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961.
Myriam Andrea Rada Torres Vincenzo Costanzo-Álvarez Milagrosa Aldana Nery Suárez Corina Campos Maria Magdalena Mackowiak-Antczak María Carmela Brandt 《Studia Geophysica et Geodaetica》2011,55(4):717-736
Rock magnetic, petrographic and dielectric analyses were carried out, in a number of archeological ceramic potsherds, in order
to characterize the different manufacturing techniques used by Prehistoric Venezuelan Amerindian potters. Samples were excavated
in 7 Venezuelan islands and ascribed to distinct cultural groups on purely stylistic grounds (i.e. Valencioide, Ocumaroide,
Dabajuroide and Unknown). Mean coercivity values were determined through a direct signal analyses (DSA) applied to isothermal
remanent magnetization (IRM) acquisition curves. Logarithmic plots of these mean coercivities are the best quantitative means
to classify diverse ceramics. The mean coercivity values seem to group the samples according to the manufacturing development.
These values also seem to discriminate the samples provenance, indeed, this plot displays a good grouping of data for samples
presumably manufactured by the same culture but excavated at different locations. Thermomagnetic cycles supply helpful information
about original ceramic firing conditions. The irreversibility parameter (IP) for thermomagnetic curves (heating and cooling),
serves as an indicator of the amount of organic matter burnt during original pottery firing. The IP for a number of pottery
samples from Venezuelan islands and mainland, with ages ranging between 300 BC to 1500 AD, might suggest a possible increasing
trend in time towards the complete reversibility line of IP = 0. Most samples, independently of age and cultural group, have
IP values that suggest that most open fires, used by primitive Venezuelan Amerindian potters, had enough ventilation and oxidizing
atmospheres. A scatter plot of maximum current depolarization temperatures versus natural remanent magnetization (NRM) suggests
a complex non-linear relationship between these two parameters most likely due to the fact that both, dielectric and rock
magnetic data, are linked to pore-related features. 相似文献
962.
Solute transport in rivers is controlled by surface hydrodynamics and by mass exchanges with distinct retention zones. Surface and hyporheic retention processes can be accounted for separately in solute transport models with multiple storage compartments. In the simplest two component model, short term storage can be associated to in-channel transient retention, e.g. produced by riparian vegetation or surface dead zones, and the long-term storage can be associated to hyporheic exchange. The STIR (Solute Transport In Rivers) multiple domain transport model is applied here to tracer test data from three very different Mediterranean streams with distinctive characteristics in terms of flow discharge, vegetation and substrate material. The model is used with an exponential residence time distribution (RTD) to represent surface storage processes and two distinct modeling closures are tested to simulate hyporheic retention: a second exponential RTD and a power-law distribution approximating a known solution for bedform-induced hyporheic exchange. Each stream shows distinct retention patterns characterized by different timescales of the storage time distribution. Both modeling closures lead to very good approximations of the observed breakthrough curves in the two rivers with permeable bed exposed to the flow, where hyporheic flows are expected to occur. In the one case where the occurrence of hyporheic flows is inhibited by bottom vegetation, only the two exponential RTD model is acceptable and the time scales of the two components are of the same magnitude. The significant finding of this work is the recognition of a strong signature of the river properties on tracer data and the evidence of the ability of multiple-component models to describe individual stream responses. This evidence may open a new perspective in river contamination studies, where rivers could possibly be classified based on their ability to trap and release pollutants. 相似文献
963.
Véronique Boily Andrea Bertolo Pierre Magnan Maria-Grazia Martinoli Hélène-Marie Thérien 《Aquatic Sciences - Research Across Boundaries》2011,73(3):345-354
The purpose of this study was to determine experimentally the effects of the quality (UV-A/UV-B ratio) and quantity (irradiance)
of natural ultraviolet radiation (UVR) on the survival of yellow perch (Perca flavescens) larvae and on the oxidative stress in their cytoplasm, estimated by the activity of the superoxide dismutase (SOD). We also
estimated the potential accumulation of photodamage in DNA by using UV dosimeters. Freshly-hatched yellow perch and UV dosimeters
were incubated in controlled conditions under a factorial combination of selective and non-selective filters offering different
levels of UVR protection and exposed to natural solar light. Larval survival was inversely related to the UVR intensity gradient,
and responded similarly to the presence of both UV-A and UV-B or UV-A only. In contrast, the responses of SOD activity and
UV dosimeter were stronger in the presence of both UV-A and UV-B than UV-A only, leading to a partial mismatch with the results
on survival. Our results, obtained under natural solar light, suggest that incident UV-A radiation, despite its lower energy
per unit photon than UV-B, could be a serious threat for yellow perch larvae. They also show that UV dosimeters and SOD activity
predict only some components of UVR risk for yellow perch larvae. The partial mismatch between UV dosimeters results and larval
survival cautions against the use of UV dosimeters alone as a proxy for UVR risk in the field. 相似文献
964.
