全文获取类型
收费全文 | 43306篇 |
免费 | 3801篇 |
国内免费 | 5593篇 |
专业分类
测绘学 | 2618篇 |
大气科学 | 5911篇 |
地球物理 | 9731篇 |
地质学 | 20894篇 |
海洋学 | 3821篇 |
天文学 | 2427篇 |
综合类 | 3820篇 |
自然地理 | 3478篇 |
出版年
2024年 | 127篇 |
2023年 | 417篇 |
2022年 | 925篇 |
2021年 | 1097篇 |
2020年 | 857篇 |
2019年 | 915篇 |
2018年 | 5532篇 |
2017年 | 4774篇 |
2016年 | 3511篇 |
2015年 | 1205篇 |
2014年 | 1208篇 |
2013年 | 1241篇 |
2012年 | 2058篇 |
2011年 | 3737篇 |
2010年 | 3118篇 |
2009年 | 3315篇 |
2008年 | 2755篇 |
2007年 | 3207篇 |
2006年 | 774篇 |
2005年 | 876篇 |
2004年 | 901篇 |
2003年 | 965篇 |
2002年 | 832篇 |
2001年 | 602篇 |
2000年 | 706篇 |
1999年 | 948篇 |
1998年 | 795篇 |
1997年 | 783篇 |
1996年 | 749篇 |
1995年 | 656篇 |
1994年 | 589篇 |
1993年 | 497篇 |
1992年 | 413篇 |
1991年 | 290篇 |
1990年 | 237篇 |
1989年 | 193篇 |
1988年 | 191篇 |
1987年 | 103篇 |
1986年 | 113篇 |
1985年 | 69篇 |
1984年 | 59篇 |
1983年 | 58篇 |
1982年 | 55篇 |
1981年 | 62篇 |
1980年 | 66篇 |
1979年 | 36篇 |
1978年 | 14篇 |
1976年 | 12篇 |
1974年 | 9篇 |
1958年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
Biogeochemical records of paleoenvironmental changes in Nainital Lake,Kumaun Himalayas,India 总被引:1,自引:0,他引:1
Preetam Choudhary Joyanto Routh Govind J. Chakrapani Bhishm Kumar 《Journal of Paleolimnology》2009,42(4):571-586
Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns
about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield
a mean sedimentation rate of ~4.7 mm year−1. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ13C, δ15N, and δ34S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC
flux varies from 1.0 to 3.5 g m−2 year−1. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m−2 year−1) N, but high (0.37–1.0 g m−2 year−1) S flux. Shifts in δ13C, δ15N, and δ34S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain
hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of
the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the
lake has remained largely unchanged, and the perceived human-induced impacts are limited. 相似文献
52.
Janice Brahney John J. Clague Brian Menounos Thomas W. D. Edwards 《Journal of Paleolimnology》2008,40(1):489-505
The level of Kluane Lake in southwest Yukon Territory, Canada, has fluctuated tens of metres during the late Holocene. Contributions
of sediment from different watersheds in the basin over the past 5,000 years were inferred from the elemental geochemistry
of Kluane Lake sediment cores. Elements associated with organic material and oxyhydroxides were used to reconstruct redox
fluctuations in the hypolimnion of the lake. The data reveal complex relationships between climate and river discharge during
the late Holocene. A period of influx of Duke River sediment coincides with a relatively warm climate around 1,300 years BP.
Discharge of Slims River into Kluane Lake occurred when Kaskawulsh Glacier advanced to the present drainage divide separating
flow to the Pacific Ocean via Kaskawulsh and Alsek rivers from flow to Bering Sea via tributaries of Yukon River. During periods
when neither Duke nor Slims river discharged into Kluane Lake, the level of the lake was low and stable thermal stratification
developed, with anoxic and eventually euxinic conditions in the hypolimnion. 相似文献
53.
荒漠土壤微生物群落结构特征研究进展 总被引:1,自引:2,他引:1
荒漠生态系统占地球陆地面积三分之一,是地球化学循环中的重要部分。荒漠干旱高温、缺乏植被、UV辐射强,曾被认为是没有生命的地方。然而在这恶劣环境中却蕴含有大量的微生物资源,尤其是荒漠土壤富集了大量微生物。微生物参与和主导整个荒漠生态系统地球生物化学循环,对于调节重要生态过程、修复和稳定荒漠生态系统起到重要作用,对其的研究生态学意义突出。本文综述了国内外对荒漠土壤微生物群落结构特征、群落功能多样性以及微生物群落、微生物与植物、微生物与环境之间相互关系的研究现状,旨在充分了解荒漠土壤微生物多样性研究,总结对荒漠微生物生态认识的不足,为荒漠微生物生态研究方向提供参考。 相似文献
54.
