Laser combustion analysis of δS of sulfosalt minerals: determination of the fractionation systematics and some crystal-chemical considerations

Laser sulfur combustion in the presence of O₂ is the most commonly used method of in situ sulfur isotope analysis. Previous workers indicated that a small but reproducible fractionation of ³⁴S/³²S exists between the product SO₂ gas and the mineral. The magnitude of this fractionation varies with bond strength, as reflected in Gibbs free energy of formation at 298.15K. The correction factors are known for common sulfides and anhydrite but not, hitherto, for stibnite and the sulfosalt minerals, which are important constituents of many classes of ore deposits. We present the correction factors for the following chemically and crystallographically well-characterized minerals: stibnite (weighted mean = −1.2‰), bournonite (+0.6‰), tetrahedrite (+1.3‰), and boulangerite (+1.4‰). The Gibbs free energies of formation of these phases have been approximated by the sulfide summation method, and the correlation of ΔG₂₉₈⁰ with the correction factor for the sulfosalts fits well with the trend previously established for simple sulfides. There is an excellent correlation between the fractionation factor and mineral composition, a parameter that does result in bond strength variations (e.g., mol fraction of PbS in sulfosalts), allowing estimation of the correction factors for simple intermediate compositions. Finally, bond strength also varies with variation in interatomic distances, and we have, therefore, investigated the behaviour of stibnite, a strongly anisotropic mineral. Our results indicate that there is a significant variation in fractionation factor, depending on crystal orientation. The fractionation factor along the prismatic b-axis, which displays the strongest chemical bonds (as monitored by the shortest bond lengths), is more negative (−1.7‰) than along the other crystallographic directions (−0.7 to −1.0‰), which is in full agreement with theoretical predictions. We demonstrate the application of the technique in unravelling source- and process-related sulfur isotope systematics in two hydrothermal vein systems in the classic mining area of the NE Rhenish Massif, both studies requiring resolution beyond the scale of conventional sulfur isotope analysis.     

Interstitial flow through preferential flow paths in the hyporheic zone of the Oberer Seebach,Austria

We investigated interstitial flow velocities in the Oberer Seebach, Austria, with NaCl tracer injections at a sediment depth of 30 cm to estimate the hydraulic conditions experienced by invertebrates inhabiting the hyporheic zone. Flow velocity measured with tracers is taken as travel time of the water along a straight line between injection and sampling points, although the water flows around sediment particles, and thus travels a somewhat longer distance. From sections of stream sediment in which the interstitial spaces were replaced by concrete, we estimated that this difference amounts, on average, to 27% and used this factor to correct the results of our velocity measurements. Corrected interstitial water velocities ranged from 0.01 to 1.32 cm s^-1 and were independent of surface discharge. We also studied spatial flow patterns in the bed sediments with long-term tracer injections. The three-dimensional distribution of tracer concentrations 24 hours after the start of the injection indicated that interstitial water preferentially flows in a complex network of areas of high hydraulic connectivity. Reynolds numbers for flow in the hyporheic pore space ranged from 0.1 to 489, implying that the flow environment varies from laminar up to the zone of transition to turbulent flow. Therefore, invertebrates may have a size-related active choice of areas where either friction drag or pressure drag predominates. The consequence of flow patterns, such as those observed in our study, is that small-scale variability of hydraulic conditions may be an important determinant of the patchy invertebrate distribution in bed sediments.     

Evaluating (and Improving) Estimates of the Solar Radial Magnetic Field Component from Line-of-Sight Magnetograms

Although for many solar physics problems the desirable or meaningful boundary is the radial component of the magnetic field \(B_{\mathrm {r}}\), the most readily available measurement is the component of the magnetic field along the line of sight to the observer, \(B_{\mathrm {los}}\). As this component is only equal to the radial component where the viewing angle is exactly zero, some approximation is required to estimate \(B_{\mathrm {r}}\) at all other observed locations. In this study, a common approximation known as the “\(\mu\)-correction”, which assumes all photospheric field to be radial, is compared to a method that invokes computing a potential field that matches the observed \(B_{\mathrm {los}}\), from which the potential field radial component, \(B_{\mathrm {r}}^{\mathrm {pot}}\) is recovered. We demonstrate that in regions that are truly dominated by a radially oriented field at the resolution of the data employed, the \(\mu\)-correction performs acceptably if not better than the potential-field approach. However, it is also shown that for any solar structure that includes horizontal fields, i.e. active regions, the potential-field method better recovers both the strength of the radial field and the location of magnetic neutral line.     

The effective stiffness of modern unreinforced masonry walls

Code design of unreinforced masonry (URM) buildings is based on elastic analysis, which requires as input parameter the effective stiffness of URM walls. Eurocode estimates the effective stiffness as 50% of the gross sectional elastic stiffness, but comparisons with experimental results have shown that this may not yield accurate predictions. In this paper, 79 shear‐compression tests of modern URM walls of different masonry typologies from the literature are investigated. It shows that both the initial and the effective stiffness increase with increasing axial load ratio and that the effective‐to‐initial stiffness ratios are approximately 75% rather than the stipulated 50%. An empirical relationship that estimates the E‐modulus as a function of the axial load and the masonry compressive strength is proposed, yielding better estimates of the elastic modulus than the provision in Eurocode 6, which calculates the E‐modulus as a multiple of the compressive strength. For computing the ratio of the effective to initial stiffness, a mechanics‐based formulation is built on a recently developed analytical model for the force‐displacement response of URM walls. The model attributes the loss in stiffness to diagonal cracking and brick crushing, both of which are taken into account using mechanical considerations. The obtained results of the effective‐to‐initial stiffness ratio agree well with the test data. A sensitivity analysis using the validated model shows that the ratio of effective‐to‐initial stiffness is for most axial load ratios and wall geometries around 75%. Therefore, a modification of the fixed ratio of effective‐to‐initial stiffness from 50% to 75% is suggested.     