Pablo J.D. Mauas Andrea P. Buccino Eduardo Flamenco 《Journal of Atmospheric and Solar》2011,73(2-3):377-382
River streamflows are excellent climatic indicators since they integrate precipitation over large areas. Here we follow up on our previous study of the influence of solar activity on the flow of the Paraná River, in South America. We find that the unusual minimum of solar activity in recent years have a correlation on very low levels in the Paraná's flow, and we report historical evidence of low water levels during the Little Ice Age. We also study data for the streamflow of three other rivers (Colorado, San Juan and Atuel), and snow levels in the Andes. We obtained that, after eliminating the secular trends and smoothing out the solar cycle, there is a strong positive correlation between the residuals of both the Sunspot Number and the streamflows, as we obtained for the Paraná. Both results put together imply that higher solar activity corresponds to larger precipitation, both in summer and in wintertime, not only in the large basin of the Paraná, but also in the Andean region north of the limit with Patagonia. 相似文献
965.
Fatemeh Jalayer Domenico Asprone Andrea Prota Gaetano Manfredi 《Bulletin of Earthquake Engineering》2011,9(4):997-1014
The post-earthquake assessment of existing structures can be further complicated by the progressive damage induced by the occurrence of a sequence of aftershocks. This work presents a simple methodology for the calculation of the probability of exceeding a certain limit state in a given interval of time. The time-decaying mean daily rate of occurrence of significant aftershock events is modeled by employing a site-specific aftershock model for the L??Aquila 2009 aftershock sequence (central Italy). The number of aftershock events occurring in a given interval of time elapsed after the main event is modeled using a non-homogenous Poisson model. An equivalent single-degree of freedom structure with cyclic stiffness degradation is used in order to evaluate the progressive damage caused by a sequence of aftershock events. Given the time history of the main-shock and the residual damage caused by it, the probability of exceeding a set of discrete limit states in a given interval of time is calculated. Of particular importance is the time-variant probability of exceeding the limit state in a 24-h (a day) interval of time which can be used as a proxy for the life-safety considerations regarding the re-occupancy of the structure and to complement the results of visual inspections for prioritizing the emergency operations. The method presented herein can also be used in an adaptive manner, progressively conditioned on the time-histories of aftershock events following the main-shock and on the corresponding residual damage caused by them. 相似文献
966.
Autonomous underwater vehicle teams for adaptive ocean sampling: a data-driven approach 总被引:1,自引:1,他引:0
Andrea?MunafòEmail author Enrico?Simetti Alessio?Turetta Andrea?Caiti Giuseppe?Casalino 《Ocean Dynamics》2011,61(11):1981-1994
The current technological developments in autonomous underwater vehicles (AUVs) and underwater communication have nowadays
allowed to push the original idea of autonomous ocean sampling network even further, with the possibility of using each agent
of the network not only as an operative component driven by external commands (model-driven) but as a reactive element able
to act in response to changing conditions as measured during the exploration (data-driven). With this paper, we propose a
novel data-driven algorithm for AUVs team for adaptive sampling of oceanic regions, where each agent shares its knowledge
of the environment with its teammates and autonomously takes decision in order to reconstruct the desired oceanic field. In
particular, sampling point selection is made in order to minimize the uncertainty in the estimated field while keeping communication
contact with the rest of the team and avoiding to repeatedly sampling sub-regions already explored. The proposed approach
is based on the use of the emergent behaviour technique and on the use of artificial potential functions (interest functions)
to achieve the desired goal at the end of the mission. In this way, there is no explicit minimization of a cost functional
at each decision step. The oceanic field is reconstructed by the application of radial basis functions interpolation of irregularly
spaced data. A simulative example for the estimation of a salinity field with sea data obtained using the Mediterranean Sea
Forecasting System is shown in the paper, in order to investigate the effect of the different uncertainty sources, including
sea currents, on the behaviour of the exploration team and ultimately on the reconstruction of the salinity field. 相似文献
967.
