A comment on ‘Far ultraviolet photometry of orion stars’ by G.R. Carruthers

It is shown that the ultraviolet flux-deficiency, recently discovered by Carruthers, in early-type supergiants compared to dwarfs of the same spectral type can be explained in terms of the effects of differences in surface-gravity and effective temperature between stars of these two luminosity classes. It thus appears that this deficiency does not represent a serious discrepancy between theory and observation.     

An archetype hydrogen atmosphere problem

Populations for the first three bound states and the continuum of hydrogen are determined for an isothermal, hydrostatic atmosphere at 20 000 K. The atmosphere is treated as being optically thin in the Balmer and Paschen continua and illuminated by continuum radiation at these wavelengths with prescribed radiation temperatures. The atmosphere is optically thick in the 2-1, 3-1, 3-2 and c-1 transitions. Three stages of approximation are treated:

1. radiative detailed balance in the 2-1, 3-1 and 3-2 transitions,
2. radiative detailed balance in the 3-1 and 3-2 transitions, and
3. all transitions out of detailed balance.

The solution of this problem is non-trivial, and presents sufficient difficulty to have caused failure of at least one rather standard technique. The problem is thus a good archetype against which new methods, or new implementations of old methods may be tested.     

The stream power river incision model: evidence,theory and beyond

The stream power incision model (SPIM) is a cornerstone of quantitative geomorphology. It states that river incision rate is the product of drainage area and channel slope raised to the power exponents m and n, respectively. It is widely used to predict patterns of deformation from channel long profile inversion or to model knickpoint migration and landscape evolution. Numerous studies have attempted to test its applicability with mixed results prompting the question of its validity. This paper synthesizes these results, highlights the SPIM deficiencies, and offers new insights into the role of incision thresholds and channel width. By reviewing quantitative data on incising rivers, I first propose six sets of field evidence that any long‐term incision model should be able to predict. This analysis highlights several inconsistencies of the standard SPIM. Next, I discuss the methods used to construct physics‐based long‐term incision laws. I demonstrate that all published incising river datasets away from knickpoints or knickzones are in a regime dominated by threshold effects requiring an explicit upscaling of flood stochasticity neglected in the standard SPIM and other incision models. Using threshold‐stochastic simulations with dynamic width, I document the existence of composite transient dynamics where knickpoint propagation locally obeys a linear SPIM (n=1) while other part of the river obey a non‐linear SPIM (n>1). The threshold‐stochastic SPIM resolves some inconsistencies of the standard SPIM and matches steady‐state field evidence when width is not sensitive to incision rate. However it fails to predict the scaling of slope with incision rate for cases where width decreases with incision rate. Recent proposed models of dynamic width cannot resolve these deficiencies. An explicit upscaling of sediment flux and threshold‐stochastic effects combined with dynamic width should take us beyond the SPIM which is shown here to have a narrow range of validity. Copyright © 2013 John Wiley & Sons, Ltd.     

Seismic response of classical monuments-a numerical perspective developed at the Temple of Apollo in Bassae,Greece

We report on the initial phase of assessing numerically the seismic vulnerability of sections of the Temple of Apollo in Bassae, in the W. Peloponnesus, Greece. The site is exposed to large subduction earthquakes and to smaller local extensional events. In this phase the only link with the seismotectonic environment is the selection of representative recorded accelerograms for numerical analysis. The analysis confirms the highly non-linear nature of the response of megalithic structures relying for their stability on the friction and cohesion between the constituent blocks. Intact sections of the Temple showed substantial resistance to strong ground motions due to their capacity to absorb energy with large relative movements of the building blocks that do not impair the stability of the structure. However, imperfections typical of the present condition of the monument, namely deterioration of the building stones and of the foundation material, substantially reduce the stability threshold. Powerful numerical tools are available to assist rational schemes for the protection of ancient structures and to subject archaeological hypotheses to numerical tests. This analysis quantifies the effects of weaknesses of the structure and allows testing the effectiveness of strengthening procedures. In the archaeological context there is a strong interaction with the ongoing geodynamic processes.     

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.     

Physical constraints in engineering seismic hazard analysis

In engineering seismic hazard probabilistic analysis, physical constraints are generally overlooked. We formulate such constraints for the general case of a site within an annular seismogenic zone. This configuration provides a first approximation of seismic hazard analysis within a broad zone undergoing crustal deformation; such zones are a common expression of continental tectonics. Applications are restricted to medium size earthquakes (M_s < 7). The formulation is applied to two cases reflecting the mid-plate (case I) and plate boundary (case II) seismotectonic environments. It is found that, for a given strain rate and for an upper bound magnitude of 6 3/4, the extreme hazard in both the environments is the same but of different character. In the plate boundary example, it is associated with widespread ground deformation while in the mid-plate example, it involves more intense ground motion. On the other hand, if the upper bound magnitude is 5 3/4, the extreme hazard is likely to be an order of magnitude less in case I than in case II. Moreover, when the extreme hazard is associated with singular conditions generated by a single fault, the assumption of a Poissonian process may not be safe for earthquake resistant design decisions.     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite + enstatite and magnesite + enstatite

We experimentally investigated the dissolution of forsterite, enstatite and magnesite in graphite-saturated COH fluids, synthesized using a rocking piston cylinder apparatus at pressures from 1.0 to 2.1 GPa and temperatures from 700 to 1200 °C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were buffered by Ni–NiO–H₂O (ΔFMQ = ??0.21 to ??1.01), employing a double-capsule setting. Fluids, binary H₂O–CO₂ mixtures at the P, T, and fO₂ conditions investigated, were generated from graphite, oxalic acid anhydrous (H₂C₂O₄) and water. Their dissolved solute loads were analyzed through an improved version of the cryogenic technique, which takes into account the complexities associated with the presence of CO₂-bearing fluids. The experimental data show that forsterite?+?enstatite solubility in H₂O–CO₂ fluids is higher compared to pure water, both in terms of dissolved silica (mSiO₂?=?1.24 mol/kg_H2O versus mSiO₂?=?0.22 mol/kg_H2O at P?=?1 GPa, T?=?800 °C) and magnesia (mMgO?=?1.08 mol/kg_H2O versus mMgO?=?0.28 mol/kg_H2O) probably due to the formation of organic C–Mg–Si complexes. Our experimental results show that at low temperature conditions, a graphite-saturated H₂O–CO₂ fluid interacting with a simplified model mantle composition, characterized by low MgO/SiO₂ ratios, would lead to the formation of significant amounts of enstatite if solute concentrations are equal, while at higher temperatures these fluid, characterized by MgO/SiO₂ ratios comparable with that of olivine, would be less effective in metasomatizing the surrounding rocks. However, the molality of COH fluids increases with pressure and temperature, and quintuplicates with respect to the carbon-free aqueous fluids. Therefore, the amount of fluid required to metasomatize the mantle decreases in the presence of carbon at high P–T conditions. COH fluids are thus effective carriers of C, Mg and Si in the mantle wedge up to the shallowest level of the upper mantle.