全文获取类型
收费全文 | 29289篇 |
免费 | 1264篇 |
国内免费 | 2227篇 |
专业分类
测绘学 | 1937篇 |
大气科学 | 2781篇 |
地球物理 | 5537篇 |
地质学 | 14531篇 |
海洋学 | 1688篇 |
天文学 | 1863篇 |
综合类 | 2575篇 |
自然地理 | 1868篇 |
出版年
2024年 | 20篇 |
2023年 | 102篇 |
2022年 | 245篇 |
2021年 | 302篇 |
2020年 | 235篇 |
2019年 | 271篇 |
2018年 | 4983篇 |
2017年 | 4269篇 |
2016年 | 2821篇 |
2015年 | 499篇 |
2014年 | 366篇 |
2013年 | 341篇 |
2012年 | 1301篇 |
2011年 | 3047篇 |
2010年 | 2379篇 |
2009年 | 2627篇 |
2008年 | 2151篇 |
2007年 | 2597篇 |
2006年 | 285篇 |
2005年 | 360篇 |
2004年 | 567篇 |
2003年 | 577篇 |
2002年 | 505篇 |
2001年 | 288篇 |
2000年 | 264篇 |
1999年 | 231篇 |
1998年 | 179篇 |
1997年 | 140篇 |
1996年 | 143篇 |
1995年 | 114篇 |
1994年 | 110篇 |
1993年 | 88篇 |
1992年 | 71篇 |
1991年 | 60篇 |
1990年 | 45篇 |
1989年 | 27篇 |
1988年 | 27篇 |
1987年 | 19篇 |
1986年 | 16篇 |
1985年 | 17篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1981年 | 22篇 |
1980年 | 22篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 9篇 |
1958年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
南海西沙西南海域表层沉积物特征 总被引:3,自引:1,他引:3
西沙西南海域表层沉积物样品矿物成分,地球化学,微体古生物分析测试结果表明:沉积物可分为7种类型,沉积环境主要是陆坡,部分为深海平原,海洋生物,海洋化学以及火山物质的沉积起到了积极的作用,陆源物质的影响较小,它们主要来源于北部大陆和南部岛礁等物源区。 相似文献
202.
203.
三峡工程对长江口区无脊椎动物资源影响的预测 总被引:1,自引:0,他引:1
长江大量径流给其河口及邻近海域带来丰富的营养物质,维持了长江河口生态系统的巨大生产力,并使之成为多种重要经济鱼虾类的索饵和产卵场。长江口生态环境的变化,对我国最大、最重要的浙江舟山渔场和江苏吕四渔场的渔业资源有举足轻重的影响。三峡工程的兴建将使长江入海径流量及其季节分配有很大改变,也不可避免地影响到河口和近海的海洋环境、经济动物的组成及资源状况。为研究长江口无脊椎动物资源结构及其分布特征,以及预测三峡工程对自然生态环境的影响,于1985年9月至1986年8月进行了无脊椎动物资源底拖网试捕调查。本文系统整理调查所得资料,结合历史资料进行分析研究。 相似文献
204.
The Magellan seamounts began forming as large submarine shield volcanoes south of the equator during the Cretaceous. These
volcanoes formed as a cluster on the small Pacific plate in a period when tectonic stress was absent. Thermal subsidence of
the seafloor led to sinking of these volcanoes and the formation of guyots as the seamounts crossed the equatorial South Pacific
(10–0°S) sequentially and ocean surface temperatures became too high for calcareous organisms to survive. Guyot formation
was completed between about 59 and 45 Ma and the guyots became phosphatized at about 39–34 and 27–21 Ma. Ferromanganese crusts
began formation as proto-crusts on the seamounts and guyots of the Magellan Seamount cluster towards the end of the Cretaceous
up to 55 Ma after the formation of the seamounts themselves. The chemical composition of these crusts evolved over time in
a series of steps in response to changes in global climate and ocean circulation. The great thickness of these crusts (up
to 15–20 cm) reflects their very long period of growth. The high Co contents of the outer parts of the crusts are a consequence
of the increasing deep circulation of the ocean and the resulting deepening of the oxygen minimum zone with time. Growth of
the Co-rich Mn crusts in the Magellan Seamount cluster can be considered to be the culmination of a long journey through time. 相似文献
205.
