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31.
32.
D. E. Martínez O. M. Quiroz Londoño H. E. Massone P. Palacio Buitrago L. Lima 《Environmental Earth Sciences》2012,65(2):411-420
Fluoride pollution is a widespread problem in Argentina, as it is in many areas of the World. Former investigations have demonstrated
that the dissolution of volcanic glass disseminated in the loess-like sediments is the main source of fluoride in the Argentine
pampas. Nevertheless, fluoride distribution is erratic and the factors controlling it are not yet well-known. A large amount
of hydrochemical data collected in the Quequen Grande river catchment is used in order to contribute to the understanding
of fluoride distribution and mobility in groundwater in the Argentine pampas. The Quequen Grande river catchment is 10,000 km2. It extends between two low hills ranges of Precambrian metamorphic rocks and sedimentary Paleozoic rocks, filled by a thick
sequence of Cenozoic sediments, mainly silts and silt-clayed, with sand layers. These sediments form a multilayer phreatic
aquifer, which is recharged from precipitation infiltration, discharging through streams directly into the sea. Fluoride concentration
in surface water (n = 353) ranges between 0 and 6.5 mg/l, with an average value of 2.14 mg/l. Groundwater concentration (n = 135) is from 0 up to 5.7 mg/l, with an average of 1.84 mg/l. Considering the statistic distribution of fluoride, two groups
of samples can be determined. A dominant group between 0 and 3 mg/l, with a mean value around 1.5 mg/l, corresponding to samples
widely distributed in the catchment; and a second one group with fluoride contents between 3 and 6 mg/l, corresponding to
a smaller area in the central-west border. Two different sources are proposed. Volcanic glass dissolution is responsible for
dominant values in the catchment, while the weathering of biotite from the Paleozoic bedrock can be assumed to account for
the higher concentrations in the second group. 相似文献
33.
David Valença Dantas Mario Barletta Jonas de Assis Almeida Ramos André Ricardo Araújo Lima Monica Ferreira da Costa 《Estuaries and Coasts》2013,36(2):237-256
This study describes the seasonal feeding habits of different size classes of Cathorops spixii and Cathorops agassizii along an estuarine ecocline and the food overlap when different size classes occur together. These species were essentially zoobenthivorous, feeding mainly on Polychaeta Nereis sp., Copepoda Pseudodiaptomus acutus, Ostracoda Asterope sp., Gastropoda: Littorinidae, and Bivalvia Mytilus sp. and Anomalocardia brasiliana. However, during their life cycle and between different habitats and seasons, their trophic guild can change to zooplanktivore. The competition for resources was observed among C. spixii and C. agassizii, but was significantly reduced due to the seasonal diference in habitat use by different ontogenetic phases in the main channel of the estuary. The food niche separation was strongly influenced by environmental fluctuations, principally of salinity, resulting from rainfall and river inflow. High abundance of some preys, such as P. acutus (all estuary), Asterope sp. (upper and middle estuary), and Littorinidae (upper and lower estuary), could influence the significant diet overlap, principally during the rainy season, and call for more detailed studies of the benthic community structure. Moreover, dietary overlap was observed mainly between smaller sizes (intraspecific and/or interspecific) or between corresponding ontogenetic phases (interspecific), suggesting some differentiation in the diet in relation to the size class. Differences in prey type and size between the different ontogenetic phases of these ariids, principally among juveniles and adults, could be related to the size of the mouth, since adults are able to successfully capture larger preys or larger quantities of particular items. 相似文献
34.
35.
New limits on the cosmic equation of state are derived from age measurements of three recently reported old high-redshift galaxies (OHRG). The results are based on a flat Friedmann–Robertson–Walker (FRW) type cosmological model driven by non-relativistic matter plus a smooth component parametrized by its equation of state p x ωρ x ( ω −1). The range of ω is strongly dependent on the matter density parameter. For ΩM ∼0.3, as indicated from dynamical measurements, the age estimates of the OHRG restrict the cosmic parameter to ω −0.27. However, if ΩM is the one suggested by some studies of field galaxies, i.e., ΩM ≃0.5, only a cosmological constant ( ω =−1) may be compatible with these data. 相似文献
36.
It is known that stellar winds from late type stars are of mixed thermal and magnetic origin. The stellar wind model presented in this work uses the hydrodynamic equations of mass and momentum conservation and closes the system of equations with a detailed energy equation. Both momentum and energy equations have terms due to the effects of Alfvén waves. A smooth transition between the two regimes for Alfvén wave propagation, the undamped and the damped modes, is achieved by considering the geometrical mean of both wave amplitudes. It will be shown that the initial push on the plasma is provided by the mechanical heating input, and that further out the Alfvén waves take over energetically. 相似文献
37.
