H ₂ excitation in turbulent interstellar clouds     

Cesare Cecchi-Pestellini  Silvia Casu  Alexander Dalgarno 《Monthly notices of the Royal Astronomical Society》2005,364(4):1309-1314

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Cosmic and Galactic neutrino backgrounds from thermonuclear sources     

Cristiano Porciani  Silvia Petroni  Giovanni Fiorentini 《Astroparticle Physics》2004,20(6):683-701

We estimate energy spectra and fluxes at the Earth’s surface of the cosmic and Galactic neutrino backgrounds produced by thermonuclear reactions in stars. The extra-galactic component is obtained by combining the most recent estimates of the cosmic star formation history and the stellar initial mass function with accurate theoretical predictions of the neutrino yields all over the thermonuclear lifetime of stars of different masses. Models of the structure and evolution of the Milky Way are used to derive maps of the expected flux generated by Galactic sources as a function of sky direction. The predicted neutrino backgrounds depend only slightly on model parameters. In the relevant 50 keV–10 MeV window, the total flux of cosmic neutrinos ranges between 20 and 65 cm⁻² s⁻¹. Neutrinos reaching the Earth today have been typically emitted at redshift z2. Their energy spectrum peaks at E0.1–0.3 MeV. The energy and entropy densities of the cosmic background are negligible with respect to the thermal contribution of relic neutrinos originated in the early universe. In every sky direction, the cosmic background is outnumbered by the Galactic one, whose integrated flux amounts to 300–1000 cm⁻² s⁻¹. The emission from stars in the Galactic disk contributes more than 95% of the signal.  相似文献   

Characterization of Wind Meandering in Low-Wind-Speed Conditions     

Luca Mortarini  Michel Stefanello  Gervasio Degrazia  Debora Roberti  Silvia Trini Castelli  Domenico Anfossi 《Boundary-Layer Meteorology》2016,161(1):165-182

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Use of Remote Imagery to Analyse Changes in Morphology and Longitudinal Large Wood Distribution in the Blanco River After the 2008 Chaitén Volcanic Eruption,Southern Chile          下载免费PDF全文

Héctor Ulloa  Andrés Iroumé  Luca Mao  Andrea Andreoli  Silvia Diez  Luis E. Lara 《Geografiska Annaler: Series A, Physical Geography》2015,97(3):523-541

The 2008 Chaitén volcanic eruption generated significant changes in the channel morphology and large wood (LW) abundance along the fluvial corridor of the Blanco River, southern Chile. Comparisons of remote sensing images from the pre‐eruption (year 2005) and post‐eruption (years 2009 and 2012) conditions showed that in a 10.2 km long study segment the Blanco River widened 3.5 times from 2005 to 2009, and that the maximum enlargement was nine times the original width. Changes in channel width were lower between the years 2012 and 2009. The sinuosity and braiding indexes also changed between 2005 and 2009. After the eruption the channel sinuosity was higher and specific river reaches developed a braided pattern, but by 2012 the channel was recovering pre‐eruption characteristics. Huge quantities of LW were introduced to the study segment; individual LW per km of channel length were 1.6 and 74.3 in 2005 and 2009, respectively, and more than 30 log jams km^?1 were observed in the year 2009. Between 2009 and 2012 the quantity of LW was very similar. Statistically significant relationships were found between the number of log jams and channel sinuosity and between the number of pieces of large wood with sinuosity and channel width. Wood was highly dynamic between 2009 and 2012: 78% of individual pieces and 48% of log jams identified in the 2009 image had moved by 2012. Finally the supervised classification of imagery associated with ArcMap tools was tested to identify large wood.  相似文献   

Benthic foraminifera as bioindicator for cold-water coral reef ecosystems along the Irish margin     

Stephan Margreth  Andres Rüggeberg  Silvia Spezzaferri 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(12):2216-2234

