全文获取类型
收费全文 | 6472篇 |
免费 | 1588篇 |
国内免费 | 2518篇 |
专业分类
测绘学 | 821篇 |
大气科学 | 1420篇 |
地球物理 | 1620篇 |
地质学 | 3900篇 |
海洋学 | 1151篇 |
天文学 | 276篇 |
综合类 | 528篇 |
自然地理 | 862篇 |
出版年
2024年 | 44篇 |
2023年 | 129篇 |
2022年 | 371篇 |
2021年 | 444篇 |
2020年 | 371篇 |
2019年 | 425篇 |
2018年 | 351篇 |
2017年 | 406篇 |
2016年 | 419篇 |
2015年 | 492篇 |
2014年 | 393篇 |
2013年 | 498篇 |
2012年 | 494篇 |
2011年 | 526篇 |
2010年 | 501篇 |
2009年 | 500篇 |
2008年 | 464篇 |
2007年 | 456篇 |
2006年 | 463篇 |
2005年 | 375篇 |
2004年 | 298篇 |
2003年 | 305篇 |
2002年 | 283篇 |
2001年 | 276篇 |
2000年 | 208篇 |
1999年 | 177篇 |
1998年 | 104篇 |
1997年 | 127篇 |
1996年 | 97篇 |
1995年 | 72篇 |
1994年 | 78篇 |
1993年 | 55篇 |
1992年 | 50篇 |
1991年 | 32篇 |
1990年 | 27篇 |
1989年 | 25篇 |
1988年 | 32篇 |
1987年 | 31篇 |
1986年 | 21篇 |
1985年 | 13篇 |
1984年 | 17篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1977年 | 7篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 578 毫秒
21.
Sulfur mineralogy and geochemistry of serpentinites and gabbros of the Atlantis Massif (IODP Site U1309) 总被引:3,自引:0,他引:3
In-situ uplifted portions of oceanic crust at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were drilled during Expeditions 304 and 305 of the Integrated Ocean Drilling Program (IODP) and a 1.4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered. Here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites, olivine-rich troctolites and diverse gabbroic rocks recovered from Hole 1309D. These data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the Atlantis Massif, which hosts the Lost City Hydrothermal Field, 5 km to the south. The oceanic crust at the central dome is characterized by Ni-rich sulfides reflecting reducing conditions and limited seawater circulation. During uplift and exhumation, seawater interaction in gabbroic-dominated domains was limited, as indicated by homogeneous mantle-like sulfur contents and isotope compositions of gabbroic rocks and olivine-rich troctolites. Local variations from mantle compositions are related to magmatic variability or to interaction with seawater-derived fluids channeled along fault zones. The concomitant occurrence of mackinawite in olivine-rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150-200 °C for late circulating fluids along local brittle faults below 700 m depth. In contrast, the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry. The serpentinites in the upper part of the hole are characterized by high total sulfide contents, high δ34Ssulfide values and low δ34Ssulfate values, which reflect a multistage history primarily controlled by seawater-gabbro interaction and subsequent serpentinization. The basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the Lost City hydrothermal system. Our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time. Heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes, temperature, oxygen and sulfur fugacities, and microbial activity. 相似文献
22.
南水北调西四环暗涵工程为京石段控制性工程,下穿北京市西四环路桥梁30余座,桥梁的安全至关重要。在暗涵施工过程中对桥梁实施了有效的监控量测技术,通过自动化监测和常规监测手段获取数据,然后对监测数据进行详细分析、对比,以研究暗涵下穿桥梁施工过程中桥梁的受力变形协调过程。结果表明:墩柱差异沉降小于允许沉降的80%;墩柱倾斜值小于允许值的30%。监控工作做到了过程预警、过程控制,规避了工程风险,对类似工程具有借鉴意义。 相似文献
23.
24.
25.
中国全新世大暖期哺乳动物与气候波动 总被引:13,自引:0,他引:13
计宏祥 《海洋地质与第四纪地质》1996,16(1):5-16
中国全新世哺乳动物群有众多地点,并已作了详细研究,绝大多数地点有^14C的年代测定,全新世一更新世的界线,作者根据哺乳动物及^14C年代测定,在12-10kaB.P之间,可作为二者之间的过渡层位,在10kaB.P。可作为全新世的开始,中国的全新世大暖期出现于8.5-3.0kaB.P。在此期间内的哺乳动物偏南种类似为明显,如亚洲象,苏门犀及爪哇犀等,在5.6-3.0kaB.P。这一时期内的气候波动剧 相似文献
26.
27.
28.
29.
LARGE-SCALE CIRCULATION CONDITIONS AFFECTINGTHE TYPHOON FORMATION OVER THENORTHWEST PACIFIC 总被引:1,自引:0,他引:1
A contrasting study of the large-scale circulation features responsible for months with many typhoons and months with tew typhoons has revealed that the frequency of typhoon formation over the Northwest Pacific is related to the following conditions:Over the Northwest Pacific, a well-defined ITCZ (Intertropical Convergence Zone) extending eastward to 160°E was displaced to 20°N. At 200 mb, an extensive anomalous anticyclonic circulation prevails over the western and central Pacific. The condition characteristic of a break in the monsoon prevailed in India. The monsoon trough at 500 mb and at the surface over the Indian Peninsula was relatively weak and was accompanied by higher-than-normal rainfall in the northern part of India and lower-than-normal rainfall over the peninsula. In addition, the polar vortex tends to be weak and move to the side of the Northern Hemisphere, opposite to the North Pacific. Finally, abnormally warm water was observed over the Central and Eastern Pacific and abnormally cold 相似文献
30.
The three-dimensional problem of the dynamics of a moored floating object under the action of regular waves is solved numerically as a boundary value problem by use of the finite-infinite element method. The cross-sectional shape of the floating body and the mooring arrangements may all be arbitrary. The mathematical formulations of the problem and procedures of the numerical method are presented in this paper. A corresponding computer program WALOAD has been developed, which is capable of computing wave forces on fixed and floating structures. Numerical computations using this program could give very accurate results, even though rather coarse meshes were used. The program is easy to use and is readily applicable in many practical situations. 相似文献