Detecting recent changes in the demographic parameters of drosophilid populations from western and central Africa

Previous genetic studies showing evidence of past demographic changes in African drosophilids suggested that these populations had strongly responded to Quaternary climate changes. We surveyed nine species of Zaprionus, a drosophilid genus mostly present in Africa, in forests located between southern Senegal and Gabon. The mitochondrial COI gene showed contrasted levels of sequence variation across species. Populations of the only cosmopolitan species of the genus, Z. indianus, and of its closely related sibling species, Z. africanus, are highly polymorphic and appear to have undergone a continuous population expansion beginning about 130,000 years ago. Five less variable species probably underwent a population expansion beginning only about 20,000–30,000 years ago. One of them, Z. taronus, was significantly structured between forest blocks. The last two species were nearly monomorphic, probably due to infection by Wolbachia. These results are similar to those obtained in three species from the melanogaster subgroup, and may be typical of the responses of African drosophilid populations to glacial cycles.     

Syn-thrust deformation across a transverse zone: the Grem–Vedra fault system (central Southern Alps, N Italy)

The lateral continuity of the E?CW trending thrust sheets developed within the Lower to Middle Triassic cover of the central Southern Alps (Orobic belt) is disturbed by the occurrence of several N?CS trending transverse zones, such as the poorly known Grem?CVedra Transverse Zone (GVTZ). The GVTZ developed during the emplacement of the up to six S-verging thrust sheets consisting of Lower to Middle Triassic units, occurring immediately south of the Orobic Anticlines. The transverse zone, active during thrust emplacement related to the early Alpine compressions which pre-date the Adamello intrusion, includes three major vertical shear zones, the Grem, Pezzel and Zuccone faults. The major structure of the transverse zone is the dextral Grem fault, forming a deep lateral ramp between thrust sheets 3 and 5. A similar evolution also occurred along the Zuccone and Pezzel faults, which show a left-lateral displacement of syn-thrust folds. The Grem fault was later reactivated as an oblique tear fault during the emplacement of the Orobic Anticlines, due to back-thrusting along out-of-sequence thrust surfaces (Clusone fault). Transpressional deformations along the fault zone are recorded by the rotation of major syn-thrust folds, which also suggest a horizontal offset close to 0.5?km. Records of the first stage of evolution of the Grem fault are better preserved along its northern segment, and structural relationships suggest that it propagated southward and downward in the growing thrust stack. The study of the meso and megascopic structures developed along the GVTZ constrains the evolution of the transverse zone, illustrating the complex deformational phenomena occurring in a transpressional regime. The GVTZ probably reflects the existence of pre-existing tectonic lineaments with a similar orientation. Evidence of pre-existing structures are not preserved in the exposed units, nevertheless the N?CS extensional fault systems that characterize the Norian to Jurassic rifting history of the Lombardian basin are valid candidates.     

A model for oxygen and sulfur isotope fractionation in sulfate during bacterial sulfate reduction processes

We present a model of bacterial sulfate reduction that includes equations describing the fractionation relationship between the sulfur and the oxygen isotope composition of residual sulfate (δ³⁴S_{SO4_residual}, δ¹⁸O_{SO4_residual}) and the amount of residual sulfate. The model is based exclusively on oxygen isotope exchange between cell-internal sulfur compounds and ambient water as the dominating mechanism controlling oxygen isotope fractionation processes. We show that our model explains δ³⁴S_{SO4_residual} vs. δ¹⁸O_{SO4_residual} patterns observed from natural environments and from laboratory experiments, whereas other models, favoring kinetic isotope fractionation processes as dominant process, fail to explain many (but not all) observed δ³⁴S_{SO4_residual} vs. δ¹⁸O_{SO4_residual} patterns. Moreover, we show that a “typical” δ³⁴S_{SO4_residual} vs. δ¹⁸O_{SO4_residual} slope does not exist. We postulate that measurements of δ³⁴S_{SO4_residual} and δ¹⁸O_{SO4_residual} can be used as a tool to determine cell-specific sulfate reduction rates, oxygen isotope exchange rates, and equilibrium oxygen isotope exchange factors. Data from culture experiments are used to determine the range of sulfur isotope fractionation factors in which a simplified set of equations can be used. Numerical examples demonstrate the application of the equations. We postulate that, during denitrification, the oxygen isotope effects in residual nitrate are also the result of oxygen isotope exchange with ambient water. Consequently, the equations for the relationship between δ³⁴S_{SO4_residual}, δ¹⁸O_{SO4_residual}, and the amount of residual sulfate could be modified and used to calculate the fractionation-relationship between δ¹⁵N_{NO3_residual}, δ¹⁸O_{NO3_residual}, and the amount of residual nitrate during denitrification.     