M. C. Lima e Cunha V. P. Pereira E. Menegotto A. C. Bastos Neto L. D. Oliveira M. L. L. Formoso 《Environmental Geology》2008,55(6):1355-1362
The vegetal species Ampelozizyphus amazonicus Ducke (Rhamnaceae Family) was chosen as a sampling medium for the lateritic surfaces of the Pitinga Mine in the Amazon region,
in order to study the biogeochemical behavior of this species and compare it with the chemical composition of a reference
plant. The Pitinga mining district is one of the largest producers of tin in the world. This district contains unique deposits
of cryolite and rare metals such as Zr, Nb, Ta, Y and REEs related to granitic bodies that intrude into the volcanic and acid
pyroclastic rocks. The results showed that the species A. amazonicus predominantly concentrates significant levels of Zr, Nb, Ta, Th, Be, Sc over U, Hf, Ga and In. These elements are characteristic
of the mineral paragenesis for the region, suggesting that this plant can provide a representative sampling medium future
geochemical exploration programs in the region. 相似文献
38.
Fault tip regions, relay ramps and accommodation zones in between major segments of extensional fault systems provide zones of additional structural and stratigraphic complexities and also significantly affect their hydraulic behaviour. The great interest for both academic and industrial purposes encouraged specific studies of fault tip regions that, in some cases, produced controversial results. We approached the study of fault tip regions by integrating structural, AMS and stratigraphic analyses of the tip of an extensional growth fault system in the Tarquinia basin, on the Tyrrhenian side of the Northern Apennines. Detailed structural mapping indicates the occurrence of systematic relationships between the orientation of the main subsidiary fault zones, the orientation and position of the two main joint sets developed in the fault damage zones, and the overprinting relationships between the two main joint sets themselves. Microstructural analysis of fault core rocks indicates a progression of deformation from soft-sediment to brittle conditions. The AMS study supports the evolution of deformation under a constantly oriented stress field. By combining this multidisciplinary information we propose an evolutionary model for the tip of the extensional growth fault system that accounts for the progressively changing sediment rheological properties, and for the time dependent subsidiary deformation pattern by invoking the interplay between the regional stress field and the local, kinematically-derived one by fault activity. We also speculate on the overall implications for fluid flow of the proposed evolutionary model. 相似文献
39.
Risers and anchor lines play important roles in offshore oil exploitation activities nowadays. For this reason the proper analysis and design of such slender structures has been of a paramount interest. The principal characteristics to be accounted for in riser and mooring line analysis are the severe nonlinearities involved and the random dynamic effects associated. The Finite Element Method (FEM) is an essential step to cope with this kind of analysis. But the use of the FEM can be computationally very expensive for the solution of the resultant nonlinear differential equations of motion, because the time-domain integration should produce sufficiently long response time-histories using small time-steps in order to obtain reliable time-series statistics of any structural response parameter, e.g., top tension in an anchor line or stresses occurring at a critical section in a steel catenary riser (SCR). This paper presents a very efficient hybrid Artificial Neural Network (ANN)–Finite Element Method (FEM) procedure to perform a nonlinear mapping of the current and past system excitations (inputs) to produce subsequent system response (output) for the random dynamic analysis of mooring lines and risers. Firstly, a quite short FEM-based time-domain response simulation is generated. Then, an ANN is used to predict the remaining structural response time-history simulation. The hybrid ANN–FEM approach can be very efficient for predicting long response time-histories. It has been observed that a 3 h response time-history can accurately be obtained with approximately the computational cost of a 500 s one, i.e., 20 times faster than a complete simulation using finite element-based solution. Roughly, this can represent a reduction of about a dozen of hours of computer time for a single mooring line analysis and about two dozens of hours (or more) for a single SCR analysis, both belonging to a deep-water floating unit. 相似文献
40.
We report on biomarker responses conducted as part of a multi-level assessment of the chronic toxicity of estuarine sediments to the amphipod Gammarus locusta. A companion article accounts for organism and population-level effects. Five moderately contaminated sediments from two Portuguese estuaries, Sado and Tagus, were assessed. Three of them were muddy and two were sandy sediments. The objective was to assess sediments that were not acutely toxic. Three of the sediments met this criterion, the other two were diluted (50% and 75%) with clean sediment until acute toxicity was absent. Following 28-d exposures, the amphipods were analysed for whole-body metal bioaccumulation, metallothionein induction (MT), DNA strand breakage (SB) and lipid peroxidation (LP). Two of the muddy sediments did not cause chronic toxicity. These findings were consistent with responses at organism and population levels that showed higher growth rates and improvement of reproductive traits for amphipods exposed to these two sediments. Two other sediments, one muddy and one sandy, exhibited pronounced chronic toxicity, affecting SB, MT induction (in muddy sediment), survival and reproduction. Potential toxicants involved in these effects were identified. The last sandy sediment exhibited some loss of DNA integrity, however growth was also enhanced. Present results, together with the organism/population-level data, and also benthic communities information, were analysed under a weight-of-evidence approach. By providing evidence of exposure (or lack of it) to contaminants in sediments, the biomarkers here applied assisted in distinguishing toxicants' impacts in test organisms from the confounding influence of other geochemical features of the sediments. 相似文献