植物与大气之间的湍流交换是地球和大气系统中能量和物质循环的一个重要环节,也是土壤-植物-大气系统的重要研究内容之一。揭示这一交换环节的物理机制,了解地气系统中能量、水分和C0.2等物质的运动和转换规律,对于农业生产、数值气象预报,环境生态和气候变化等研究工作有诸多方面的意义,是地学界所共同关心的问题。近十年来,植物与大气之间相互作用的研究有了许多重要进展,传统的局地湍流交换模式的局限性已逐渐被认识,一些新的方法在这一领域中出现,其中高阶闭合模式和流轨模式是比较重要的探索途径。 相似文献
55.
56.
Natural Resources Research - High-intensity coal mining in grasslands has negative impact on the surface ecology of the mining area and its surroundings. This impact shows spatio-temporal... 相似文献
57.
The application of the saddlepoint approximation to reliability analysis of dynamic systems is investigated. The failure event in reliability problems is formulated as the exceedance of a single performance variable over a prescribed threshold level. The saddlepoint approximation technique provides a choice to estimate the cumulative distribution function (CDF) of the performance variable. The failure probability is obtained as the value of the complement CDF at a specif ied threshold. The method requires computing the saddlepoint from a simple algebraic equation that depends on the cumulant generating function (CGF) of the performance variable. A method for calculating the saddlepoint using random samples of the performance variable is presented. The applicable region of the saddlepoint approximation is discussed in detail. A 10-story shear building model with white noise excitation illustrates the accuracy and effi ciency of the proposed methodology. 相似文献
58.
The Ocean Drilling Program (ODP) initiated drilling at Site 1256D in the Guatemala Basin, about 1,000 km off the East Pacific
Rise to penetrate plutonic rocks, anticipated to be relatively shallow in this region, formed at an ultra-fast spreading rate.
IODP Expedition E312 successfully drilled into gabbros at ~1,150 m in basement. Multi-channel seismic traces show weak laterally
coherent sub-basement reflections at borehole depths. Synthetic reflectivity seismograms were computed using a Ricker wavelet
and impedance profiles from borehole sonic logs. These seismograms show significant sub-basement amplitude peaks. A zero-offset
vertical seismic profile, shot on E312, was processed to investigate the authenticity of these reflections and their relationship
to borehole geology. A dual scheme of the median filtering and F–K dip filtering was used. Tests with synthetic seismograms
indicate the approach is effective at reasonable SNR levels. Downgoing energy is clearly identified but negligible upgoing
energy is visible over random noise. These results indicate that lava flows and igneous contacts in upper ocean crust have
significant topography on lateral scales less than the Fresnel Zone (~300 m) due to igneous and tectonic processes. 相似文献
59.
Behshad Jodeiri Shokri Faramarz Doulati Ardejani Hamidreza Ramazi 《Natural Resources Research》2016,25(3):347-363
In this paper, an abandoned waste coal pile, which is resulted from Alborz-Sharghi coal washing plant, NE of Iran was mineralogically and geochemically characterized to evaluate pyrite oxidation, acid mine drainage (AMD) generation, and trace element mobility. After digging ten trenches and vertical sampling, a quantitative method including the atomic absorption test, and the quality-based methods including optical study were carried out for determination of pyrite fractions in the waste pile. The geochemical results revealed that the fraction of remaining pyrite increased with depth, indicating that pyrite oxidation is limited to the shallower depths of the pile which were confirmed by variations of sulfate, pH, EC, and carbonate with depth of the pile. To evaluate the trend of trace elements and mineralogical constituents of the waste particles, the samples were analyzed by using XRD, ICP-MS, and ICP-OES methods. The results showed the secondary and neutralizing minerals comprising gypsum have been formed below the oxidation zone. Besides, positive values of net neutralization potential indicated that AMD generation has not taken in the waste pile. In addition, variations of trace elements with depth reveal that Pb and Zn exhibited increasing trends from pile surface toward the bottom sampling trenches while another of them such as Cu and Ni had decreasing trends with increasing depth of the waste pile. 相似文献
60.