全文获取类型
收费全文 | 438篇 |
免费 | 26篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 4篇 |
大气科学 | 53篇 |
地球物理 | 95篇 |
地质学 | 145篇 |
海洋学 | 70篇 |
天文学 | 43篇 |
综合类 | 1篇 |
自然地理 | 54篇 |
出版年
2022年 | 2篇 |
2021年 | 5篇 |
2020年 | 10篇 |
2019年 | 10篇 |
2018年 | 21篇 |
2017年 | 16篇 |
2016年 | 17篇 |
2015年 | 12篇 |
2014年 | 15篇 |
2013年 | 37篇 |
2012年 | 21篇 |
2011年 | 32篇 |
2010年 | 30篇 |
2009年 | 36篇 |
2008年 | 32篇 |
2007年 | 26篇 |
2006年 | 23篇 |
2005年 | 12篇 |
2004年 | 4篇 |
2003年 | 14篇 |
2002年 | 8篇 |
2001年 | 5篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有465条查询结果,搜索用时 328 毫秒
21.
Paolo?A.?SossiEmail author Julie?Prytulak Hugh?St.?C.?O’Neill 《Contributions to Mineralogy and Petrology》2018,173(4):27
Vanadium has multiple oxidation states in silicate melts and minerals, a property that also promotes fractionation of its isotopes. As a result, vanadium isotopes vary during magmatic differentiation, and can be powerful indicators of redox processes at high temperatures if their partitioning behaviour can be determined. To quantify the partitioning and isotope fractionation factor of V between magnetite and melt, piston cylinder experiments were performed in which magnetite and a hydrous, haplogranitic melt were equilibrated at 800 °C and 0.5 GPa over a range of oxygen fugacities (\({f_{{{\text{O}}_{\text{2}}}}}\)), bracketing those of terrestrial magmas. Magnetite is isotopically light with respect to the coexisting melt, a tendency ascribed to the VI-fold V3+ and V4+ in magnetite, and a mixture of IV- and VI-fold V5+ and V4+ in the melt. The magnitude of the fractionation factor systematically increases with increasing log\({f_{{{\text{O}}_{\text{2}}}}}\) relative to the Fayalite–Magnetite–Quartz buffer (FMQ), from ?51Vmag-gl = ? 0.63?±?0.09‰ at FMQ ? 1 to ? 0.92?±?0.11‰ (SD) at ≈?FMQ?+?5, reflecting constant V3+/V4+ in magnetite but increasing V5+/V4+ in the melt with increasing log\({f_{{{\text{O}}_{\text{2}}}}}\). These first mineral-melt measurements of V isotope fractionation factors underline the importance of both oxidation state and co-ordination environment in controlling isotopic fractionation. The fractionation factors determined experimentally are in excellent agreement with those needed to explain natural isotope variations in magmatic suites. Furthermore, these experiments provide a useful framework in which to interpret vanadium isotope variations in natural rocks and magnetites, and may be used as a potential fingerprint the redox state of the magma from which they crystallise. 相似文献
22.
Neo-rural populations and their relations with local decision makers in rural Québec: collaboration or conflict? 总被引:1,自引:0,他引:1
The socio-demographic recomposition of the countryside is affecting local interactions and power relations. Understanding these relationships remains a challenge, as the studies to date are often limited to conflicts between neo-rural populations (newcomers) and long-time country residents over partial issues, without including decision makers. To go beyond this conflictual and fragmentary perspective, the objective of this article is to present an overall picture of both cooperative and oppositional relations between four groups, namely, newcomers, long-time rural residents, leaders of local organizations and municipal officials, in regard to all the issues that concern them. The data are based on interviews with these various actors in two contrasting rural areas of Québec (Canada). After looking at the newcomers’ mixed assessment of their participation in community life, we concentrate on areas of collaboration and/or conflict between all the actors regarding demographic, economic, sociocultural, political, environmental and agricultural issues. Three main trends emerge, revealing unexpected ways of interacting, complex power relations and antagonistic conceptions of rural spaces and their future development. 相似文献
23.
A good response spectrum method, used for calculating the seismic design response for design inputs defined in terms of response spectra, should be able to include the effect of the higher modes, even though they may be truncated and not explicitly considered in the analysis. It should also be able to include the intercorrelation of the retained modes as well as their correlation with truncated modes. Herein one such method based on a suitable combination of the mode displacement and mode acceleration formulations of structural dynamics is proposed. Several numerical examples are presented to demonstrate the efficiency of the proposed approach vis-a-vis some other commonly used response spectrum approaches. 相似文献
24.
