全文获取类型
收费全文 | 632篇 |
免费 | 37篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 46篇 |
大气科学 | 82篇 |
地球物理 | 219篇 |
地质学 | 171篇 |
海洋学 | 30篇 |
天文学 | 83篇 |
综合类 | 7篇 |
自然地理 | 32篇 |
出版年
2022年 | 4篇 |
2021年 | 14篇 |
2020年 | 21篇 |
2019年 | 11篇 |
2018年 | 22篇 |
2017年 | 26篇 |
2016年 | 30篇 |
2015年 | 24篇 |
2014年 | 32篇 |
2013年 | 55篇 |
2012年 | 40篇 |
2011年 | 35篇 |
2010年 | 28篇 |
2009年 | 35篇 |
2008年 | 28篇 |
2007年 | 22篇 |
2006年 | 19篇 |
2005年 | 17篇 |
2004年 | 12篇 |
2003年 | 11篇 |
2002年 | 6篇 |
2001年 | 15篇 |
2000年 | 14篇 |
1999年 | 4篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1991年 | 3篇 |
1990年 | 7篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1978年 | 6篇 |
1976年 | 5篇 |
1975年 | 7篇 |
1969年 | 2篇 |
1966年 | 2篇 |
1964年 | 2篇 |
1957年 | 2篇 |
1956年 | 3篇 |
1954年 | 4篇 |
1951年 | 2篇 |
排序方式: 共有670条查询结果,搜索用时 140 毫秒
1.
AbstractThe concept of a bioeconomy has been placed central in formation of a Swedish National Forest Program (NFP). Drawing on Hajer’s conceptual framework of storylines, we present a discourse analysis of the working group reports underlying the establishment of the NFP strategy. We ask what stories about Swedish forests come to dominate the NFP process, how well they reflect the commitment of balancing economic, social and environmental interests, and what role the concept of a bioeconomy, has on the formation of these stories. Storylines of Swedish forests in the bioeconomy unite wider European discourses on the bioeconomy and climate change with historical Swedish forest policy discourses, revitalizing a discourse coalition comprising the state and the industry. Particular to the Swedish discourse is the strong emphasis on creating consensus around a single story of the forest-based bioeconomy. 相似文献
2.
3.
Karst aquifers represent important water resources in many parts of the world. Unfortunately, karst aquifers are characterised
by high contamination risks. This paper presents a travel time based method for the estimation of karst groundwater vulnerability.
It considers (1) physics-based lateral flow within the uppermost weathered zone (epikarst) in a limestone-dominated region
and (2) high velocities of vertical infiltration at discrete infiltration points (e.g. sinkholes) or lines (e.g. dry valleys,
faults). Consequently, the Transit Time Method honours the actual flow path within the unsaturated zone of a karst aquifer
system. A test site in Northern Jordan was chosen for the demonstration of the assessment technique, i.e. the catchment area
of the Qunayyah Spring north of the capital Amman. The results demonstrate that zones of highest vulnerability lie within
valleys and nearby main fault zones. It also reveals that regions, categorised as protected areas by other methods due to
thick unsaturated zones, contribute to a major degree to the total risk. 相似文献
4.
Until the mid-1980s, transport policy was considered by many as one of the least successful domains of the European integration project. However, from the early 1990s onwards, there are clear signs of a single European transport policy, along with the accompanying implementation of infrastructure projects. What is the explanation for such a change in pace? This paper aims to offer insight in these processes by looking at the mechanisms which form and transform this policy domain. To understand the state of a policy domain and its dynamics over time an institutional approach is taken. Two concepts in political science, ‘policy arrangements’ and ‘supranational governance’ are combined and used as a framework to analyse the European transport policy domain. This analysis describes the development of several elements: organisations, rules, the transnational society, power, resources, and the central transport discourse. It demonstrates that all of these elements have developed from an intergovernmental setting towards a more supranational one. This development was slow in the first decennia when European transport policy was rather passive, but it picked up speed in the 1980s and 1990s. In the pivotal year of 1985, pressure from the transnational society resulted in a rapid change of the rules, the resources and the discourse. 相似文献
5.
