The age and palæoenvironmental significance of the Kalahari Sands in western Zimbabwe: a thermoluminescence reconnaissance study

Thermoluminescence (TL) dating, has been applied to 10 samples from the æolian Kalahari Sands of western Zimbabwe using the total bleach (additive dose) and regeneration techniques on coarse-grain quartz separates. The results suggest that the main phases of sand accumulation occurred between 10 and 96 ka. This Upper Pleistocene age is consistent with current opinions and argues against an important period of Holocene æolian activity. The oldest sands were found in the Victoria Falls area where they overlie a ferricrete horizon. Based on our age determinations, we were able to constrain the age of this horizon to between 96±8 and 160±23 ka, which approximately coincides with the Last Integlaciation in the high latitudes. The youngest ages were obtained for sands from the southeastern part of the Hwange dune field but our limited sampling permits assigning only a minimum age of ca 20 ka to the ferricrete horizon observed in this area. Thus, we cannot ascertain if this ferricrete horizon is isochronous with the one at Victoria Falls or if it developed during a younger humid phase.Dune building activity most probably occurred in recurrent cycles of aridity interspersed with periods during which æolian activity was limited or non-existent. Unfortunately, besides the ferricrete horizons, no other stratigraphical evidence of climatic fluctuation has been observed in the study area. However, it is possible that palæosols have been entirely eroded before the subsequent deposition of new sediment, resulting in an incomplete stratigraphical record. The possibilities and limitations of luminescence dating for elucidating phases of climatic fluctuation in such situations are discussed briefly. In addition, we noticed serious discrepancies between the results yielded by the different techniques that were used for determining the dose rate. Hence, further work is needed to validate the accuracy of our dose rate measurements that may cause our TL ages to be about 30% too young.     

The H− equilibrium using coupled rate equations for H−, H,H+, H2, and H2 +

We formulate rate equations for the reaction network coupling H, H^–, H⁺, H₂, and H₂ ⁺. We attempt to systematize the notation, and to write the equations in a form suitable for modern computational methods of handling the coupled rate equations and radiative transfer equations, for both dynamical and static atmospheres. We have accounted for more processes than are generally considered in most current work; some of these may have an impact on the equilibrium of H^– (hence its opacity) and on charge conservation (hence the proton density) in the atmospheres of solar-type stars.Operated by the Association of Universities for Research in Astronomy, Inc. under Contract AST 78-17292 with the National Science Foundation.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Europium redox equilibria in aqueous solution

Thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous Eu²⁺ and Eu³⁺ over a wide range of temperatures and pressures. At low temperatures, near earth surface conditions, the aqueous geochemistry of europium should be dominated by the trivalent state, except possibly in the most reducing, alkaline pore waters of anoxic marine sediments. However, at temperatures greater than about 250°C and elevated pressures, divalent europium should predominate. Even with significant amounts of complexing, trivalent europium is not stable at elevated temperatures and pressures. Consequently, under most hydrothermal and metamorphic conditions, europium in aqueous solution should be divalent. At intermediate temperatures, around 100°C, significant activities of both Eu²⁺ and Eu³⁺, and related complexes, can occur in aqueous solutions, depending on the oxidation state and the pH of the solutions, and the activities of potential ligands such as sulfate, carbonate and chloride. The predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high-temperature barites of hydrothermal and metamorphic origin reported by Guichard et al. [5] and with the observed depletion of europium as a result of high-temperature sericitization of feldspar-bearing assemblages discovered by Alderton et al. [1]. It is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high-temperature hydrothermal alteration processes, such as in the mid-oceanic ridge systems, because of the expected stability of divalent europium during these processes.     