Cold-water coral ecosystems building cold-water carbonate mounds occur worldwide and are especially developed along the European margin, from northern Norway to the Gulf of Cadiz. A remarkable mound province is documented southwest of Ireland along the Porcupine and Rockall Banks. In this area carbonate mounds are formed in water depths between 500 and 1200 m and are often densely settled by cold-water coral ecosystems offering many ecological niches for benthic foraminifera. We investigated total (unstained) benthic foraminiferal assemblages from surface sediments (0–1 cm, >63 μm size fraction) of this region with the aim to trace their distribution patterns and to test if they can be used as bioindicators for facies characterization in different parts of carbonate mound systems. Our quantitative data were further statistically treated with non-metric multidimensional scaling (nMDS) based on Bray–Curtis similarity matrix to highlight community patterns that were not readily apparent. Our results indicate that different benthic foraminiferal assemblages characterize different facies along cold-water carbonate mounds and are related to the environmental conditions and available substrates. The following facies can be described: (1) the Off-Mound Facies is dominated by uvigerinids and other infaunal species; (2) the Dropstone Facies is characterized by infaunal Globocassidulina subglobosa and attached-epifaunal Cibicidoides sp.; (3) the Dead Coral Facies is characterised by epifaunal species (e.g., Planulina ariminensis, Hanzawaia boueana) and infaunal species (Spiroplectinella wrightii, Angulogerina angulosa, Epistominella vitrea); (4) the Living Coral Facies includes both infaunal and epifaunal species, but is dominated by the epifaunal Discanomalina coronata; and (5) the Sandwave Facies contains high abundances of epifaunal species including D. coronata. Based on this distribution, we propose D. coronata, as an indicator species to identify active mounds and/or living cold-water coral ecosystems. Our results also emphasise the importance of studying the small size fractions that yield many infaunal species. A causal link exists between distribution patterns of benthic foraminifera and cold-water coral facies, thus providing an independent tool to identify and describe the different facies in this setting.  相似文献   

Enhanced Atlantic Meridional Overturning Circulation supports the Last Glacial Inception     

Abel Guihou  Sylvain Pichat  Aline Govin  Silvia Nave  Elisabeth Michel  Jean-Claude Duplessy  Philippe Telouk  Laurent Labeyrie 《Quaternary Science Reviews》2011,30(13-14):1576-1582

The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the climate system. However, its role during climate change is still poorly constrained particularly during an Interglacial to Glacial climate transition and the associated global cooling. We present here the first reconstruction of the evolution of the vertical structure of the rate of the AMOC from the Last Interglaciation to the subsequent glaciation (128,000–60,000 years ago) based on sedimentary (²³¹Pa/²³⁰Th) records. We show a deep AMOC during the interglacial warmth Marine Isotope Stage (MIS) 5.5 and a shallower glacial one during glacial MIS 4. The change between these two patterns occurred mostly during the glacial inception, i.e. the transition from MIS 5.5 to MIS 5.4. Our data show that AMOC was enhanced during this latter transition as a consequence of a large increase of the overturning rate of the Intermediate Waters, above 2500 m. We suggest that this AMOC pattern required a reinforced Gulf Stream-North Atlantic Current system that ultimately supported ice-sheet growth by providing heat and moisture to the Northern high latitudes. From MIS 5.4 to MIS 5.1, the AMOC was broadly continuous below 2000 m and supported periods of ice-sheet growth. As a result, a glacial AMOC is triggered at the beginning of MIS 4 due to the extension of ice-sheet and the subsequent reorganization of deep-water formation. This study highlights the role of intermediate waters as a major player during climate change.  相似文献   

Dissolved and particulate heavy metals distribution in coastal lagoons. A case study from Mar Chiquita Lagoon,Argentina     

María Ornela Beltrame  Silvia G. De Marco  Jorge E. Marcovecchio 《Estuarine, Coastal and Shelf Science》2009

Mar Chiquita Coastal Lagoon is located on the Atlantic coast of Argentina, and it has been declared a Biosphere Reserve under the UNESCO Man and Biosphere Programme (MAB). This coastal lagoon constitutes an estuarine environment with a very particular behaviour and it is ecologically important due to its biological diversity. The aim of the present study was to evaluate the distribution and geochemical behaviour of several heavy metals in this coastal system, focusing on their distribution in both the dissolved phase (<0.45 μm) and the suspended particulate matter. Therefore, the general hydrochemical parameters (salinity, temperature, turbidity, pH and dissolved oxygen) and concentration of total particulate and dissolved metals (Cd, Cu, Ni, Zn, Fe, Pb, Cr and Mn) were measured along 2 years (2004–2006) at two different sites. As regards their distribution, hydrological parameters did not present any evidence of deviation with respect to historical values. Suspended particulate matter showed no seasonal variation or any relationship with the tide, thus indicating that in this shallow coastal lagoon neither tides nor freshwater sources regulate the particulate matter input. Heavy metals behaviour, both in dissolved and particulate phases did not reveal any relationship with tide or seasons. Mar Chiquita Coastal Lagoon showed a large input of dissolved and particulate metals, which is probably due to intensive agriculture within the drainage basin of this system.  相似文献   

Robust vulnerability analysis of nuclear facilities subject to external hazards     

Silvia?ToloEmail authorView author&#;s OrcID profile  Edoardo?Patelli  Michael?Beer 《Stochastic Environmental Research and Risk Assessment (SERRA)》2017,31(10):2733-2756

