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MARIANNE LlLLIESKÖLD BJÖRN SUNDQVIST 《Boreas: An International Journal of Quaternary Research》1994,23(4):473-478
The relative age of glacial deposits on Jameson Land was investigated by weathering-based techniques. Two related methods, measurement of rind thickness and of Schmidt hammer rebound values. were added to a dating programme of luminescence-, amino acid- and 14 C-analyses. Rind thicknesses differ significantly between areas which represent three glacial phascs, spanning a time interval from late Saale to Late Weichselian. Rebound values are more susceptible to external factors than the rind values but may, if these external factors are known, serve as a complement to the interpretation of rind development. 相似文献
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The effects of subsurface hydrology on the evolution of erosive processes inside layered colluvium mantles in unchannelled valleys are investigated. Tensiometer nests are installed in different morphological conditions with their specific depths controlled by the subsurface structure of the colluvium mantle. Daily readings (during one year) and physical and mechanical analyses of the different colluvium layers are carried out. In spite of a highly discontinuous subsurface structure, present topography is the major factor controlling subsurface flow in this landscape. Paleotopographic features, usually resulting from the lateral mobilization of the previous topographic hollow axis, can locally control the preferential expansion of the saturated zone towards areas other than the present hollow axis. During rainy periods, pressure head at the base of the hollow axis increases towards a maximum value which is not increased by subsequent precipitation inputs. New water input is dissipated by direct precipitation onto the saturated zone and exfiltration by seepage at the gully head walls, precluding the development of the excess pore pressures required to trigger failure by landsliding. 相似文献
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Résumé L'essentiel du minerai actuellement exploité dans le district minier des Malines (Gard, France) provient d'amas plombo-zincifères apparamment constitués par le remplissage de cavités paléo-karstiques post-stéphaniennes développées dans la dolomie cambrienne, à proximité de la surface de discordance qui la sépare de sa couverture mésozoïque. L'étude détaillée des minéralisations du substrat carbonaté révèle l'existence de concentrations également plombo-zincifères, mais stratoïdes et syngénétiques de la dolomie qui les supporte. En effet, ces minéralisations ont enregistré tous les phénomènes ayant affecté la dolomie encaissante: figures de sédimentation et de déformation synmétamorphe (orogénie varisque), fracturation post-schisteuse antérieure à la discordance permo-triasique, ... Il est proposé de considérer que ce type de minéralisation alimenta en métaux de base, voire en soufre, les pièges karstiques post-hercyniens: on en trouve notamment remanié à l'état d'éléments brèchiques dans les remplissages paléo-karstiques.
Most of the ores extracted from the Les Malines mining district come from Pb-Zn deposits apparently formed by the in-filling of paleo-karstic caves. These cavities derive from cambrian dolomites solution by meteoric water circulations below the unconformity separating the triassic and/or mesozoic strata from the hercynien basement. Detailed study of the mineralizations located in the carbonate basement reveals besides the karstic in-filling stratoid ore bodies apparently syngenetic of their dolomitic bearing rock. The mineralizations record all the main geological events which affected the dolomitic rock since its deposition i.e. sedimentary features, synmetamorphic deformation, post-schistosity fracturation prior to the permo-triassic unconformity ... It is proposed to consider that these early stratoid mineralizations supplied the post-hercynian karstic traps with base metals and sulfur. It is to be noticed that these cambrian ores are found reworked as breccia elements in the karstic infilling.相似文献
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Francisco Jose da Silva Dias Belmiro Mendes Castro Luiz Drude Lacerda 《Ocean Dynamics》2018,68(8):967-985
This article characterizes the spatial and temporal current variations, in the subtidal and tidal ranges, during the rainy and dry seasons, at the continental shelf off the Jaguaribe River, through measurements of continuous current field data from an acoustic Doppler current profiler (ADCP) mooring during 124 days, from June 12 to October 14, 2009. To support this dataset, we collected corresponding data from a meteorological station located at the estuary. The spatial variation showed that highest current speeds occur near the coast, with an offset of a NNW coastal jet, decreasing intensity, monotonically, towards offshore up to 0.1 ms?1. In the rainy season, small inversions of the wind field were observed, lasting 2 to 3 days on average and were accompanied by the direction of surface currents only. In the dry season, the period of reversal of wind fields and currents lasted 14 and 35 h, respectively. The analysis of empirical orthogonal functions in rainy and dry seasons showed that the continental shelf is predominantly barotropic, where the second and third modes explained only 7% of the total variance, during the dry season. The tidal currents are more intense in the direction normal to the coast, showing a semidiurnal tidal regime. Energy distribution between tidal currents and currents of longer periods showed that for the component parallel to bathymetry, subtidal frequency currents are dominant, contributing to more than 70% of the variance. For the normal component to the coastline bathymetry, there is a significant increase of power concerning tidal currents, at all depths, so they contribute with about 55% of the total variance. 相似文献
