全文获取类型
收费全文 | 3837篇 |
免费 | 595篇 |
国内免费 | 1262篇 |
专业分类
测绘学 | 52篇 |
大气科学 | 140篇 |
地球物理 | 742篇 |
地质学 | 3219篇 |
海洋学 | 872篇 |
天文学 | 20篇 |
综合类 | 189篇 |
自然地理 | 460篇 |
出版年
2024年 | 22篇 |
2023年 | 89篇 |
2022年 | 133篇 |
2021年 | 155篇 |
2020年 | 182篇 |
2019年 | 188篇 |
2018年 | 172篇 |
2017年 | 163篇 |
2016年 | 163篇 |
2015年 | 182篇 |
2014年 | 262篇 |
2013年 | 235篇 |
2012年 | 226篇 |
2011年 | 289篇 |
2010年 | 212篇 |
2009年 | 280篇 |
2008年 | 253篇 |
2007年 | 255篇 |
2006年 | 268篇 |
2005年 | 212篇 |
2004年 | 214篇 |
2003年 | 183篇 |
2002年 | 150篇 |
2001年 | 158篇 |
2000年 | 157篇 |
1999年 | 132篇 |
1998年 | 127篇 |
1997年 | 127篇 |
1996年 | 102篇 |
1995年 | 78篇 |
1994年 | 63篇 |
1993年 | 47篇 |
1992年 | 44篇 |
1991年 | 28篇 |
1990年 | 32篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 19篇 |
1985年 | 24篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有5694条查询结果,搜索用时 15 毫秒
51.
52.
53.
重矿物组合、含量变化和特征矿物的分布及变化规律是沿海泥沙来源和运移趋势判断的重要手段之一。通过对廉州湾南部海域海底表层沉积物的重矿物分析 ,发现该区重矿物分布以北海地角为界 ,其百分含量和特征矿物南北有别 ,可能分别代表不同的物质来源区。其中廉州湾北部南流江流域来沙是该区主要物源 ,运移趋势为自 NE向 SW;地角西南岸段、岭南侧海岸侵蚀及银滩来沙也为该区提供了部分物源 ,运移趋势为绕过冠头岭沿海岸向 N方向运移。由于缺乏北海陆域陆相地层重矿物含量等相关资料 ,暂未做物质来源区的具体判断 相似文献
54.
55.
藻类功能食品的物质基础与研究开发现状 总被引:3,自引:0,他引:3
对藻类功能食品的研究和开发现状作综述报道。涉及的主要内容有:作为功能食品物质基础的食物纤维、PUFA、维生素、矿物质和微量元素、氨基酸以及其它有生理活性的物质。 相似文献
56.
High-resolution measurements of chromophoric dissolved organic matter in the Mississippi and Atchafalaya River plume regions 总被引:4,自引:0,他引:4
Chromophoric dissolved organic matter (CDOM) was measured in the spring and summer in the northern Gulf of Mexico with the ECOShuttle, a towed, instrumented, undulating vehicle. A submersible pump mounted on the vehicle supplied continuously flowing, uncontaminated seawater to online instruments in the shipboard laboratory and allowed discrete samples to be taken for further analysis. CDOM in the northern Gulf of Mexico was dominated by freshwater inputs from the Mississippi River through the Birdfoot region and to the west by discharge from the Atchafalaya River. CDOM was more extensively dispersed in the high-flow period in the spring but in both time periods was limited by stratification to the upper 12 m or so. Thin, subsurface CDOM maxima were observed below the plume during the highly stratified summer period but were absent in the spring. However, there was evidence of significant in situ biological production of CDOM in both seasons.The Mississippi River freshwater end member was similar in spring and summer, while the Atchafalaya end member was significantly higher in the spring. In both time periods, the Atchafalaya was significantly higher in CDOM and dissolved organic carbon (DOC) than the Mississippi presumably due to local production and exchange within the coastal wetlands along the lower Atchafalaya which are absent along the lower Mississippi. Nearshore waters may also have higher CDOM due to outwelling from coastal wetlands. High-resolution measurements allow the differentiation of various water masses and are indicative of rapidly varying (days to weeks) source waters. Highly dynamic but conservative mixing between various freshwater and marine end members apparently dominates CDOM distributions in the area with significant in situ biological inputs (bacterial degradation of phytoplankton detritus), evidence of flocculation, and minor photobleaching effects also observed. It is clear that high-resolution measurements and adaptive sampling strategies allow a more detailed examination of the processes that control CDOM distributions in river-dominated systems. 相似文献
57.
