全文获取类型
收费全文 | 304篇 |
免费 | 44篇 |
国内免费 | 60篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 6篇 |
地球物理 | 93篇 |
地质学 | 160篇 |
海洋学 | 77篇 |
天文学 | 13篇 |
综合类 | 22篇 |
自然地理 | 32篇 |
出版年
2022年 | 5篇 |
2021年 | 16篇 |
2020年 | 16篇 |
2019年 | 14篇 |
2018年 | 7篇 |
2017年 | 14篇 |
2016年 | 15篇 |
2015年 | 16篇 |
2014年 | 14篇 |
2013年 | 16篇 |
2012年 | 19篇 |
2011年 | 20篇 |
2010年 | 16篇 |
2009年 | 13篇 |
2008年 | 20篇 |
2007年 | 28篇 |
2006年 | 34篇 |
2005年 | 14篇 |
2004年 | 14篇 |
2003年 | 11篇 |
2002年 | 16篇 |
2001年 | 11篇 |
2000年 | 15篇 |
1999年 | 6篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1989年 | 1篇 |
1986年 | 1篇 |
1980年 | 2篇 |
排序方式: 共有408条查询结果,搜索用时 15 毫秒
81.
82.
河北平原地下水氘过量参数特征 总被引:11,自引:0,他引:11
氘过量参数是由Dansg aar d提出的一个新概念,它被定义为:d= δD-8δ18O。河北平原地下水氘过量参数有三个特征: ( 1)地下热水的氢和氧同位素组成显示出热交换的态势,d 值随地下水年龄增大而减少。( 2)在同一地区,d 值随着地下水埋深加大而增大。( 3)在同一含水层内,沿着地下水的路径,从补给区到承压区, d 值随着地下水年龄增大而增大。我们认为, d 值虽然是地下水年龄的函数,但最好和3 H、3H- 3He、14 C、36 Cl和4 He测年结果结合使用。 相似文献
83.
Yetang Hong Dongsheng Liu Hongbo Jiang Liping Zhou J. Beer Bing Hong Yongxuan Zhu Handing Li Xuetian Leng Xiaoguang Qin Wang Yu Qinghua Lin Yiqiang Zeng 《中国科学D辑(英文版)》2000,43(2):217-224
There have been a number of investigations for examining the possible link between long-term climate variability and solar
activity. A continuous δ18O record of peat cellulose covering the past 6 000 years and the response of climate variation inferred from the proxy record
to solar forcing are reported. Results show that during the past 5 000 years the abrupt climate variations, including 17 warming
and 17 cooling, and a serious of periodicities, such as 86, 101, 110, 127, 132, 140, 155, 207, 245, 311, 820 and 1 050 years,
are strikingly correlative to the changes of solar irradiation and periodicity. These observations are considered as further
evidence for a close relationship between solar activity and climate variations on time scales of decades to centuries. 相似文献
84.
Two stages of isotopic exchanges experienced by the Ertaibei granite pluton, northern Xinjiang, China 总被引:1,自引:0,他引:1
LIU Wei 《中国科学D辑(英文版)》2000,43(6):605-616
18O/16O and D/H of coexisting feldspar, quartz, and biotite separates of twenty samples collected from the Ertaibei granite pluton,
northern Xinjiang, China are determined. It is shown that the Ertaibei pluton experienced two stages of isotopic exchanges.
The second stage of18O/16O and D/H exchanges with meteoric water brought about a marked decrease in the δ18O values of feldspar and biotite from the second group of samples. The D/H of biotite exhibits a higher sensitivity to the
meteoric water alteration than its18O/16O. However, the first stage of18O/16O exchange with the18O-rich aqueous fluid derived from the dehydration within the deep crust caused the Δ18OQuartz-Feldspar reversal. It is inferred that the dehydration-melting may have been an important mechanism for anatexis. It is shown that
the deep fluid encircled the Ertaibei pluton like an envelope which serves as an effective screen to the surface waters. 相似文献
85.
Summary. Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review
we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial r?le in the
cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity
can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve
many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields,
outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metal-poor
galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and
the physical processes that are at work in these objects. The question on how they are related and may evolve from one class
of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion
of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe,
we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in
the universe. We discuss what do QSO absorption lines and known distant galaxies tell us already? We illustrate the importance
of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator
and discuss the possibility to detect nearly metal-free galaxies at high redshift from Ly emission.
Received 19 August 1999 / Published online: 15 February 2000 相似文献
86.
