全文获取类型
收费全文 | 513篇 |
免费 | 30篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 46篇 |
地球物理 | 130篇 |
地质学 | 200篇 |
海洋学 | 39篇 |
天文学 | 59篇 |
综合类 | 4篇 |
自然地理 | 54篇 |
出版年
2022年 | 3篇 |
2021年 | 6篇 |
2020年 | 8篇 |
2019年 | 19篇 |
2018年 | 14篇 |
2017年 | 17篇 |
2016年 | 16篇 |
2015年 | 17篇 |
2014年 | 20篇 |
2013年 | 27篇 |
2012年 | 22篇 |
2011年 | 24篇 |
2010年 | 13篇 |
2009年 | 19篇 |
2008年 | 19篇 |
2007年 | 16篇 |
2006年 | 17篇 |
2005年 | 14篇 |
2004年 | 20篇 |
2003年 | 18篇 |
2002年 | 15篇 |
2001年 | 8篇 |
2000年 | 6篇 |
1999年 | 7篇 |
1998年 | 8篇 |
1997年 | 12篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 14篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 5篇 |
1973年 | 7篇 |
1967年 | 2篇 |
排序方式: 共有548条查询结果,搜索用时 15 毫秒
61.
Mineral separates of pure magnetite from the upper zone of the Bushveld Complex have been analysed for Cr. Detailed sampling within layers of massive magnetite indicates that the Cr content frequently shows an extremely rapid but regular depletion with height (for example, by a factor of seven over 85 cm of massive magnetite), with sudden reversals of variable magnitude. Rayleigh Law fractionation, from a homogeneous liquid of the volume now seen as cumulate rocks overlying the sampled horizons, cannot satisfy these observed concentration gradients. A diffusion-controlled crystallization model is preferred. In another vertical section of massive magnetite the Cr content remains constant with increasing height, and is interpreted as indicating a steady state situation where the rate of depletion of Cr by magnetite fractionation is balanced by the rate of diffusive addition into the crystallization zone. Reversals in Cr content sometimes occur in the middle of pure magnetite layers and are attributed to convective overturn in the magma chamber. These results provide geochemical support for the model of convection cells and bottom crystallization in large magma reservoirs as proposed by Jackson [20]. Depletion of compatible elements in this bottom layer causes chemical inhomogeneity in the magma.Sections, composed of several layers of magnetite sandwiched between magnetite-bearing gabbro, have also been studied. In one case, a steady decrease in Cr content with increasing height in the separated magnetite was observed; in another, several irregular reversals were found. There appears to be no systematic relation between convection cycles, the depletion in Cr and the formation of pure magnetite layers, indicating that the chemical composition of the magma does not control the production of monomineralic layers. The fluctuating pressure model envisaged by Cameron [16] for the formation of such layers is entirely consistent with these data. 相似文献
62.
Erik Thorson Brown Robert F. Stallard Matthew C. Larsen Didier L. Bourls Grant M. Raisbeck Franoise Yiou 《Earth and Planetary Science Letters》1998,160(3-4):723-728
Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic 10Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using 10Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos. 相似文献
63.
Norman K. Grant James L. Powell Flora R. Burkholder John V. Walther Max L. Coleman 《Earth and Planetary Science Letters》1976,31(2):209-223
Strontium and oxygen isotope measurements on the alkali basalt-trachyte-phonolite suite of St. Helena show that some of the late-fractionated rocks are enriched in 87Sr and depleted in 18O relative to the older basalts. The data rule out both the formation of the late-fractionated rocks by the partial melting of hydrothermally altered oceanic crust and the contamination of the volcanic rocks by oceanic sediment. It also appears to be incompatible with models based either on the melting of previously fractionated and crystallized liquids in the volcanic pile, or the long-term fractionation of lavas over several millions of years in a sub-volcanic magma chamber.It is concluded that hydrothermal interaction with meteoric water is the most important cause of the 18O depletion. If the interaction occurred at widely differing temperatures, and involved meteoric and seawaters, it might conceivably have caused both the oxygen and strontium isotope heterogeneities. 相似文献
64.
65.
The Winnipeg and Deadwood formations form deep clastic reservoirs in Saskatchewan, Canada, with temperatures exceeding 40 °C over most of southern Saskatchewan and reaching 100 °C in southwestern Saskatchewan. At these temperatures, the formations have geothermal potential for development of direct use and electricity generation systems. Numerous disposal wells operating at rates of 30 L/s or more are currently installed in these formations, suggesting that electricity could be generated at rates exceeding 2 megawatts of electrical output (MWe) from individual wells. These basal clastic units, thus, could provide significant energy supply over a broad region of Saskatchewan. 相似文献
66.
Guenther Eichhorn Alberto Accomazzi Carolyn S. Grant Michael J. Kurtz Stephen S. Murray 《Astrophysics and Space Science》2002,282(1):299-340
The Astrophysics Data System (ADS) provides access to the astronomical literature through the World Wide Web. It is a NASA
funded project and access to all the ADS services is free to everybody world-wide. The ADS Abstract Service allows the searching
of four databases with abstracts in Astronomy, Instrumentation, Physics/Geophysics, and the LANL Preprints with a total of
over 2.2 million references. The system also provides access to reference and citation information, links to on-line data,
electronic journal articles, and other on-line information. The ADS Article Service contains the full articles for most of
the astronomical literature back to volume 1. It contains the scanned pages of all the major journals (Astrophysical Journal,
Astronomical Journal, Astronomy & Astrophysics, Monthly Notices of the Royal Astronomical Society, and Solar Physics), as
well as most smaller journals back to volume 1.The ADS can be accessed through any web browser without signup or login. Alternatively
an email interface is available that allows our users to execute queries via email and to retrieve scanned articles via email.
This might be interesting for users on slow or unreliable links, since the email system will retry sending information automatically
until the transfer is complete. There are now 9 mirror sites of the ADS available in different parts of the world to improve
access. The ADS is available at: http://ads.harvard.edu
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
67.
68.
69.
Stream temperature is a complex function of energy inputs including solar radiation and latent and sensible heat transfer. In streams where groundwater inputs are significant, energy input through advection can also be an important control on stream temperature. For an individual stream reach, models of stream temperature can take advantage of direct measurement or estimation of these energy inputs for a given river channel environment. Understanding spatial patterns of stream temperature at a landscape scale requires predicting how this environment varies through space, and under different atmospheric conditions. At the landscape scale, air temperature is often used as a surrogate for the dominant controls on stream temperature. In this study we show that, in regions where groundwater inputs are key controls and the degree of groundwater input varies in space, air temperature alone is unlikely to explain within-landscape stream temperature patterns. We illustrate how a geologic template can offer insight into landscape-scale patterns of stream temperature and its predictability from air temperature relationships. We focus on variation in stream temperature within headwater streams within the McKenzie River basin in western Oregon. In this region, as in other areas of the Pacific Northwest, fish sensitivity to summer stream temperatures continues to be a pressing environmental issue. We show that, within the McKenzie, streams which are sourced from deeper groundwater reservoirs versus shallow subsurface flow systems have distinct summer temperature regimes. Groundwater streams are colder, less variable and less sensitive to air temperature variation. We use these results from the western Oregon Cascade hydroclimatic regime to illustrate a conceptual framework for developing regional-scale indicators of stream temperature variation that considers the underlying geologic controls on spatial variation, and the relative roles played by energy and water inputs. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
70.