Balmer absorption variability and the disc wind of V592 Cassiopeiae

We present results from time-series optical spectroscopy of the low-inclination, nova-like cataclysmic variable, V592 Cas. The data span the wavelength range from ∼4000 to 5000  Å, and include Balmer lines (Hβ to Hε) as well as He i and He ii . The Balmer lines are generally characterized by shallow absorption troughs with superimposed narrower central emission components. The absorption troughs are variable on time-scales of at least ∼20 min, but the fluctuations are asymmetric such that they are more dominant on the blueward side of the profile out to ∼−2000 km s⁻¹. Fourier analysis reveals modulation periods for emission radial velocities and absorption changes of ∼0.114 and 0.103 d; we support earlier suggestions that the latter is likely to be 1-day alias of the former, which is essentially the system orbital period. However, there is a phase lag of ∼0.3 between absorption and emission changes. The Balmer absorption changes are compared to the predicted behaviour caused by the (stream overflowing) SW Sex phenomenon. There are several discrepancies in this scenario, including the highly asymmetric nature of blueward changes, the velocity amplitudes, and the phase relation to the emission changes. The role of a disc wind in V592 Cas is also discussed; in particular, with this data set, we cannot rule in or rule out the possibility that axisymmetry of the outflow is broken because it is seated on a warped/tilted inner disc, which is implied by previous evidence for negative superhumps in this system.     

Multi-Frequency Studies of Intrinsic Intraday Variability

Intraday variability is detected in Blazars throughout the electromagnetic spectrum. Only radio emission is affected by interstellar scintillation. The characteristics of variability observed at different frequencies are often similar and suggest that intrinsic variations also contribute to the changes detected in the radio wavelength regime. This in turn implies very high brightness temperatures. Unambiguous signatures of scintillation and of intrinsic changes have been identified. The properties of intrinsic variations are reviewed to illustrate their potential contribution to variability in the radio domain. Such intrinsic changes need to be discriminated against when studying scintillation in quasars and BL Lac objects. Observations of scintillating sources and of sources with significant intrinsic IDV suggest interesting modifications to the standard paradigm of AGN. This revised version was published online in July 2006 with corrections to the Cover Date.     

Proposal for selection of control,preventive and rehabilitation measures based on degradation land level and geoindicators in the fortaleza metropolitan region,State of Ceará, Brazil

Preventive, controlling, rehabilitative and corrective measures to improve the environmental quality of the hydrographic basins of the Fortaleza metropolitan region in the state of Ceará (Brazil) are presented based on geological and geotechnical data, land degradation rates and sources, and other geoindicators. The region is affected by coastal and stream erosion, gravitational mass movement, dunes, silting, and water pollution, sanitary landfills in inappropriate sites, decommissioned and abandoned aggregate mining, and swamp areas. Geoindicators were used to classify each of the 12 hydrographic basins as possessing low (4), intermediate (3), or high (5) degradation levels. A group of 59 environmental management measures related to ecological, bioengineering, mechanical, policy and administrative, agricultural and clean-up methods was selected. From this general group a set of 31 measures was chosen considering goals, efficacy for specific environmental problems, suitability for large or small areas, behavior under different annual climatic conditions, environmental problems of the region, geological and geotechnical limitations and degradation level of the basins. A set of the more suitable geoindicators was selected for monitoring efficiency of the measures for each basin.     

Post-storm mid-latitude green aurora and electron precipitation

Unusual auroral emission at mid-latitudes, showing nearly exclusively the green oxygen line (557.7 nm) and occurring during the early part of the recovery phase after strong magnetic storms is described. The emission has a life-time of up to several hours, consists of cloud-like patches and appears quite isolated at medium latitudes with no simultaneous aurorae at higher latitudes. The name “post-storm mid-latitude green aurora” is proposed for this emission. For the event observed during the night of 29–30 August 1978, additional ionospheric measurements from heights below the mid-latitude aurora ($\text{?}{}_{\min }$, A3-LF data) are available from nearby observations. Our investigation shows that the emission was observed just at the beginning of a post-storm effect (PSE) in ionospheric absorption. The optical and absorption data have been used to extract information on precipitating high-energy electrons, assumed to be the cause of both the optical emisson and the excessive absorption. During the night in question precipitating electrons with fluxes above the quiet-time level and energies upto at least 200 keV were found in a region extended in latitude (2.7 < L < 3.3) and probably even more extended in longitude. Latitudinally narrow bands, elongated along shells of constant L, with extremely high fluxes of 10–20keV electrons (according to our estimates at least 5. 10⁷ el cm^?2s^?1) were embedded within this region.     

Is there an Insect Signal for the "Little Ice Age"?

The downturn in temperature in the late medieval period is likely to have had significant impact upon insect distribution. Despite the amount of study fossil insect faunas have been afforded, there is presently little convincing evidence of climatic, rather than human impact upon faunas during the "Little Ice Age". This probably reflects as much the paucity of suitable sites, as the overarching scale of habitat destruction by Man.