Lead analysis of sediment cores from seven Connecticut lakes 总被引:2,自引:0,他引:2
Lead analyses were performed on sediment cores from seven Connecticut lakes by dilute acid leaching. The absolute concentrations of lead varied from core to core, but the lead profiles were similar in all of the cores, and each depicted a point where the concentration of lead sharply increased above background levels. The background lead concentrations observed in the bottom portions of the cores ranged from 12-54 g Pb/g sediment with a mean of 30 g Pb/g, and increased to a mean maximum of 310 g Pb/g. In six of the lakes significant increases above background levels were not noted until between 1924 and 1952, suggesting that, at least for Connecticut lakes, the point in a lake sediment core where the concentration of lead began to increase was after ca. 1924, and on average about 1930. Lead concentrations declined in the surface sediments of only three of the lakes, mostly since the 1980's, and presumably in response to the phasing out of leaded gasoline. However, significant declines in lead concentrations were not observed in the remaining cores, possibly due to increased motor vehicle use within the airshed of the lakes and/or changes in the export of lead to the lakes from their watersheds. 相似文献
25.
A design study has been conducted to explore the use of structural cladding panels with energy-dissipating cladding-to-frame connections for seismic-resistant design. The study identifies several issues involved in the modelling and analysis of frames with energy-dissipating cladding-to-frame connections, establishes concepts for design, and provides a preliminary assessment of the force and deformation demands that are likely to be placed on panels and connections. Non-linear dynamic analyses indicate that the clad frames perform well, based on observations about maximum interstorey drifts, maximum plastic hinge rotations in the frames, and maximum ductility demands on the cladding-to-frame connections. 相似文献
26.
Robert J. Beare Malcolm K. Macvean Albert A. M. Holtslag Joan Cuxart Igor Esau Jean-Christophe Golaz Maria A. Jimenez Marat Khairoutdinov Branko Kosovic David Lewellen Thomas S. Lund Julie K. Lundquist Anne Mccabe Arnold F. Moene Yign Noh Siegfried Raasch Peter Sullivan 《Boundary-Layer Meteorology》2006,118(2):247-272
Results are presented from the first intercomparison of large-eddy simulation (LES) models for the stable boundary layer (SBL), as part of the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study initiative. A moderately stable case is used, based on Arctic observations. All models produce successful simulations, in as much as they generate resolved turbulence and reflect many of the results from local scaling theory and observations. Simulations performed at 1-m and 2-m resolution show only small changes in the mean profiles compared to coarser resolutions. Also, sensitivity to subgrid models for individual models highlights their importance in SBL simulation at moderate resolution (6.25 m). Stability functions are derived from the LES using typical mixing lengths used in numerical weather prediction (NWP) and climate models. The functions have smaller values than those used in NWP. There is also support for the use of K-profile similarity in parametrizations. Thus, the results provide improved understanding and motivate future developments of the parametrization of the SBL. 相似文献
27.
Abstract— We have measured the titanium isotopic compositions of 23 silicon carbide grains from the Orgueil (CI) carbonaceous chondrites for which isotopic compositions of silicon, carbon, and nitrogen and aluminum‐magnesium systematics had been measured previously. Using the 16 most‐precise measurements, we estimate the relative contributions of stellar nucleosynthesis during the asymptotic giant branch (AGB) phase and the initial compositions of the parent stars to the compositions of the grains. To do this, we compare our data to the results of several published stellar models that employ different values for some important parameters. Our analysis confirms that s‐process synthesis during the AGB phase only slightly modified the titanium compositions in the envelopes of the stars where mainstream silicon carbide grains formed, as it did for silicon. Our analysis suggests that the parent stars of the >1 μm silicon carbide grains that we measured were generally somewhat more massive than the Sun (2–3 M⊙) and had metallicities similar to or slightly higher than solar. Here we differ slightly from results of previous studies, which indicated masses at the lower end of the range 1.5–3 M⊙ and metallicities near solar. We also conclude that models using a standard 13C pocket, which produces a good match for the main component of s‐process elements in the solar system, overestimate the contribution of the 13C pocket to s‐process nucleosynthesis of titanium found in silicon carbide grains. Although previous studies have suggested that the solar system has a significantly different titanium isotopic composition than the parent stars of silicon carbide grains, we find no compelling evidence that the Sun falls off of the array defined by those stars. We also conclude that the Sun does lie on the low‐metallicity end of the silicon and titanium arrays defined by mainstream silicon carbide grains. 相似文献
28.