Identifying the source of atmospheric moisture over arid deserts using stable isotopes (2H and 18O) in precipitation 下载免费PDF全文
Wenbo Rao Wenbing Zhang Bin Yong Hongbing Tan Karina T. Meredith Ke Jin Fangwen Zheng Shuai Wang 《水文研究》2018,32(3):436-449
Precipitation is a major component of the hydrologic cycle in arid desert areas. To date, however, few studies have been conducted on investigating the isotope characteristics and moisture sources of precipitation in arid desert environments. The Alxa Desert Plateau is a critical arid desert area in North China. This study is the first to analyse the stable isotopic composition of precipitation to identify the sources of atmospheric moisture over this plateau. Our results show that the δD and δ18O values of precipitation across the plateau change greatly at both daily and monthly timescales, and exhibit seasonal variations. Among the main meteorological parameters, atmospheric temperature is the most predominant factor controlling the isotopic composition and the δD–δ18O relationship of local precipitation. Analyses of the precipitation isotopes with the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model reveal that (a) the westerly and polar moisture sources are the dominant controls on summer and winter precipitation and (b) the evaporation of local lake water significantly affects winter precipitation even though it only represents a small amount. Based on the isotope data of 2013–2016 precipitation, a local meteoric water line (LMWL) is derived: δD = (8.20 ± 0.22)183;δ18O + (8.15 ± 2.16)‰ for the study site. Compared to the global meteoric water line, the LMWL has a greater slope and lower d‐excess. This can be explained by admixing of atmospheric moisture resulting from the evaporation of local lake water. Based on this LMWL, we are able to trace that groundwater of the Badain Jaran Desert originates from the surrounding mountains with altitudes of <4,000 m. The newly derived LMWL shows that the recharge altitudes of desert groundwater are overestimated on the basis of the previous LMWLs. This study not only provides insights into the hydrological cycle but also offers guidance for water resource management in arid desert areas of China. Additionally, this study provides techniques that can be applied to the analyses of precipitation isotopes in similar arid regions of the world. 相似文献
87.
最近 40年 ,硝酸盐 (NO-3 )已成为最常见的地下水污染源之一。本文介绍了用CaO除去CO2 与H2 O的测定氮同位素比值的燃烧管方法和用AgNO3 +C(石墨 )生成CO2 的测定NO-3 中氧同位素比值的燃烧法 ,它们是 1995年以后发展起来的最新测试技术。 相似文献
88.
Study of evaporation and recharge in desert soil using environmental tracers, New Mexico, USA 总被引:4,自引:0,他引:4
The purpose of this study is to investigate the rates and mechanisms of recharge and evaporation in soils of a desert environment
using two environmental tracers (chloride and oxygen-18). The profiles of chloride concentration and oxygen-18 enrichment
in soil-water, together with the depth distribution of water content in soil, reveal information about long-term recharge
and instantaneous evaporation processes without needing to know the physical properties of the soil. Three holes were hand-augured,
in different desert settings in southeastern New Mexico. The chloride concentration profiles were used, with the chloride
mass balance method, to estimate long-term recharge rates in these three holes as 0.5, 0.8, and 2.4 mm yr–1. Analysis using a bimodal flow and transport model shows that possibly 85% of the recharge occurs via movement of water through
preferred pathways in the root zone. Preferential flow was evident in all three sampling sites. Clay layers have a noticeable
effect on the development of water content distribution and thus on oxygen-18 enrichment and chloride concentration profiles.
The spatial variation in clay layering partly explains the variation in recharge rate estimates.
Received: 13 October 1995 183; Accepted: 15 November 1995 相似文献
89.
全新世早期弱夏季风事件的精确定位及机制探讨--以湖南莲花洞LHD5石笋为例 总被引:2,自引:0,他引:2
全新世早期是太阳辐射加强、全球温度上升,并伴随着冰盖消融的重要时期,而其间发生的冷事件以及亚洲季风区的弱夏季风事件的成因一直是全新世早期研究的重点。对亚洲季风-海洋-极地联系研究有着重要的意义。通过分析湖南莲花洞LHD5石笋28个U/Th年龄和535个氧同位素数据重建了全新世亚洲季风演化特征,其中全新世早期分辨率达8年。LHD5石笋记录到YD结束时间为11748±30 a B.P.,全新世开始于11684±39 a B.P.,转换时间约为64年,与格陵兰gicc05记录在误差范围内一致。LHD5石笋记录到全新世早期6次弱夏季风事件,事件年龄中心点分别为11461±34 a B.P.、10354±36 a B.P.、9957±25 a B.P.、9062±36 a B.P.、8744±23 a B.P.、8144±24 a B.P.,其δ18O值的波动幅度分别为1.08‰、0.94‰、0.66‰、0.90‰、0.55‰、1.02‰,这些弱季风事件在亚洲季风区具有普遍的区域意义。除8.2 ka事件之外,10 ka B.P.之前的弱季风事件除了受到太阳活动的影响,还受到北大西洋IRD事件的影响,而之后更多地受到太阳活动和ITCZ南移的影响。 相似文献
90.
地区大气降水线(LMWL)是进行区域水循环研究的基准。受监测站点数量的限制,大多数地区无法获得,只能参考最近监测站点的降水线或全球大气降水线(GMWL),或者需要较长时间的监测后才能获得。传统的获取方法存在较多的限制。本研究以北京平原区为例,提出了一种获取地区大气降水线(LMWL)的新方法:通过对不同深度的第四系地下水进行取样测试获取同位素数据,然后拟合获得。通过与传统方法所得的本区大气降水线对比,表明本次所得方程的相关系数更大,标准误差小,样本量大,拟合效果更好。同时可以节约大量的监测时间与人力物力,且可获得长时间序列的历史数据,较为简单实用。该方法也存在一定的局限,要求取样时尽量选取颗粒细小的冲积平原地带。 相似文献