A.C. Fischer J.J. Kroon T.G. Verburg T. Teunissen H.Th. Wolterbeek 《Marine Chemistry》2007,107(4):533-546
Iron chemistry in seawater has been extensively studied in the laboratory, mostly in small-volume sample bottles. However, little has been reported about iron wall sorption in these bottles. In this paper, radio-iron 55Fe was used to assess iron wall adsorption, both in terms of capacity, affinity and kinetics. Various bottle materials were tested. Iron sorption increased from polyethylene/polycarbonate to polymethylmetacrylate (PMMA)/high-density polyethylene/polytetrafluoroethylene to glass/quartz, reaching equilibrium in a 25–70 h period. PMMA was studied in more detail: ferric iron (Fe(III)) adsorbed on the walls of the bottles, whereas ferrous iron (Fe(II)) did not. Considering that in seawater the inorganic iron pool mostly consists of ferric iron, the wall will be a factor that needs to be considered in bottle experiments.The present data indicate that for PMMA with specific surface (S)-to-volume (V) ratio S/V, both iron capacity (42 ± 16 × 10− 9 mol/m2 or 1.7 × 10− 9 mol/L recalculated for the S/V-specific PMMA bottles used) and affinity (log KFe'W = 11.0 ± 0.3 m2/mol or 12.4 ± 0.3 L/mol, recalculated for the S/V-specific PMMA bottles used) are of similar magnitude as the iron capacity and -affinity of the natural ligands in the presently used seawater and thus cannot be ignored.Calculation of rate constants for association and dissociation of both Fe'L (iron bound to natural occurring organic ligands) and Fe'W (iron adsorbed on the wall of vessels) suggests that the two iron complexes are also of rather similar kinetics, with rate constants for dissociation in the order of 10 −4–10− 5 L/s and rate constants for association in the order of 108 L/(mol s). This makes that iron wall sorption should be seriously considered in small-volume experiments, both in assessments of shorter-term dynamics and in end-point observations in equilibrium conditions. Therefore, the present data strongly advocate making use of iron mass balances throughout in experiments in smaller volume set-ups on marine iron (bio) chemistry. 相似文献
6.
7.
This paper presents a continuous version of the model of distribution dynamics to analyse the transition dynamics and implied
long-run behaviour of the EU-27 NUTS-2 regions over the period 1995–2003. It departs from previous research in two respects:
first, by introducing kernel estimation and three-dimensional stacked conditional density plots as well as highest density regions
plots for the visualisation of the transition function, based on Hyndman et al. (J Comput Graph Stat 5(4):315–336, 1996), and second, by combining Getis’ spatial filtering view with kernel estimation to explicitly account for the spatial dimension of the
growth process. The results of the analysis indicate a very slow catching-up of the poorest regions with the richer ones,
a process of shifting away of a small group of very rich regions, and highlight the importance of geography in understanding
regional income distribution dynamics.
相似文献
Manfred M. FischerEmail: |
8.
Lutz Nasdala Wolfgang Hofmeister Nicholas Norberg James M. Martinson Fernando Corfu Wolfgang Dörr Sandra L. Kamo Allen K. Kennedy Andreas Kronz Peter W. Reiners Dirk Frei Jan Kosler Yusheng Wan Jens Götze Tobias Häger Alfred Kröner John W. Valley 《Geostandards and Geoanalytical Research》2008,32(3):247-265
We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ~ 840 μg g?1 U (Th/U ~ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 1018 g?1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn. 相似文献
9.
E. Fischer 《Astrophysics and Space Science》1993,207(2):203-219
Based on the assumption, that potential energy of matter in a mass filled space contributes a negative term to the energy tensor, solutions of the Einstein field equations are possible that exhibit no singularities, since the action of gravity changes sign when the density of potential energy exceeds the density of mass-energy. The solution, in which potential energy and mass-energy are in balance, is identical with Einstein's static universe. It is shown that all the observational facts, that are usually considered as confirming the big bang model, as the cosmological red shift, the abundances of light elements and the existence of the microwave background radiation, can be understood also in a static world model, when it is taken into account that due to the finite velocity of gravitational interaction all moving quanta lose momentum to the gravitational tensor potential. As in the static cosmological model the overwhelming fraction of the total mass exists in form of a hot intergalactic plasma. The model gives a simple explanation for the diffuse x-ray background and a solution to the missing mass problem without invoking any kind of new physics or of yet undiscovered particles. Also the causality problem and the curvature problem posed by the energy density of the quantum mechanical vacuum fields find a natural solution. 相似文献
10.
Current evidence indicates that the Martian surface is abundant with water presently in the form of ice, while the atmosphere was at one time more massive with a past surface pressure of as much as 1 atm of CO2. In an attempt to understand the Martian paleoclimate, we have modeled a past CO2H2O greenhouse and find global temperatures which are consistent with an earlier presence of liquid surface water, a finding which agrees with the extensive evidence for past fluvial erosion. An important aspect of the CO2H2O greenhouse model is the detailed inclusion of CO2 hot bands. For a surface pressure of 1 atm of CO2, the present greenhouse model predicts a global mean surface temperature of 294°K, but if the hot bands are excluded, a surface temperature of only 250°K is achieved. 相似文献