Since the 1997 local ban on ocean dumping of dredged sediments, the States of New York and New Jersey have pursued a policy of environmentally sound solutions to the management of dredged material, including beneficial use of stabilized dredged material (SDM) in transportation applications. A pilot study was initiated in 1998 to evaluate the use of SDM in the construction of highway embankments. Utilizing 80,000 cubic yards of dredged material, two embankments were constructed from SDM on a commercial development area adjacent to the Harbor. Geotechnical properties and handling of SDM were evaluated both during and one year post - construction. This article presents the evaluation of the embankments themselves, including constructability and performance. The results demonstrate that SDM satisfies most of the geotechnical criteria for fill construction, except those for durability, requiring proper coverage and protection similar to that provided for fills constructed on cohesive soils. This same characteristic precludes long term stockpiling of SDM prior to final placement, limiting applications to those that have schedules overlapping with dredging projects. Increased costs for the use of SDM can be as high as $8 per cubic yard over traditional fills; however, this cost may be recouped through management fees collected from dredging projects. 相似文献
58.
Cross-flow ultrafiltration (CFF) is often used to obtain separation and concentration of colloids from bulk natural water samples. Application of the ultrafiltration permeation model allows the quantitative determination of the low molecular weight material (LMW, < 1 kDa) and colloids in bulk dissolved organic matter (DOM) from measurements of time series permeate samples obtained from CFF. Detailed analysis of a Yukon River water sample shows that DOM absorption coefficient and fluorescence follow the permeation model and that the complex spectral optical properties of LMW DOM can be reconstructed from CFF data. A combination of measured and modeled data indicates that the LMW contribution to bulk DOM optical properties obtained from CFF can be grossly underestimated by the use of a low concentration factor (CF, the ratio of initial sample volume to retentate volume). Even at a relatively high CF of 19, optical properties of LMW DOM calculated from measurements of the retentate or integrated permeate would underestimate true values by 5–36%. In the Yukon River sample, LMW dissolved organic carbon represented 26% of the bulk concentration, but only 3–14% of the colored DOM was in the LMW fraction while 31–33% of bulk DOM florescence was due to LMW DOM. The contrasting optical properties of LMW and colloidal DOM support the concept that analysis of bulk DOM absorption and fluorescence properties reveals information about DOM molecular weight. 相似文献
59.
福建近海主要经济渔业生物营养级和有机碳含量研究 总被引:2,自引:0,他引:2
本文研究了福建近海52种鱼类、6种虾类、4种蟹类、1种虾蛄和4种头足类的食性和营养级及其有机碳含量。结果表明:营养级以鱼类最高,为2.1~3.7级,平均2.65级;头足类为2.04~2.88级,平均2.60级;蟹类2.38~2.73级,平均2.58级;虾类最低,为1.64~2.21级,平均1.96级.干样有机碳含量百分率也以鱼类最高,为33.0%~76.2%(m/m),平均46.25%(m/m);头足类为27.80%~46.16%(m/m),平均40.24%(m/m);虾类为30.60%~38.40%(m/m),平均34.90%(m/m);蟹类最低,为25.00%~30.78%(m/m),平均28.24%(m/m),鲜样有机碳含量百分率还是以鱼类最高,为4.80%~29.55%(m/m),平均13.75%(m/m);蟹类为8.90%~12.68%(m/m),平均10.74%(m/m);虾类为6.97%~11.53%(m/m),平均9.55%(m/m);头足类最低仅为7.38%~8.52%(m/m),平均7.95%(m/m),干样有机碳含量百分率和鲜样有机碳含量百分率与营养级均成正相关,但前者的相关性高于后者。 相似文献
60.
胶州湾海水中颗粒有机碳含量的分布与变化 总被引:6,自引:1,他引:6
本文报道了胶州湾表层海水及C_3和D_3站中层和底层水中的颗粒有机碳(POC)的分布与变化规律。结果表明,胶州湾POC年平均浓度为155~410μg/L;平面分布有明显的规律性,近岸和河口一带的含量较高,湾中部较低,湾口诸站为最低;垂直分布有个别数值偏高或偏低现象,但没有明显的规律;季节变化较明显,初春较高,秋季较低。 相似文献