Natural acid rock drainage associated with black shale in the Yukon Territory,Canada

Investigated herein are water and sediment geochemistry, and metal attenuation processes associated with natural acid rock drainage originating from black shale formations in the Macmillan Pass area, Clear Lake prospect and Engineer Creek by the Dempster Highway in the Yukon Territory, Canada. The most metalliferous water having pH 3.0, 150 mg/L Zn, 39 mg/L Ni, 2.8 mg/L Cu and 9.1 mg/L As was found in a tributary stream of Engineer Creek with no known mineral deposits occurring in the vicinity. For all three study areas, the water and sediment geochemistry is significantly affected by the local lithology and prevailing metal attenuation processes. Despite their anomalous acidity and metal contents, the natural acid streams contribute only a small fraction of the contaminant loadings to the major water courses because of their low flows. Dilution, neutralization, sorption and co-precipitation are identified as the major mechanisms attenuating aqueous transport of potentially deleterious metals. However, microbial mediation in metal attenuation is also evident in low-flow systems. The wide variation of water and sediment geochemistry along a flow path renders the establishment of background metal values difficult. In assessing environmental impacts, it may be more practical to consider metal loadings on a watershed scale than to rely on a comparison with operationally defined background concentrations.     

A near infrared speckle imaging study of T Tauri Stars

The results of a speckle imaging survey of T Tauri stars suggest that most, if not all, young low mass stars have companions. Repeated observations of these young binary stars have revealed orbital motion in the closest pairs (0.3), proving that these systems are indeed gravitationally bound and providing the basis for mass estimates in the upcoming years. These mass estimates are necessary to distinguish between the various binary star formation mechanisms that have been proposed to date.     

Porous rock deformation and fluid flow — numerical FE-simulation of the coupled system

Elastic compaction is characterized by the interaction of the deforming rock matrix and fluid flow for a given environment. The basic theory was developed by Biot. However, Biot's theory has to be extended in order to account for the effective pressure dependent, tensorial nature of permeability.Fluid flow in a deformable rock matrix is controlled by a nonlinear, time dependent permeability. Permeability is defined as a function of the rock matrix pressure as well as the excess fluid pressure. A two-dimensional Finite Element approximation of this process has been developed. Based on numerical experiments the consequences of structural and physical heterogeneities of permeability are investigated and discussed.The numerical calculations emphasize the temporal and spatial significance of permeability variations due to the interaction of rock matrix deformation and fluid flow. The influence of these permeability changes on transient fluid flow cannot be understood without the tool of numerical simulation.

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Zusammenfassung Das Phänomen der elastischen Kompaktion ist durch die Wechselwirkung zwischen deformierender Gesteinsmatrix und Flüssigkeitsbewegung bei entsprechenden mechanischen Bedingungen charakterisiert. Die grundlegende Theorie hierzu wurde von Biot entwickelt. Um dem druckabhängigen, tensoriellen Charakter der Permeabilität für eine geeignete Beschreibung der Wechselwirkung gerecht zu werden, ist eine Erweiterung der Biot'schen Theorie notwendig.In einem zweidimensionalen Finite Elemente Programm wird der Fließvorgang durch eine nichtlineare, zeitabhängige Permeabilität bestimmt, die eine Funktion von Gesteinsdruck und Fluidüberdruck ist. Anhand von Modellexperimenten wird der Einfluß struktureller und physikalischer Permeabilitätsvariationen untersucht und diskutiert. Die numerischen Rechnungen machen deutlich, daß Permeabilitätsänderungen durch die Wechselwirkung von Gesteinsmatrix und Fluid sowohl von räumlicher als auch von zeitlicher Signifikanz sind. Ihr Einfluß auf die entsprechenden Fließvorgänge ist allgemein kaum ohne das Hilfsmittel numerischer Simulation richtig erfaßbar.

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Résumé Le phénomène de compaction élastique est caractérisé par l'interaction entre la matrice de la roche et le flux de fluide, dans des conditions mécaniques déterminées. La théorie en a été développée par Biot. Un développement de cette théorie s'avère nécessaire si on veut tenir compte de la nature tensorielle de la perméabilité.Le flux de fluide dans une matière rocheuse déformable a été représenté en termes de perméabilité non linéaire fonction du temps dans un programme d'éléments finis bidimensionnel. La perméabilité est une fonction de la pression de la roche et de la surpression du fluide.Les effets de variations structurelles et physiques de la perméabilité sont étudiées et discutées à partir d'expériences modélisées. Les calculs numériques montrent la signification spatiale et temporelle des variations de perméabilité. Les processus non linéaires ainsi mis en évidence n'auraient guère pu l'être sans l'emploi de simulations quantitatives.

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Development of ground-testable phase fresnel lenses in silicon

Diffractive optics, such as Phase Fresnel Lenses (PFL's), offer the potential to achieve excellent imaging performance in the x-ray and gamma-ray photon regimes. In principle, the angular resolution obtained with these devices can be diffraction limited. Furthermore, improvements in signal sensitivity can be achieved as virtually the entire flux incident on a lens can be concentrated onto a small detector area. In order to verify experimentally the imaging performance, we have fabricated PFL's in silicon using gray-scale lithography to produce the required Fresnel profile. These devices are to be evaluated in the recently constructed 600-meter x-ray interferometry testbed at NASA/GSFC. Profile measurements of the Fresnel structures in fabricated PFL's have been performed and have been used to obtain initial characterization of the expected PFL imaging efficiencies.     

Southern Hemisphere Westerly Winds have modulated the formation of laminations in sediments in Lago Fagnano (Tierra del Fuego,Argentina) over the past 6.3 ka

Tierra del Fuego in Argentina is a unique location to examine past Holocene wind variability since it intersects the core of the Southern Hemisphere Westerly Winds (SHWW). The SHWW are the most powerful prevailing winds on Earth. Their variation plays a role in regulating atmospheric CO₂ levels and rainfall amounts and distribution, both today and in the past. We obtained a piston core (LF06-PC8) from Bahía Grande, a protected sub-basin at the southern margin of Lago Fagnano, the largest lake in Tierra del Fuego. This article focuses on the uppermost 185 cm of this core, corresponding to laminated sediment from the last ~6.3 ka. Laminations consist of millimetre-scale paired dark and light layers. Previous studies and new geochemical analysis show that the dark and light layers are characterized by differing concentrations of Mn and Fe. We attribute the distribution of Mn and Fe to episodic hypolimnic oxic–anoxic variations. The age model suggests an approximately bidecadal timescale for the formation of each layer pair. We propose a new model of these redox changes with the SHWW variations. The most likely phenomenon to produce complete water-column mixing is thermobaric instability, which occurs in colder winters with low-intensity SHWW (El Niño-like conditions). In contrast, windier winters are characterized by higher temperatures and reduced mixing in the water column, facilitating a decline in oxygen concentration. Laminations, and the inferred presence of periodic hypolimnion redox changes, are common features of the past ~6.3 ka. Geochemical proxy variability is compatible with an intensification of El Niño/Southern Oscillation activity during the past ~2 ka.     

Dark Matter and Chemical Evolution of dSph Galaxies

We calculate chemical evolution models for 4 dwarf spheroidal satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii for the gas heated by supernovae to be retained until the observed stellar population has formed. Systems showing extended star formation histories however (Carina and Leo I), are consistent with the idea that their tidally limited dark haloes provide the necessary gravitational potential wells to retain their gas. This revised version was published online in September 2006 with corrections to the Cover Date.