Matteo Tregnaghi Andrea Marion Andrea Bottacin‐Busolin Simon J. Tait 《地球表面变化过程与地形》2011,36(13):1761-1769
In this paper a modelling approach is presented to predict local scour under time varying flow conditions. The approach is validated using experimental data of unsteady scour at bed sills. The model is based on a number of hypotheses concerning the characteristics of the flow hydrograph, the temporal evolution of the scour and the geometry of the scour hole. A key assumption is that, at any time, the scour depth evolves at the same rate as in an equivalent steady flow. The assumption is supported by existing evidence of geometrical affinity and similarity of scour holes formed under different steady hydraulic conditions. Experimental data are presented that show the scour hole development downstream of bed sills due to flood hydrographs follow a predictable pattern. Numerical simulations are performed with the same input parameters used in the experimental tests but with no post‐simulation calibration. Comparison between the experimental and model results indicates good correspondence, especially in the rising limb of the flow hydrograph. This suggests that the underlying assumptions used in the modelling approach are appropriate. In principle, the approach is general and can be applied to a wide range of environments (e.g. bed sills, step‐pool systems) in which scouring at rapid bed elevation changes caused by time varying flows occurs, provided appropriate scaling information is available, and the scour response to steady flow conditions can be estimated. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
968.
Stefano Zanchetta Paolo D’Adda Andrea Zanchi Valentina Barberini Igor M. Villa 《Journal of Geodynamics》2011
The integration of structural analyses with 40Ar/39Ar dating of fault-related pseudotachylytes provides time constraints for the reconstruction of the Alpine evolution of the central portion of the South Alpine orogenic wedge. In the northern sector of the belt a Variscan basement is stacked southward on the Permian to Mesozoic cover along regional faults (Orobic and Porcile thrusts). Fault zones, slightly postdating a first folding event of Alpine age, experienced a complex evolution through the ductile and brittle deformation regime, showing greenschist facies mylonites overprinted by a penetrative cataclastic deformation. Generation of fault-related pseudotachylyte veins marks the onset of brittle conditions, lasting up to the youngest episodes of fault activity. 40Ar/39Ar dating of the pseudotachylyte matrix of 9 samples give two separated age clusters: Late Cretaceous (80–68 Ma) and latest Palaeocene to Middle Eocene (55–43 Ma). These new data provide evidence that the pre-Adamello evolution of the central Southern Alps was characterised by the superposition of different tectonic events accompanying the exhumation of the deepest part of the belt through the brittle–ductile transition. The oldest pseudotachylyte ages demonstrate that south-verging regional thrusting in the central Southern Alps was already active during the Late Cretaceous, concurrently with the development of a synorogenic foredeep basin where the Upper Cretaceous Lombardian Flysch was deposited. 相似文献
969.
Pyrite plays the central role in the environmental issue of acid rock drainage. Natural weathering of pyrite results in the release of sulphuric acid which can lead to further leaching of heavy and toxic metals from other associated minerals. Understanding how pyrite reacts in aqueous solution is critical to understanding the natural weathering processes undergone by this mineral. To this end an investigation of the effect of solution redox potential (Eh) and various anions on the rate of pyrite leaching under carefully controlled conditions has been undertaken.Leaching of pyrite has been shown to proceed significantly faster at solution Eh of 900 mV (SHE) than at 700 mV, at pH 1, for the leach media of HCl, H2SO4 and HClO4. The predominant effect of Eh suggests electrochemical control of pyrite leaching with similar mechanism(s) at Eh of 700 and 900 mV albeit with different kinetics. Leach rates at 700 mV were found to decrease according to HClO4 > HCl > H2SO4 while at 900 mV the leach rate order was HCl > HClO4 > H2SO4. Solution Fe3+ activity is found to continually increase during all leaches; however, this is not accompanied by an increase in leach rate.Synchrotron based photoemission electron microscopy (PEEM) measurements showed a localised distribution of adsorbed and oxidised surface species highlighting that pyrite oxidation and leaching is a highly site specific process mediated by adsorption of oxidants onto specific surface sites. It appears that rates may be controlled, in part, by the propensity of acidic anions to bind to the surface, which varies according to , thus reducing the reactive or effective surface area. However, anions may also be involved in specific reactions with surface leach products. Stoichiometric dissolution data (Fe/S ratio), XPS and XRD data indicate that the highest leach rates (in HCl media at 900 mV Eh) correlate with relatively lower surface S abundance. Furthermore, there are indications that solution Cl− assists oxidation especially at higher Eh through the prevention of surface S0 buildup at reactive surface sites. 相似文献