全文获取类型
收费全文 | 246篇 |
免费 | 5篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 29篇 |
地球物理 | 41篇 |
地质学 | 90篇 |
海洋学 | 11篇 |
天文学 | 59篇 |
自然地理 | 15篇 |
出版年
2021年 | 2篇 |
2020年 | 2篇 |
2018年 | 3篇 |
2017年 | 5篇 |
2016年 | 4篇 |
2015年 | 3篇 |
2014年 | 5篇 |
2013年 | 19篇 |
2012年 | 5篇 |
2011年 | 13篇 |
2010年 | 7篇 |
2009年 | 16篇 |
2008年 | 11篇 |
2007年 | 8篇 |
2006年 | 7篇 |
2005年 | 6篇 |
2004年 | 14篇 |
2003年 | 15篇 |
2002年 | 11篇 |
2001年 | 4篇 |
2000年 | 7篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1969年 | 4篇 |
1968年 | 1篇 |
1907年 | 2篇 |
排序方式: 共有251条查询结果,搜索用时 15 毫秒
1.
Summary
Atmospheric backscattering from aerosol particulates has been measured over the Atlantic at 10.6 μm wavelength with an airborne,
coherent heterodyne, lidar, and corresponding air mass back trajectories have been calculated. These back trajectories (usually
extended up to 10 days prior to the backscatter measurement) have shown very diverse origins for the air parcels at different
altitudes. In many cases it has been possible to attribute the observed levels of scattering to these origins over oceanic,
arctic, continental, industrial etc. regions. This is illustrated by 6 flight records: out of Ascension Island in the South
Atlantic; over the Azores in the mid North Atlantic; over the UK and the North Sea; and in the Arctic along 71° North. In
each of these regions the profiles of backscatter versus altitude show highly variable features; remarkably different origins
for air masses at different altitudes are evident from the corresponding back trajectory analyses. It is thus possible for
the first time to present probable explanations for the different levels of scattering observed at different altitudes.
Received July 14, 2000 Revised May 14, 2001 相似文献
2.
A. P. M. Vaughan L. A. Dowling F. J. G. Mitchell S.‐E. Lauritzen A. M. McCabe P. Coxon 《第四纪科学杂志》2004,19(6):577-590
Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
3.
4.
Neoliberalism and natural resource management: Agri-environmental standards and the governing of farming practices 总被引:1,自引:0,他引:1
Private standards and certification schemes are widely acknowledged as playing an increasingly important role in agri-environmental governance. While much of the existing research concludes that these mechanisms consolidate the global extension of neoliberalism – enhancing the power of corporate actors to the detriment of smaller producers – we argue that this overlooks the complex ways in which standards are used by governments and farmers in the governing of farming practices. Focusing specifically on a process standard – Environmental Management Systems (EMS) – promoted by the Australian government as a way of verifying the ‘clean and green’ status of agricultural exports, we examine how one regional group of producers has sought to use EMS standards in practice. Our analysis of a case study in the state of Victoria appears to confirm that EMS was a successful instrument for the extension of neoliberal governance, reinforcing the production of neoliberal subjectivities and practices amongst farmer participants and enabling the government to compensate for gaps in environmental provision. However, it would be a mistake to interpret the development of this EMS scheme as an example of naïve farmers manipulated by the state. In practice, farmers used the opportunities provided by government funding to undertake actions which expressed their own agri-environmental values and practices. Establishment of an EMS and associated eco-label enabled producers to demonstrate and extend their capacity to act as good environmental stewards. Our research highlights how the local application of environmental standards negotiates and shapes, rather than simply contributes to, neoliberal rule. 相似文献
5.
Recent Rapid Regional Climate Warming on the Antarctic Peninsula 总被引:15,自引:1,他引:15
David G. Vaughan Gareth J. Marshall William M. Connolley Claire Parkinson Robert Mulvaney Dominic A. Hodgson John C. King Carol J. Pudsey John Turner 《Climatic change》2003,60(3):243-274
The Intergovernmental Panel on Climate Change (IPCC) confirmed that mean global warming was 0.6 ± 0.2 °C during the 20th century and cited anthropogenic increases in greenhouse gases as the likely cause of temperature rise in the last 50 years. But this mean value conceals the substantial complexity of observed climate change, which is seasonally- and diurnally-biased, decadally-variable and geographically patchy. In particular, over the last 50 years three high-latitude areas have undergone recent rapid regional (RRR) warming, which was substantially more rapid than the global mean. However, each RRR warming occupies a different climatic regime and may have an entirely different underlying cause. We discuss the significance of RRR warming in one area, the Antarctic Peninsula. Here warming was much more rapid than in the rest of Antarctica where it was not significantly different to the global mean. We highlight climate proxies that appear to show that RRR warming on the Antarctic Peninsula is unprecedented over the last two millennia, and so unlikely to be a natural mode of variability. So while the station records do not indicate a ubiquitous polar amplification of global warming, the RRR warming on the Antarctic Peninsula might be a regional amplification of such warming. This, however, remains unproven since we cannot yet be sure what mechanism leads to such an amplification. We discuss several possible candidate mechanisms: changing oceanographic or changing atmospheric circulation, or a regional air-sea-ice feedback amplifying greenhouse warming. We can show that atmospheric warming and reduction in sea-ice duration coincide in a small area on the west of the Antarctic Peninsula, but here we cannot yet distinguish cause and effect. Thus for the present we cannot determine which process is the probable cause of RRR warming on the Antarctic Peninsula and until the mechanism initiating and sustaining the RRR warming is understood, and is convincingly reproduced in climate models, we lack a sound basis for predicting climate change in this region over the coming century. 相似文献
6.
