The influence of particle size of the dispersed mineral fraction on the settlement of marine and estuarine muds

Considering the sedimentation behaviour of suspensions, four states of different sediment-water mixtures are usually distinguished, these being (in increasing concentration) dilute suspensions, concentrated suspensions, fluid muds and mud deposits. As the concentration values delimiting these states vary considerably from one cohesive sediment to another, a procedure for their quantitative determination is proposed here. This paper deals mainly with the effects of hindered settling in interacting concentrated suspensions and of sedimentation in fluid muds. In contrast to the settling velocity of individual particles in a dilute suspension, which must be studied statistically as a stochastic variable denoted W, hindered settling and sedimentation velocities can be described by a scalar denoted V, as solids at any particular level of concentrated suspensions and fluid muds settle at the same velocity. On the basis of settlement tests carried out in this study and published data on organic-rich cohesive sediments, a concentration-dependent empirical law for a permeability coefficient (k) has been generated for cohesive sediments, using data from ten estuarine and nine marine environments, based on the one-dimensional Kynch theory of sedimentation. Based on the median diameters of the dispersed mineral fraction, the main provenances of the sediments are: fine to very fine clay from tropical marine/estuarine environments, medium clay to very fine silt from estuaries in western France, and fine to coarse silt from marine (harbour) environments in the Normandy region of France. A general trend for the influence of the grain size of the mineral fraction on the permeability coefficient has been established. It is demonstrated that the concentrations delimiting the different states of sediment-water mixtures can also be related to the grain size of the mineral fraction. Thus, hindered settling and sedimentation processes of muds, similar to the marine and estuarine cohesive sediments considered in this paper, can be studied as generic problems parameterized through a defined median diameter of the dispersed mineral fraction. Results for Loire estuary sediments are presented separately, based on specific tests to analyse the influence of experimental conditions on settlement. Moreover, the concentration values delimiting the different sediment-water mixture states have been largely established for this estuary.     

The active fault system of SW Alps

Historical and active seismicity in the south-western Alps (France and Italy) shows the recurrence of relatively high-magnitude earthquakes (M ≥ 5.8), like the one that recently affected the Italian Apennine range (M = 6.3 on the 30th March 2009). However, up-to-date detailed mapping of the active fault network has been poorly established. The evaluation of seismological hazard in particular in the highly populated French and Italian coastal region cannot be done without this. Here, we present a detailed study of the main active fault system, based on geological observations along the south-western flank of the Alpine arc. This N140° right-lateral strike-slip active fault system runs along the edge of the Argentera-Mercantour range and can be followed down to the Mediterranean Sea. It is evidenced by (1) Holocene offsets of glacial geomorphology witnessing ongoing fault activity since 10 ka, (2) widespread recent (10–20 Ma) pseudotachylytes featuring long term activity of the faults, (3) active landslides along the main fault zone, (4) geothermal anomalies (hot springs) emerging in the active faults, (5) ongoing low-magnitude seismic activity and (6) localization of the main historical events. In the light of our investigations, we propose a new tectonic pattern for the active fault system in the south-western Alps.     

Coversand and Pleistocene palaeosols in the Landes region,southwestern France

New sections in the coversand of the Landes region, southwestern France, show at least two main depositional phases corresponding to the Upper Pleniglacial and the Lateglacial, which are separated by palaeosols. The lower palaeosol, a gleyic to histic cryosol overlying a net of sand wedges and dated to ca. 23 ¹⁴C ka BP, testifies to a short occurrence of permafrost. Impeded drainage due to the frozen subsoil is assumed to be the main factor involved in lowered aeolian transport and soil formation. Pollen analysis indicates a shrub tundra‐type environment. The overlying coversand unit is associated with small transverse ridges or sheet‐like deposits, and corresponds to the maximal extension of the sands, Upper Pleniglacial in age. An incipient podzol developed on the dunes under a boreal pine forest, and has been dated to 11.5–12 ¹⁴C ka BP, i.e. to the Allerød period. This has been buried by the second coversand unit during the Younger Dryas, typified by abundant denivation features and root imprints. Although preliminary, the chronology of sand deposition in the Landes region appears thus to be roughly similar to that found for the other European coversands, showing that all were the result of similar western European climatic changes, i.e. repeated episodes of increasing aridity related to the Upper Pleniglacial and the Younger Dryas episode. Copyright © 2008 John Wiley & Sons, Ltd.     

Can Jupiters be found by monitoring Galactic bulge microlensing events from northern sites?

In 1998 the EXPORT team monitored microlensing event light curves using a charge-coupled device (CCD) camera on the IAC 0.8-m telescope on Tenerife to evaluate the prospect of using northern telescopes to find microlens anomalies that reveal planets orbiting the lens stars. The high airmass and more limited time available for observations of Galactic bulge sources make a northern site less favourable for microlensing planet searches. However, there are potentially a large number of northern 1-m class telescopes that could devote a few hours per night to monitor ongoing microlensing events. Our IAC observations indicate that accuracies sufficient to detect planets can be achieved despite the higher airmass.     

