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James S. Bolton Martin G. Haehnelt Matteo Viel Robert F. Carswell 《Monthly notices of the Royal Astronomical Society》2006,366(4):1378-1390
The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi-stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic H i and He ii Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a Λ cold dark matter (ΛCDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H i and He ii ionization rates. We further show that the large spatial variations and the anticorrelation with H i absorber density observed in the ratio of the measured He ii to H i column densities can be explained in a model where the H i ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the He ii ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the He ii ionization rate. A significant contribution to UV emission at the He ii photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in He ii opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He ii Lyα forest spectra at z ∼ 2–3.5 probe the tail end of the reionization of He ii by QSOs. 相似文献
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Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions 总被引：19，自引：0，他引：19
Larry D. Hinzman Neil D. Bettez W. Robert Bolton F. Stuart Chapin Mark B. Dyurgerov Chris L. Fastie Brad Griffith Robert D. Hollister Allen Hope Henry P. Huntington Anne M. Jensen Gensuo J. Jia Torre Jorgenson Douglas L. Kane David R. Klein Gary Kofinas Amanda H. Lynch Andrea H. Lloyd A. David McGuire Frederick E. Nelson Walter C. Oechel Thomas E. Osterkamp Charles H. Racine Vladimir E. Romanovsky Robert S. Stone Douglas A. Stow Matthew Sturm Craig E. Tweedie George L. Vourlitis Marilyn D. Walker Donald A. Walker Patrick J. Webber Jeffrey M. Welker Kevin S. Winker Kenji Yoshikawa 《Climatic change》2005,72(3):251-298
The Arctic climate is changing. Permafrost is warming, hydrological processes are changing and biological and social systems
are also evolving in response to these changing conditions. Knowing how the structure and function of arctic terrestrial ecosystems
are responding to recent and persistent climate change is paramount to understanding the future state of the Earth system
and how humans will need to adapt. Our holistic review presents a broad array of evidence that illustrates convincingly; the
Arctic is undergoing a system-wide response to an altered climatic state. New extreme and seasonal surface climatic conditions
are being experienced, a range of biophysical states and processes influenced by the threshold and phase change of freezing
point are being altered, hydrological and biogeochemical cycles are shifting, and more regularly human sub-systems are being
affected. Importantly, the patterns, magnitude and mechanisms of change have sometimes been unpredictable or difficult to
isolate due to compounding factors. In almost every discipline represented, we show how the biocomplexity of the Arctic system
has highlighted and challenged a paucity of integrated scientific knowledge, the lack of sustained observational and experimental
time series, and the technical and logistic constraints of researching the Arctic environment. This study supports ongoing
efforts to strengthen the interdisciplinarity of arctic system science and improve the coupling of large scale experimental
manipulation with sustained time series observations by incorporating and integrating novel technologies, remote sensing and
modeling. 相似文献
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Centrifuge tests were carried out to determine the effect of different alcohols and one glycol on a thin consolidated disc
of clay. The evolution of changes in the clay hydraulic conductivity with time was investigated and other structural changes
due to chemical attack were monitored. The findings presented here demonstrate that the hydraulic conductivity of the clay
appear to be generally related to the polarity of the chemicals and the dielectric constant, with the exception of butanol.
In the case of butanol at low flow rate and low stress level, the action of the chemical caused the clay to crack, with a
consequent large increase in flow. This effect on the clay hydraulic conductivity could be used for improving pollution remediation
especially when alcohols are associated with gasoline. 相似文献
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S Swart JJ Zietsman JC Coetzee DG Goslett A Hoek D Needham 《African Journal of Marine Science》2016,38(4):525-538
South Africa’s small-pelagic fishery is a socio-economically important component of the country’s commercial fisheries sector, second in value only to the demersal trawl fishery. Management of this sector relies on infrequent hydro-acoustic surveys, which provide measures of anchovy Engraulis encrasicolus and sardine Sardinops sagax biomass used in the assessments of stock status and in the development of management plans for the sustainable utilisation of these resources. We demonstrate how technological capabilities in ocean robotics at the Council for Scientific and Industrial Research (CSIR) could augment the current resource-intensive hydro-acoustic ship-based survey programme and create opportunities for expanding its spatial and temporal resolution. We successfully implement and demonstrate an autonomous wave glider, fitted with a hydro-acoustic sensor and compare the data to a collocated ‘traditional’ ship-based acoustics survey. In the future these autonomous systems approaches could be seen as a means to lessen the cost burden of the ship-based survey, while at the same time with the added advantage of continuous collection over much wider spatial and temporal domains. This could enable a more reflexive stock management approach taking into account the seasonal characteristics of the fishery and its ecosystem. Gliders thus have potential to increase dramatically the quantity of information available to fisheries managers, thereby reducing uncertainty and contributing to improved management of valuable fish resources. They are likely to contribute to improved knowledge of the ecology of small pelagic fish species off the coast of South Africa in a changing climate and should potentially also permit the collection of biomass data for other marine resources currently not routinely monitored. 相似文献
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Anne A. Weekes Christian E. Torgersen David R. Montgomery Andrea Woodward Susan M. Bolton 《水文研究》2015,29(3):356-372