Increasing Aridity is Enhancing Silver Fir Abies Alba Mill.) Water Stress in its South-Western Distribution Limit

Tree populations located at the geographical distribution limit of the species may provide valuable information about the response of tree growth to climate warming across climatic gradients. Dendroclimatic information was extracted from a network of 10 silver-fir (Abies alba) populations in the south-western distribution limit of the species (Pyrenees, NE Iberian Peninsula). Ring-width chronologies were built for five stands sampled in mesic sites from the Main Range in the Pyrenees, and for five forests located in the southern Peripheral Ranges where summer drought is more pronounced. The radial growth of silver-fir in this region is constrained by water stress during the summer previous to growth, as suggested by the negative relationship with previous September temperature and, to a lesser degree, by a positive relationship with previous end of summer precipitation. Climatic data showed a warming trend since the 1970s across the Pyrenees, with more severe summer droughts. The recent warming changed the climate-growth relationships, causing higher growth synchrony among sites, and a higher year-to-year growth variation, especially in the southernmost forests. Moving-interval response functions suggested an increasing water-stress effect on radial growth during the last half of the 20th century. The growth period under water stress has extended from summer up to early autumn. Forests located in the southern Peripheral Ranges experienced a more intense water stress, as seen in a shift of their response to precipitation and temperature. The Main-Range sites mainly showed a response to warming. The intensification of water-stress during the late 20th century might affect the future growth performance of the highly-fragmented A. alba populations in the southwestern distribution limit of the species.     

Standard states for the activities of mineral surface sites and species

A standard state commonly used for the activities of surface sites and surface species is the hypothetical 1.0 Molar standard state, which is implied by the use of molarity-based equilibrium constants. An undesirable practical consequence is that the magnitudes of such equilibrium constants are directly dependent on properties of the solid sorbent such as the site density and surface area. For reactions forming binuclear complexes, the magnitudes of the equilibrium constants even depend on the amount of solid. Although widely used, such equilibrium constants cannot be directly compared with each other without correction for differences in the properties or the amount of the solid. In the present study, new more general and useful standard states are proposed, leading to equilibrium constants independent of the surface area, site density, and the amount of the solid sorbent. Analytical relationships between the old and the new standard states enable conversion of equilibrium constants from one standard state to the other. These results have implications for several different types of surface complexation studies, including studies that correlate and compare equilibrium adsorption constants for different solids, sensitivity-analysis studies of the fitting of surface charge data as functions of pH and ionic strength, and studies employing correlations involving aqueous equilibrium constants for the purpose of predicting equilibrium constants for surface reactions.     

Modeling level selection for seismic analysis of concrete‐filled steel tube/moment‐resisting frames by using fragility curves

The seismic behavior of plane moment‐resisting frames (MRFs) consisting of I steel beams and concrete‐filled steel tube (CFT) columns is investigated in this study. More specifically, the effect of modeling details of each individual component of CFT‐MRFs, such as the composite CFT columns, the beam‐column connections, the panel zones, and the steel I beams on their seismic behavior, is studied through comparisons against available experimental results. Then, fragility curves are constructed for three typical CFT‐MRFs, designed according to European codes, for various levels of modeling sophistication through nonlinear time‐history analyses. On the basis of these fragility curves, one can select the appropriate modeling level of sophistication that can lead to the desired seismic behavior for a given seismic intensity. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical modeling of zero-offset laboratory data in a strong topographic environment: results for a spectral-element method and a discretized Kirchhoff integral method

Accurate simulation of seismic wave propagation in complex geological structures is of particular interest nowadays. However conventional methods may fail to simulate realistic wavefields in environments with great and rapid structural changes, due for instance to the presence of shadow zones, diffractions and/or edge effects. Different methods, developed to improve seismic modeling, are typically tested on synthetic configurations against analytical solutions for simple canonical problems or reference methods, or via direct comparison with real data acquired in situ. Such approaches have limitations, especially if the propagation occurs in a complex environment with strong-contrast reflectors and surface irregularities, as it can be difficult to determine the method which gives the best approximation of the “real” solution, or to interpret the results obtained without an a priori knowledge of the geologic environment. An alternative approach for seismics consists in comparing the synthetic data with high-quality data collected in laboratory experiments under controlled conditions for a known configuration. In contrast with numerical experiments, laboratory data possess many of the characteristics of field data, as real waves propagate through models with no numerical approximations. We thus present a comparison of laboratory-scaled measurements of 3D zero-offset wave reflection of broadband pulses from a strong topographic environment immersed in a water tank with numerical data simulated by means of a spectral-element method and a discretized Kirchhoff integral method. The results indicate a good quantitative fit in terms of time arrivals and acceptable fit in amplitudes for all datasets.