Natural hazards have the potential to trigger complex chains of events in technological installations leading to disastrous effects for the surrounding population and environment. The threat of climate change of worsening extreme weather events exacerbates the need for new models and novel methodologies able to capture the complexity of the natural-technological interaction in intuitive frameworks suitable for an interdisciplinary field such as that of risk analysis. This study proposes a novel approach for the quantification of risk exposure of nuclear facilities subject to extreme natural events. A Bayesian Network model, initially developed for the quantification of the risk of exposure from spent nuclear material stored in facilities subject to flooding hazards, is adapted and enhanced to include in the analysis the quantification of the uncertainty affecting the output due to the imprecision of data available and the aleatory nature of the variables involved. The model is applied to the analysis of the nuclear power station of Sizewell B in East Anglia (UK), through the use of a novel computational tool. The network proposed models the direct effect of extreme weather conditions on the facility along several time scenarios considering climate change predictions as well as the indirect effects of external hazards on the internal subsystems and the occurrence of human error. The main novelty of the study consists of the fully computational integration of Bayesian Networks with advanced Structural Reliability Methods, which allows to adequately represent both aleatory and epistemic aspects of the uncertainty affecting the input through the use of probabilistic models, intervals, imprecise random variables as well as probability bounds. The uncertainty affecting the output is quantified in order to attest the significance of the results and provide a complete and effective tool for risk-informed decision making.  相似文献   

Proposal of design rules for ductile X-CBFS in the framework of EUROCODE 8     

Silvia Costanzo  Mario D'Aniello  Raffaele Landolfo 《地震工程与结构动力学》2019,48(1):124-151

Cross concentrically braced frames (X-CBFs) are commonly used as primary seismic resisting system, owing to their large lateral stiffness, simplicity of design, and relatively low constructional cost. Current EN 1998-1 provides design rules theoretically aiming at developing ductile global plastic mechanism, namely enforcing plastic deformations in the diagonal members, while the remaining structural members and connections should elastically behave. However, as widely demonstrated by many existing studies, the design and the corresponding seismic performance of EC8-compliant X-CBFs are generally affected by several criticisms, eg, difficulties in sizing of diagonal members, massive and non-economical structures, and poor seismic behavior. In light of these considerations, the research activity presented in this paper is addressed to revise the design rules and requirements given EN 1998-1 for X-CBFs to simplify the design process and to improve the ductility and the dissipative capacity of this structural system. Hence, design rules are proposed for the next version of EN 1998-1 and numerically validated by means of nonlinear dynamic analyses.  相似文献   

Demography and biomass of the seagrass Zostera marina in a Mexican Coastal Lagoon     

Miriam Poumian-Tapia  Silvia E. Ibarra-Obando 《Estuaries and Coasts》1999,22(4):837-847

From January 1987 to February 1988 the annual biomass cycle and demography of the seagrass Zostera marina were assessed in San Quintin Bay, a shallow coastal lagoon on the Pacific coast of Baja California, Mexico. Shoot density and aboveground biomass were sampled monthly along two intertidal transects parallel to the shore. Belowground biomass was sampled every 2 mo. Shoot density differed between transects, ranging from 929±71 (SE) in July to 279 ±80 shoots m⁻² in December, at the deeper transect (I). At the shallow transect (II) there was not a significant difference through time, and a mean of 737 shoots m⁻² was calculated. Lateral shoots were present year round and represented between 1% and 30% of total density at transect I and between 3% and 25% at transect II. Reproductive shoots were present from March to September at both transects, with a density range of 77±28 shoots m⁻² (March) to 9±3 shoots m⁻² (September), and represented 5% of total shoot density. Neither aboveground biomass nor LAI (Leaf Area Index) differed between transects, with values ranging between 77±14.5 g dry wt m⁻² (October 1987) and 13±2.4 g dry wt m⁻² (February 1988) for aboveground biomass, and between 0.6±0.2 m² leves m⁻² substrate (January) and 2.7±0.3 m² leaves m⁻² substrate (September) for LAI. Neither root biomass nor rhizome biomass differed between transects, or as a function of time; the mean value for roots was 17 g dry wt m⁻² and for rhizomes 29 g dry wt m⁻². Belowground biomass represented 54% of total biomass. We found a significant correlation between aboveground biomass and LAI (r=0.949 for transect I, and 0.926 for transect II) as well as between total biomass (aboveground and belowground) and LAI (r=0.814), which allows us to consider using LAI as a predictor of these variables. Biomass changes were related to changes in shoot weight (r=0.676 at transect I; 0.582 at transect II), more than to changes in shoot number. Water temperature was found to be the driver of biomass changes in the aboveground compartment.  相似文献