A study of the factors and processes involved in the sedimentation of Tarbela reservoir,Pakistan

This study concerns Tarbela dam reservoir which is a major resource of Pakistan in terms of electricity generation and irrigation supplies. Rapid filling of reservoir due to sediment transported and deposited by Indus River is described and analysed in this article. Causes of sediment deposition and their impact on dam’s function and life are studied. The main characteristic parameters of the site are presented. The morphological evolution of the reservoir bed induced by dam construction, between the year 1974 when the River bed was in its original state before dam construction, and the situation of the river bed after the deposition of the sediment in the year 2006, is examined. A numerical model developed in this study (Khawaja and Sanchez in Tarbela dam: a numerical model for sediment management in the reservoir; coastal and maritime mediterranean conference, 1st edn, Hammamet, pp 111–114, 2009) allows obtaining the above-stated provisional evolution of the reservoir over the coming years. Finally, five potential remedial solutions are thoroughly discussed.     

Easily retrievable objects among the NEO population

Asteroids and comets are of strategic importance for science in an effort to understand the formation, evolution and composition of the Solar System. Near-Earth Objects (NEOs) are of particular interest because of their accessibility from Earth, but also because of their speculated wealth of material resources. The exploitation of these resources has long been discussed as a means to lower the cost of future space endeavours. In this paper, we consider the currently known NEO population and define a family of so-called Easily Retrievable Objects (EROs), objects that can be transported from accessible heliocentric orbits into the Earth’s neighbourhood at affordable costs. The asteroid retrieval transfers are sought from the continuum of low energy transfers enabled by the dynamics of invariant manifolds; specifically, the retrieval transfers target planar, vertical Lyapunov and halo orbit families associated with the collinear equilibrium points of the Sun–Earth Circular Restricted Three Body problem. The judicious use of these dynamical features provides the best opportunity to find extremely low energy Earth transfers for asteroid material. A catalogue of asteroid retrieval candidates is then presented. Despite the highly incomplete census of very small asteroids, the ERO catalogue can already be populated with 12 different objects retrievable with less than 500 m/s of $\Delta v$ Δ v . Moreover, the approach proposed represents a robust search and ranking methodology for future retrieval candidates that can be automatically applied to the growing survey of NEOs.     

Uptake and Accumulation of Arsenic in Different Organs of Carrot Irrigated with As‐Rich Water

Irrigation with arsenic (As)‐rich water in agricultural soil may increase high levels of As in crops and cause food chain contamination. In this study, a greenhouse experiment was established using Spanish agricultural soil (Valladolid and Segovia provinces), that are extensively cultivated for carrot plant, to investigate the process of As uptake, bioaccumulation, and translocation of As from root to shoot and leaves in carrot plant. Arsenic concentrations in different organs of carrot plant, rhizosphere soil, and soil solutions were determined by hydride generation atomic absorption spectrometry (AAS). High concentrations of As in irrigation water, and the alkaline and sandy character of this soil enhanced As uptake in carrot plants indicating the potential health risk from consumption of carrots cultivated in these areas. Bioaccumulation of As into the leaves and roots increased with increase of As concentration in irrigation water. Both roots and leaves demonstrated a higher accumulation rate of As at an As concentration of 41 than 131 µg L^?1 in the soil solution. The ratios of As_root/As_leaves showed no statistically significant differences for the different irrigation treatments, and had an average value of 0.36 indicating the high magnitude of As translocation from roots to leaves in carrot plants. The leaves of carrots had a higher affinity for As than roots did. The correlation between As uptake by leaves or roots of carrots and the soluble As in rhizosphere soil did not demonstrate a linear or a plateau curve, indicating a slow but continuous constant As absorption which could be prolonged over time with high potential environmental risks.     

Phase reddening on near-Earth asteroids: Implications for mineralogical analysis,space weathering and taxonomic classification

Phase reddening is an effect that produces an increase of the spectral slope and variations in the strength of the absorption bands as the phase angle increases. In order to understand its effect on spectroscopic observations of asteroids, we have analyzed the visible and near-infrared spectra (0.45–2.5 μm) of 12 near-Earth asteroids observed at different phase angles. All these asteroids are classified as either S-complex or Q-type asteroids. In addition, we have acquired laboratory spectra of three different types of ordinary chondrites at phase angles ranging from 13° to 120°. We have found that both, asteroid and meteorite spectra show an increase in band depths with increasing phase angle. In the case of the asteroids the Band I depth increases in the range of ～2° < g < 70° and the Band II depth increases in the range of ～2° < g < 55°. Using this information we have derived equations that can be used to correct the effect of phase reddening in the band depths. Of the three meteorite samples, the (olivine-rich) LL6 ordinary chondrite is the most affected by phase reddening. The studied ordinary chondrites have their maximum spectral contrast of Band I depths at a phase angle of ～60°, followed by a decrease between 60° and 120° phase angle. The Band II depths of these samples have their maximum spectral contrast at phase angles of 30–60° which then gradually decreases to 120° phase angle. The spectral slope of the ordinary chondrites spectra shows a significant increase with increasing phase angle for g > 30°. Variations in band centers and band area ratio (BAR) values were also found, however they seems to have no significant impact on the mineralogical analysis. Our study showed that the increase in spectral slope caused by phase reddening is comparable to certain degree of space weathering. In particular, an increase in phase angle in the range of 30–120° will produce a reddening of the reflectance spectra equivalent to exposure times of ～0.1 × 10⁶–1.3 × 10⁶ years at about 1 AU from the Sun. This increase in spectral slope due to phase reddening is also comparable to the effects caused by the addition of different fractions of SMFe. Furthermore, we found that under some circumstances phase reddening could lead to an ambiguous taxonomic classification of asteroids.