T.?Flaherty T.?H.?DruittEmail author H.?Tuffen M.?D.?Higgins F.?Costa A.?Cadoux 《Contributions to Mineralogy and Petrology》2018,173(9):75
The rhyodacitic magma discharged during the 30–80 km3 DRE (dense rock equivalent) Late Bronze Age (LBA; also called ‘Minoan’) eruption of Santorini caldera is known from previous studies to have had a complex history of polybaric ascent and storage prior to eruption. We refine the timescales of these processes by modelling Mg–Fe diffusion profiles in orthopyroxene and clinopyroxene crystals. The data are integrated with previously published information on the LBA eruption (phase equilibria studies, melt inclusion volatile barometry, Mg-in-plagioclase diffusion chronometry), as well as new plagioclase crystal size distributions and the established pre-LBA history of the volcano, to reconstruct the events that led up to the assembly and discharge of the LBA magma chamber. Orthopyroxene, clinopyroxene and plagioclase crystals in the rhyodacite have compositionally distinct rims, overgrowing relict, probably source-derived, more magnesian (or calcic) cores, and record one or more crystallization (plag???opx?>?cpx) events during the few centuries to years prior to eruption. The crystallization event(s) can be explained by the rapid transfer of rhyodacitic melt from a dioritic/gabbroic region of the subcaldera pluton (mostly in the 8–12 km depth range), followed by injection, cooling and mixing in a large melt lens at 4–6 km depth (the pre-eruptive magma chamber). Since crystals from all eruptive phases yield similar timescales, the melt transfer event(s), the last of which took place less than 2 years before the eruption, must have involved most of the magma that subsequently erupted. The data are consistent with a model in which prolonged generation, storage and segregation of silicic melts were followed by gravitational instability in the subcaldera pluton, causing the rapid interconnection and amalgamation of melt-rich domains. The melts then drained to the top of the pluton, at fluxes of up to 0.1–1 km3 year??1, where steep vertical gradients of density and rheology probably caused them to inject laterally, forming a short-lived holding chamber prior to eruption. This interpretation is consistent with growing evidence that some large silicic magma chambers are transient features on geological timescales. A similar process preceded at least one earlier caldera-forming eruption on Santorini, suggesting that it may be a general feature of this rift-hosted magmatic system. 相似文献
7.
8.
R. A. D. Pattrick G. van der Laan D. J. Vaughan C. M. B. Henderson 《Physics and Chemistry of Minerals》1993,20(6):395-401
L-edge X-ray absorption spectroscopy employing a synchrotron radiation source has been used to study the electronic structure and valency of Cu in the chemically and structurally complex tetrahedrite group of minerals. Mechanical mixtures of Cu2+O and Cu+FeS2 were used to estimate the relative cross sections of Cu2+ and Cu+; the absorption of Cu2+ at 931 eV is 25 times greater than that of Cu+ at 945 eV. Using this calibration, Cu2+/Cu ratios were found to vary from 0.00 to 0.054 in the tetrahedrite samples studied; the highest proportion of Cu2+ occurs in synthetic tetrahedrites with a composition close to Cu12Sb4S13. This study reveals the utility of the technique for determining the valence state of copper in complex minerals, allowing the crystal chemistry to be more fully characterised. 相似文献
9.
W. E. Schaaf D. S. Peters L. Coston-Clements D. S. Vaughan C. W. Krouse 《Estuaries and Coasts》1993,16(4):697-702
Pollution effects on fish populations were estimated with a simulation model, using Leslie matrices. Results from changing only first-year survival rate (So) have already been published (Schaaf et al. 1987). This paper explores the effects of perturbing both So and the adult survival rate (Si) for 12 spatial-temporal stocks. Most stocks examined are more sensitive to permanent change in Si than to changes in So. The relative importance of these two rates in determining the population growth rate (λ) depends upon the age distribution of the expected lifetime egg production of age i females (Vi). In turn, the vector Vi, as measured by its mean and standard deviation, is shown to vary among geographic or temporal stocks of a single species. Hence, we quantify the impact on population size of destroying a fixed percentage of habitat, relative to where and when it occurs (i.e., relative impact on Si and So). For example, destroying 1% of the Atlantic menhaden habitat would reduce λ by 0.8% and the population by 8.0% in 10 yr, if the impact affected only adults (e.g., offshore in winter). If the 1% habitat destruction all occurred in the estuaries, affecting juveniles as well, λ would be reduced by almost 5%, and in 10 yr drive the population down to 58% of its former equilibrium. We show that knowledge of the mean and variance of the age distribution of Vi permits prediction of relative sensitivity among species to pollution. Within species, this knowledge of Vi permits comparison of the effects of impacting different life stages and at different times and places. 相似文献
10.
Michael D. Higgins 《Geostandards and Geoanalytical Research》1984,8(1):31-34
Eighteen international reference standards, mostly geochemical standards, have been analysed for boron using prompt-gamma neutron activation analysis (PGNAA). There is good agreement between the results for samples with high boron abundances and the few published values using PGNAA and other methods. However, correlation is poor at low boron abundances, especially in those standards with low boron/sodium ratios. Corrections for sodium interference on boron have been critically evaluated and the data corrected using two different methods. 相似文献