Paul Finegan Luis León Vintró Peter I. Mitchell Dominique Boust Aurélien Gouzy Peter J. Kershaw Julie A. Lucey 《Continental Shelf Research》2009,29(16):1995-2010
Recent studies have shown that accumulation of 99Tc in seabed sediments labelled by authorised radioactive liquid discharges into the NE Irish Sea from the Sellafield reprocessing complex is greater than previously thought. In this paper, new data on 99Tc concentration profiles in subtidal and intertidal sediments from the eastern and western Irish Sea are provided with a view to elucidating the processes responsible for the incorporation and retention of 99Tc in the seabed. The data show that substantial amounts of 99Tc have accumulated in the fine-grained subtidal sediments off the Cumbrian coast, particularly after increased releases from Sellafield following the commissioning of the Enhanced Actinide Removal Plant (EARP) in 1994. In all the cores taken in this area, 99Tc has been found to be present to depths in excess of 30 cm. Analysis of 137Cs and 241Am profiles, together with other supporting geochemical data, show a high degree of homogenisation of the sediments down to these depths as a result of physical and biological processes, and confirm that incorporation of 99Tc into the sediment compartment is actually the result of mixing and reworking, rather than active sediment accumulation. In contrast, active deposition of material transported from this mixed pool of sediment appears to be the dominant mechanism controlling 99Tc profiles in intertidal areas close to the Sellafield discharge outfall.Data obtained from the analysis of subtidal sediment cores from the western Irish Sea mud basin suggest that similar mixing processes to those occurring in the subtidal sediments of the eastern Irish Sea are also active in this area. Time-series data on 99Tc concentrations in surficial sediments from this basin, gathered in the period 1988–2004, inclusive, show a clear increase in concentrations, by a factor of ~2, between samples collected pre-EARP and post-EARP. The constancy of 99Tc concentrations in surface sediments throughout the 1980s and the early-1990s suggests that little redissolution and export of 99Tc occurred over this extended period. A similar observation applies to the post-EARP period, when concentrations remained relatively constant despite the reported steady decrease in 99Tc concentrations in the overlying waters.This apparent lack of remobilisation is consistent with data from sequential extraction analyses, which indicate that the bulk of the 99Tc is strongly bound to non-labile geochemical phases, with only a small proportion associated with exchangeable and acido-soluble phases. Further, these analyses show that 99Tc is not associated with oxygen-sensitive and highly-reactive acid-volatile sulphides (AVS) to any significant extent. 相似文献
29.
Non-fishery use of artificial reefs has been given attention in recent years. The primary concern associated with non-fishery artificial reefs is their effects to the surrounding ecosystems. This study examined the infaunal communities around two non-fishery artificial reefs (the sunken vessels YO257 and Sea Tiger) in Mamala Bay, Hawaii. Infaunal community structures at these artificial reefs were relatively similar to one at a nearly natural patch reef. A large amount of basalt gravel around YO257 associated with its deployment operation possibly had an effect on the surrounding community by increasing pore space. Polychaete assemblages were compared with existing data throughout the bay, and this revealed that the variation in sediment grain sizes and depths seemed to play some role in structuring the polychaete communities. Nevertheless, the infaunal communities around the artificial reefs were typical and within the range of natural variation in Mamala Bay, supporting their beneficial uses in ecotourism. 相似文献
30.
Seth A. TEITLER Julie M. PAQUE Jeffrey N. CUZZI Robert C. HOGAN 《Meteoritics & planetary science》2010,45(7):1124-1135
Abstract– The variation in sizes of chondrules from one chondrite to the next is thought to be due to some sorting process in the early solar nebula. Hypotheses for the sorting process include chondrule sorting by mass and sorting by some aerodynamic mechanism; one such aerodynamic mechanism is the process of turbulent concentration (TC). We present the results of a series of statistical tests of chondrule data from several different chondrites. The data do not clearly distinguish between various options for the sorting parameter, but we find that the data are inconsistent with being drawn from lognormal or (three‐parameter) Weibull distributions in chondrule radius. We also find that all but one of the chondrule data sets tested are consistent with being drawn from the TC distribution. 相似文献