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If we assume that the periodic brightness variations ina Kuiper Belt lightcurve are determined only by theiraspherical shapes and the observing geomtry, the fractionof detectable Kuiper Belt lightcurves and the lightcurveamplitude distribution can be used to constrain the shapesof Kuiper Belt objects. The results indicate that most KuiperBelt objects (～85%) have shapes that are close to spherical(a/b ≤ 1.5$), but there is a small but significant fraction(～12%) possessing highly aspherical shapes (a/b ≥ 1.7).The distribution cannot be well fitted by a gaussian and is betterapproximated by a power law. 相似文献
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Franciane de Carvalho Gomes Jos Marcus Godoy Maria Luiza D.P. Godoy Zenildo Lara de Carvalho Ricardo Tadeu Lopes Joan Albert Sanchez-Cabeza Luiz Drude de Lacerda Julio Cesar Wasserman 《Marine pollution bulletin》2009,59(4-7):123
Three sediment cores were sampled at Sepetiba Bay and four cores at Ribeira Bay, Rio de Janeiro State, Brazil. Sediment accumulation rates and chronologies were obtained from 210Pb activity-depth profiles. Sediment accumulation rates in Ribera Bay ranged from 1.2 mm y−1 in the inner bay to 2.6 mm y−1 close to its entrance. In Sepetiba Bay two sediment accumulation rates were observed: a lower rate of 0.35 cm y−1 before the 1960s and 0.76 cm y−1 since then. The cause of this change is due to the construction of the Santa Cecília impoundment (1955) that brings water from the Paraíba do Sul Basin into the Guandu River, which increased its flow from the original 20 m3 s−1 to 160 m3 s−1. Concentration of 44 elements was obtained by ICP-MS after total dissolution of samples from two selected cores. The relative differences between the concentrations of crustal elements, such as Al, Fe and Ti are only about 20% (p < 0.05). Cd and Zn are 15 and four times larger in Sepetiba Bay than in Ribeira Bay, respectively. Other major and minor elements show lower statistically significant differences. The enrichment factors and metal inventories show that Sepetiba Bay can be considered polluted with Bi, Cd, Cr, Cu, Sb and Zn. Particularly, Cd and Zn present concentrations three and four times higher than the Brazilian existing limits. 相似文献
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Along contrasting environmental conditions (e.g., degree of trace metal contamination and mangrove forest structural development), sediments of Laguncularia racemosa-dominated mangrove stands in Guanabara Bay (SE Brazil) presented a trend of trace metal accumulation in forms with low potential of remobilization and biotic uptake. Concurrently, a relatively low transfer of sediment-bound metals to L. racemosa leaves was observed, which may moderate the metal export from the forests via leaf litter transport and the metal availability to enter in food chains based on leaf consumption. 相似文献
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V. M. Reshetnyk Yu. V. Skorov P. Lacerda P. Hartogh L. Rezac 《Solar System Research》2018,52(3):266-281
We consider the estimates of the main forces acting on dust particles near a cometary nucleus. On the basis of these estimates, the motion of dust particles of different structure and mass is analyzed. We consider the following forces: (1) the cometary nucleus gravity, (2) the solar radiation pressure, and (3) the drag on dust particles by a flow of gas produced in the sublimation of cometary ice. These forces are important for modeling the motion of dust particles relative to the cometary nucleus and may substantially influence the dust transfer over its surface. In the simulations, solid silicate spheres and homogeneous ballistic aggregates are used as model particles. Moreover, we propose a technique to build hierarchic aggregates—a new model of quasi-spherical porous particles. A hierarchic type of aggregates makes it possible to model rather large dust particles, up to a millimeter in size and larger, while no important requirements for computer resources are imposed. We have shown that the properties of such particles differ from those of classical porous ballistic aggregates, which are usually considered in the cometary physics problems, and considering the microscopic structure of particles is of crucial significance for the analysis of the observational data. With the described models, we study the dust dynamics near the nucleus of comet 67P/Churyumov–Gerasimenko at an early stage of the Rosetta probe observations when the comet was approximately at 3.2 AU from the Sun. The interrelations between the main forces acting on dust aggregates at difference distances from the nucleus have been obtained. The dependence of the velocity of dust aggregates on their mass has been found. The numerical modeling results and the data of spaceborne observations with the Grain Impact Analyzer and Dust Accumulator (GIADA) and the Cometary Secondary Ion Mass Analyzer (COSIMA) onboard the Rosetta probe are compared at a quantitative level. 相似文献