尤勒提群中部首次发现了大量深水遗迹化石,主要包括Glockeria Ksiazkiewicz1968,Helminthoida sp.,Megagrapton sp.,Paleodictyon sp.,Paleodictyon(Glenodictyum)Croaticum Ulchman1995,Planolites sp.,Protopaleodictyon sp.,Scalaritubamissouriensis Weller1899,Spirophycus sp.等,代表典型深海环境的Nereites遗迹相。根据对温古尔剖面的研究,比尤勒提群下部为浅海陆棚相砂泥质灰岩、粉砂质泥岩、粉砂岩、泥晶灰岩等;而含丰富遗迹化石的比尤勒提群中部为一套深海海底扇沉积的浊积岩系夹少量硅质岩,并且在浊积扇的不同位置所产遗迹组合类型也明显不同,扇根以觅食迹为主,含大量穿相分子,扇中开始出现牧食迹,扇梢则出现特征的耕作迹;比尤勒提群上部为浅海陆棚-滨海相沉积的硅质灰岩、泥晶灰岩和砂质灰岩及粉砂质泥岩、钙质砂岩等。该套遗迹化石组合与深海浊积事件密切相关,同时表明塔里木板块西北缘早二叠世时期存在一个陆棚浅海-深海盆地沉积环境,晚二叠世时期海水向西退去形成陆相沉积。 相似文献
997.
内蒙古喀喇沁早中生代闪长岩的岩石学,地球化学及其成因 总被引:7,自引:3,他引:4
本文研究了内蒙古喀喇漏沁地区含镁铁质-超镁铁质堆晶岩和基性麻粒岩捕虏体的闪长岩的同位素年代学、岩石学及地球化学,得到以下几点认识:(1)闪长岩全岩和单矿物K-Ar年龄为219~223Ma。(2)闪长岩与其中的堆晶岩和基性麻粒岩捕虏无成因联系;(3)各类闪长岩可能是太古宙斜长角闪岩高度部分熔融的产物;(4)闪长岩的形成与研究区早中生代的底侵作用有关,反映了伸展构造背景。 相似文献
998.
Liu Yunhuan Li Yong Shao Tiequan Zhu Zhixin Yu Bo Wang Zhuo Zhang Wanqian Li Rongxi 《Frontiers of Earth Science》2007,1(1):1-11
The appearance of coelenterates marks the real beginning of metazoan evolution. It therefore has a prominent position in the
origin and evolutionary history of organisms, and is also a pivotal question of evolutionary biology. Punctatus is an extinct, ancient marine animal from the early stage of the Cambrian explosion, occurring at the lowermost Cambrian
of both Kuanchuanpu (Ningqiang, Shaanxi) and Maidiping (Emei, Sichuan) areas. Punctatus has been studied for many years since the discovery of its fragments. Systematic and phylogenetic analysis has long been
limited because of the rarity of complete specimens. In order to improve research into Punctatus, more than ten thousand globular fossils were recovered by means of “Chemistry Retting”. On the basis of the study of these
globular fossils, a series of Punctatus fossils with cone parts and finely preserved soft-tissue mouthparts and fossilized metazoan embryo were recovered. Through
research on characteristics such as shape, modality and structure of these fossils symbiotic with Punctatus emeiensis, the author found many possible embryo fossils including the evidence of gastrula-stage animal fossils. The sequence of fetation
might have appeared on the corolliform oral region of Punctatus emeiensis. A study of the soft tissues, functional morphology and the sequence of embryo fossils shows evidence that Punctatus resembles coelenterate polyps in systematic classification. Perhaps it also represents an ancestor of the coelenterate with
an original tentacle. A comparison with the real “tentacle animal” found in the Chengjiang Fauna, shows that the original
tentacle is very tiny and its function range is limited. This reveals the primitive nature of the animal. Although the original
tentacle is so small, it does exist, representing the first big step towards the real flexible tentacle with a strong function
from the early evolving tentacle.
Translated from Journal of Acta Palaeontologica Sinica, 2006, 45(2): 182–194 [译自: 古生物学报] 相似文献
999.
1000.
Konglan Shao Jianping Zhang Hongbo Zheng Zhaoyan Gu Bing Xu Qing Yang Keyang He Huayu Lu 《Boreas: An International Journal of Quaternary Research》2023,52(4):507-516
Extensive fires pose catastrophic threats to both human and natural ecosystems. Understanding the history of fire, particularly Holocene palaeofire activity in densely populated areas, is essential for predicting future fire risks and developing effective fire management policies. The complexity of fire activity is influenced by various factors, including climate and anthropogenic activities. In this study, we analysed microcharcoal from the top 35.36 m of a well-dated sediment core HMD1401 in Ningshao Plain, eastern China. We combined our findings with phytolith and diatom evidence to obtain a comprehensive understanding of variations in Holocene fire activity and its controls. The results showed that there was higher fire activity during the early and late Holocene and less fire activity during the mid-Holocene. More frequent fire occurred from c. 10 000–7000 cal. a BP and was primarily caused by abundant biomass and high seasonal flammability due to increased annual temperature and precipitation and warm but dry winter climate. Fire occurrences between c. 7000–2000 cal. a BP remained at a low level, except for the periods c. 5900–5600 cal. a BP and c. 5300 cal. a BP, which may have been caused by extreme climate events. The impact of fire caused by human activity was significantly enhanced during the last two millennia. 相似文献