Few systematic studies of valley‐scale geomorphic drivers of streamflow regimes in complex alpine headwaters have compared response between catchments. As a result, little guidance is available for regional‐scale hydrological research and monitoring efforts that include assessments of ecosystem function. Physical parameters such as slope, elevation range, drainage area and bedrock geology are often used to stratify differences in streamflow response between sampling sites within an ecoregion. However, these metrics do not take into account geomorphic controls on streamflow specific to glaciated mountain headwaters. The coarse‐grained nature of depositional features in alpine catchments suggests that these landforms have little water storage capacity because hillslope runoff moves rapidly just beneath the rock mantle before emerging in fluvial networks. However, recent studies show that a range of depositional features, including talus slopes, protalus ramparts and ‘rock‐ice’ features may have more storage capacity than previously thought. To better evaluate potential differences in streamflow response among basins with extensive coarse depositional features and those without, we examined the relationships between streamflow discharge, stable isotopes, water temperature and the amplitude of the diurnal signal at five basin outlets. We also quantified the percentages of colluvial channel length measured along the stepped longitudinal profile. Colluvial channels, characterized by the presence of surficial, coarse‐grained depositional features, presented sediment‐rich, transport‐limited morphologies that appeared to have a cumulative effect on the timing and volume of flow downstream. Measurements taken from colluvial channels flowing through depositional landforms showed median recession constants (Kr) of 0.9–0.95, δ18O values of ≥?14.5 and summer diurnal amplitudes ≤0.8 as compared with more typical surface water recession constant values of 0.7, δ18O ≤ ?13.5 and diurnal amplitudes >2.0. Our results demonstrated strong associations between the percentage of colluvial channel length within a catchment and moderated streamflow regimes, water temperatures, diurnal signals and depleted δ18O related to groundwater influx. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
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The amount of capital required to transition energy systems to low-carbon futures is very large, yet analysis of energy systems change has been curiously quiet on the role of capital markets in financing energy transitions. This is surprising given the huge role finance and investment must play in facilitating transformative change. We argue this has been due to a lack of suitable theory to supplant neoclassical notions of capital markets and innovation finance. This research draws on the notion from Planetary economics: Energy, climate change and the three domains of sustainable development, by Grubb and colleagues, that planetary economics is defined by three ‘domains’, which describe behavioural, neoclassical, and evolutionary aspects of energy and climate policy analysis. We identify first- and second-domain theories of finance that are well established, but argue that third-domain approaches, relating to evolutionary systems change, have lacked a compatible theory of capital markets. Based on an analysis of electricity market reform and renewable energy finance in the UK, the ‘adaptive market hypothesis' is presented as a suitable framework with which to analyse energy systems finance. Armed with an understanding of financial markets as adaptive, scholars and policy makers can ask new questions about the role of capital markets in energy systems transitions.Policy relevanceThis article explores the role of financial markets in capitalising low-carbon energy systems and long-term change. The authors demonstrate that much energy and climate policy assumes financial markets are efficient, meaning they will reliably capitalise low-carbon transitions if a rational return is created by subsidy regimes or other market mechanisms. The authors show that the market for renewable energy finance does not conform to the efficient markets hypothesis, and is more in line with an ‘adaptive’ markets understanding. Climate and energy policy makers that design policy, strategy, and regulation on the assumption of efficient financial markets will not pay attention to structural and behavioural constraints on investment; they risk falling short of the investment levels needed for long-term systems change. In short, by thinking of financial markets as adaptive, the range of policy responses to enable low-carbon investment can be much broader. 相似文献
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Sudipta Neogi Edward W. Bolton Sumit Chakraborty 《Contributions to Mineralogy and Petrology》2014,168(2):1-22
A numerical code has been developed to track the distribution of trace elements in crustal rocks undergoing melting. The model handles diffusion with moving boundaries and accounts for the processes of diffusion, dissolution and precipitation in a partially molten system. Among the various input parameters for modelling, source composition (i.e. modal abundance) variations, diffusion coefficients and partition coefficients are found to exert a significant control on the melt chemistry. The other inputs such as melt reaction stoichiometry, kinetics of melting and grain size of protolith have lesser influence. Exploration of the general behaviour indicates that for systems in which disequilibrium melting of the kind considered in this paper occurs, trace element concentrations may be used to constrain the composition of the protolith or the timescales of melting, depending on the specific circumstances. After exploring some general features of melting in a pelitic system, the model is applied to calculate trace element distributions in migmatites from the Lesser Himalayan rocks in Sikkim, India. We focus on the distribution of trace elements during the initial stages of melt formation. These partially molten rocks show disequilibrium distribution of trace elements, and the numerical code is capable of quantitatively reproducing many of the observed patterns. The results of the modelling indicate that the observed melts in this zone were formed within 50,000 years and that segregation of melts (into leucosome and restite) was complete between 50,000 and 250,000 years. These short timescales may point to deformation-enhanced melt segregation at least on a hand specimen scale. It is important to distinguish between timescales of segregation over these scales and timescales of removal of melt on an outcrop scale to form plutons—the latter, requiring higher degrees of melting and larger distances of migration, take longer. 相似文献
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