Experimental study of gold-hydrosulphide complexing in aqueous solutions at 350-500°C, 500 and 1000 bars using mineral buffers

The solubility of gold was measured in KCl solutions (0.001-0.1 m) at near-neutral to weakly acidic pH in the presence of the K-feldspar-muscovite-quartz, andalusite-muscovite-quartz, and pyrite-pyrrhotite-magnetite buffers at temperatures 350 to 500°C and pressures 0.5 and 1 kbar. These mineral buffers were used to simultaneously constrain pH, f(S₂), and f(H₂). The experiments were performed using a CORETEST flexible Ti-cell rocking hydrothermal reactor enabling solution sampling at experimental conditions. Measured log m(Au) (mol/kg H₂O) ranges from −7.5 at weakly acid pH to −5.9 in near-neutral solutions, and increases slightly with temperature. Gold solubility in weakly basic and near-neutral solutions decreases with decreasing pH at all temperatures, which implies that Au(HS)₂ is the dominant Au species in solution. In more acidic solutions, solubility is independent of pH. Comparison of the experimentally measured solubilities with literature values for Au hydrolysis constants demonstrates that at 350°C dominates Au aqueous speciation at the weakly acidic pH and f(S₂)/f(H₂) conditions imposed by the pyrite-pyrrhotite-magnetite buffer. In contrast, at temperatures >400°C becomes less important and predominates in weakly acid solutions. Solubility data collected in this study were used to calculate the following equilibrium reaction constants:

     

On estimating frequency-magnitude relations from heterogeneous catalogs

The paper studies the effect of magnitude errors on heterogeneous catalogs, by applying the apparent magnitude theory (seeTinti andMulargia, 1985a), which proves to be the most natural and rigorous approach to the problem. Heterogeneities in seismic catalogs are due to a number of various sources and affect both instrumental as well as noninstrumental earthquake compilations.The most frequent basis of heterogeneity is certainly that the recent instrumental records are to be combined with the historic and prehistoric event listings to secure a time coverage, considerably longer than the recurrence time of the major earthquakes. Therefore the case which attracts the greatest attention in the present analysis is that of a catalog consisting of a subset of higher quality data, generallyS ₁, spanning the interval T ₁ (the instrumental catalog), and of a second subset of more uncertain magnitude determination, generallyS ₂, covering a vastly longer interval T ₂ (the historic and/or the geologic catalog). The magnitude threshold of the subcatalogS ₁ is supposedly smaller than that ofS ₂, which, as we will see, is one of the major causes of discrepancy between the apparent magnitude and the true magnitude distributions. We will further suppose that true magnitude occurrences conform to theGutenberg-Richter (GR) law, because the assumption simplified the analysis without reducing the relevancy of our findings.The main results are: 1) the apparent occurrence rate exceeds the true occurrence rate from a certain magnitude onward, saym _GR; 2) the apparent occurrence rate shows two distinct GR regimes separated by an intermediate transition region. The offset between the two regimes is the essential outcome ofS ₁ being heterogeneous with respect toS ₂. The most important consequences of this study are that: 1) it provides a basis to infer the parameters of the true magnitude distribution, by correcting the bias deriving from heterogeneous magnitude errors; 2) it demonstrates that the double GR decay, that several authors have taken as the incontestable proof of the failure of the GR law and of the experimental evidence of the characteristic earthquake theory, is instead perfectly consistent with a GR-type seismicity.     

Robust numerical implementation of a 3D rate-dependent model for reservoir geomechanical simulations

A 3D elasto-plastic rate-dependent model for rock mechanics is formulated and implemented into a Finite Element (FE) numerical code. The model is based on the approach proposed by Vermeer and Neher (A soft soil model that accounts for creep. In: Proceedings of the International Symposium “Beyond 2000 in Computational Geotechnics,” pages 249-261, 1999). An original strain-driven algorithm with an Inexact Newton iterative scheme is used to compute the state variables for a given strain increment.The model is validated against laboratory measurements, checked on a simplified test case, and used to simulate land subsidence due to groundwater and hydrocarbon production. The numerical results prove computationally effective and robust, thus allowing for the use of the model on real complex geological settings.     

Assessing the Vibrational Frequencies of the Cathedral of Cologne (Germany) by Means of Ambient Seismic Noise Analysis

Ambient seismic noise measurements were conducted inside the Cathedral of Cologne (Germany) for assessing its frequencies of vibration and for checking whether these occur in the range where soil amplification is expected. If this is the case, damages may increase in case of an earthquake due to an increased structural response of the building. Analysis of the ratio between the horizontal and vertical components of the spectra recorded at stations located inside the building as well as the ratio between the corresponding components of the spectra recorded simultaneously inside the building and at a reference station placed in the basement of the cathedral indicated several modes of vibration. Facilitated by these results an assessment of the seismic vulnerability was attempted for a 2D ground motion scenario using the finite element method.     

A Block-Based Theoretical Model Suited to Gravitational Sliding

An original theoretical model has been devised to simulate mass flow over hill slopes due to gravitational sliding. The sliding mass is discretized into a sequence of contiguous blocks which are subjected to gravitational forces, to bottom friction and to surface resistance stresses that are generally negligible for subaerial flows, but are relevant for submarine slides. The blocks interact with each other while sliding down the hill flanks because of internal forces that dissipate mechanical energy and produce a momentum exchange between the individual blocks, yet conserving the total momentum of the mass. Internal forces are expressed in terms of interaction coefficients depending on the instantaneous distance between the block centers of mass, which is a measure of the deformation experienced by the blocks: the functional dependence includes three parameters, namely the interaction intensity ¯, the deformability parameter and the shape parameter , by means of which a wide range of interaction types can be fully accounted for. The time integration is performed numerically by solving the equations for the block velocities and positions at any time ti by means of the block accelerations at the previous time t_i-1, and by subsequently updating the block accelerations, which allows to proceed iteratively to the following times. The model has been tested against laboratory results available from literature and by means of several numerical experiments involving a simplified geometry both for the sliding body and the basal surface, with the purpose of clarifying the influence of the model parameters on the slide dynamics. The model improves the performance of the existing kinematic models for slides, moreover preserving an equivalent numerical simplicity. Future applications and possible improvements of this model are suggested.