Ryoko Tokeshi Kaoru Ichikawa Satoshi Fujii Kenji Sato Shoichiro Kojima 《Journal of Oceanography》2007,63(4):711-720
A method to extract geostrophic current in the daily mean HF radar data in the Kuroshio upstream region is established by
comparison with geostrophic velocity determined from the along-track altimetry data. The estimated Ekman current in the HF
velocity is 1.2% (1.5%) and 48° (38°)-clockwise rotated with respect to the daily mean wind in (outside) the Kuroshio. Furthermore,
additional temporal smoothing is found necessary to remove residual ageostrophic currents such as the inertial oscillation.
After removal of the ageostrophic components, the HF geostrophic velocity agrees well with that from the altimetry data with
rms difference 0.14 (0.12) m/s in (outside) the Kuroshio. 相似文献
206.
The origin and biogeochemistry of organic matter in surface sediments of Lake Shihwa and Lake Hwaong
To understand the origin and biogeochemistry of the organic matter in surface sediments of Lake Shihwa and Lake Hwaong, organic
nitrogen, inorganic nitrogen, labile organic carbon, and residual organic carbon contents as well as stable isotope ratios
for carbon and nitrogen were determined by KOBr-KOH treatment. Ratios of organic carbon to organic nitrogen (Corg/Norg) (mean = 24) were much higher than ratios of organic carbon to total nitrogen (Corg/Ntot) (mean=12), indicating the presence of significant amounts of inorganic nitrogen in the surface sediments of both lakes.
Stable isotope ratios for organic nitrogen were, on average, 5.2‰ heavier than ratios of inorganic nitrogen in Lake Shihwa,
but those same ratios were comparable in Lake Hwaong. This might be due to differences in the origin or the degree of degradation
of sedimentary organic matter between the two lakes. In addition, stable isotope ratios for labile organic carbon were, on
average, 1.4‰ heavier than those for residual organic carbon, reflecting the preferential oxidation of13C-enriched organic matter. The present study demonstrates that KOBr-KOH treatment of sedimentary organic matter can provide
valuable information for understanding the origin and degradation state of organic matter in marine and brackish sediments.
This also suggests that the ratio of Corg/Norg and stable isotope ratios for organic nitrogen can be used as indexes of the degree of degradation of organic matter. 相似文献
207.
208.
209.
210.
Calculating Pollution Indices by Heavy Metals in Ecological Geochemistry Assessment and a Case Study in Parks of Beijing 总被引:4,自引:0,他引:4
Pollution index is a powerful tool for ecological geochemistry assessment. The commonly used pollution indices by heavy metals in soils and sediments were classified as two types of single index and integrated index in an algorithm point of view. Four single indices of contamination factor (or concentration factor), ecological risk factor, enrichment factor, and index of geo-accumulation were illustrated, and the reference values for calculating single indices were distinguished into background levels and threshold pollution values. Eight integrated indices were divided into two groups. One group is suitable for the normal distribution single indices including the sum, average, weighted average,vector modulus, and Nemerow pollution indices, and the other for log-normal distribution including the product, root of product, and weighted power product pollution indices. Using background levels as reference values, five contamination classes were divided, and the terminologies are suggested for the single and integrated indices to unify the assessment results. Software of EGAPI was developed in a single document interface to calculate the four single and eight integrated indices by heavy metals to assess the quality of soil and sediment ecological geochemistry. Pollution indices by heavy metals of Cu,Pb, and Zn in soils in parks of Beijing were calculated using EGAPI software, and these five contamination classes and terminologies suggested in this study were evaluated and used. Results ofintegrated indices of Cu, Pb, and Zn in soils indicated that the soil qualities are unpolluted as a whole and varied from low polluted to unpolluted status from the center to the outskirts of Beijing City. 相似文献