Influence de l'hydrodynamique sur l'intrusion saline en aquifère karstique côtier

A new method of interpretation of the $Q\text{–}C$ hysteresis (discharge–salinity) during high water is proposed using the data recorded on coastal brackish karstic springs with salinity inversely proportional to the discharge. The method, based on the $Q_{\mathrm{mer}}\text{–}{C}_{\mathrm{T}}$ (seawater discharge–salinity) hysteresis, identifies the effect of the hydrodynamic changes on the flow and transport. Three phenomena are characterized: the emptying of the karstic conduit, the dilution by freshwater and the control of the seawater intrusion in the conduit by the hydraulic head variations. To cite this article: B. Arfib et al., C. R. Geoscience 338 (2006).     

Dual polarization radar Lagrangian parameters: a statistics-based probabilistic nowcasting model

The aim of this study is to present a statistics-based Lagrangian nowcasting model to predict intense rainfall convective events based on dual polarization radar parameters. The data employed in this study are from X-band radar collected during the CHUVA-Vale campaign from November 2011 to March 2012 in southeast Brazil. The model was designed to catch the important physical characteristics of storms, such as the presence of supercooled water above 0 °C isotherm, vertical ice crystals in high levels, graupel development in the mixed-phase layer and storm vertical growth, using polarimetric radar in the mixed-phase layer. These parameters are based on different polarimetric radar quantities in the mixed phase, such as negative differential reflectivity (Z _DR) and specific differential phase (K _DP), low correlation coefficient (ρ _hv) and high reflectivity Z _h values. Storms were tracked to allow the Lagrangian temporal derivation. The model is based on the estimation of the proportion of radar echo volume in the mixed phase that is likely to be associated with intense storm hydrometeors. Thirteen parameters are used in this probabilistic nowcasting model, which is able to predict the potential for future storm development. The model distinguishes two different categories of storms, intense and non-intense rain cell events by determining how many parameters reach the “intense” storm threshold.     

A geotechnical perspective of raw earth building

Much research has been devoted over the past 30 years to the development of construction materials that can lower the environmental and economic costs of buildings over their entire life by reducing embodied energy, minimizing air conditioning needs and cutting down demolition waste. In this respect, raw earth is an attractive material because it is natural and largely available. In its simplest form, this material consists of a compacted mixture of soil and water which is put in place with the least possible transformation. Raw earth construction has been practised in ancient times but has only recently been rediscovered thanks to modern technology, which has improved fabrication efficiency. If properly manufactured, raw earth exhibits comparable mechanical characteristics and better hygro-thermal properties than concrete or fired bricks. After a brief historical overview, we discuss the advantages of raw earth construction in terms of environmental impact, energy consumption and indoor air quality together with the main obstacles to its wider dissemination. We also review the hydro-thermo-mechanical behaviour of raw earth in the context of the recent geotechnical literature, by examining the dependency of key parameters such as strength, stiffness and moisture retention on: (a) material variables (e.g. particle size and mineralogy), (b) manufacturing variables (e.g. density and stabilization) and (c) environmental variables (e.g. pore suction, ambient humidity and temperature).     

Differential host use affects fecundity of the gastropod Crepidula onyx

For epibiotic or symbiotic marine invertebrates, alternative host species may differ substantially in quality, and under some circumstances such differences in host quality may lead to the evolution of increased host specificity. However, the fitness consequences of alternative hosts for epibiotic or symbiotic marine invertebrates have rarely been quantified. In Southern California, the gastropod Crepidula onyx is often found as an epibiont on either bay mussels (Mytilus galloprovincialis) or cone snails (Conus californicus). These hosts differ greatly in maximum size, with possible effects on size at sex change and final size in Cr. onyx, and thus on fecundity. Further, Cr. onyx on the two hosts differ in shell shape, possibly affecting the size of the ctenidium, which Cr. onyx uses for suspension feeding. We examined these potential effects of host use on fitness components in Cr. onyx. Epibionts on mussels reached much larger average sizes than did those on cone snails; further, epibionts on mussels often completed sex change at much larger sizes than did those on cone snails. On average, mussel epibionts had threefold higher average fecundities than did cone snail epibionts. Although there was a slight difference in shell shape between epibionts on the two host species, there was no difference in the scaling of ctenidium area with body size for Cr. onyx from the two hosts. The large average differences in fecundity in epibionts associated with the two alternative hosts suggests that there may be strong selection on host choice at larval settlement.     

Chemical mass balance of calcrete genesis on the Toledo granite (Spain)

The chemical mass balance of calcrete genesis is studied on a typical sequence developed in granite, in the Toledo mountains, Central Spain.

Field evidence and petrographic observations indicate that the texture and the bulk volume of the parent rock are strictly preserved all along the studied calcrete profile.

Microscopic observations indicate that the calcitization process starts within the saprolite, superimposed on the usual mechanisms of granite weathering: the fresh rock is first weathered to secondary clays, mainly smectites, which are then pseudomorphically replaced by calcite. Based on this evidence, chemical mass transfers are calculated, assuming iso-volume transformation from the parent rock to the calcrete.

The mass balance results show the increasing loss of matter due to weathering of the primary phases, from the saprolite towards the calcrete layers higher in the sequence. Zr, Ti or Th, which are classically considered as immobile during weathering, are also depleted along the profile, especially in the calcrete layer. This results from the prevailing highly alkaline conditions, which could account for the simultaneous precipitation of CaCO₃ and silicate dissolution.

The calculated budget suggests that the elements exported from the weathering profile are provided dominantly by the weathering of plagioclase and biotite. We calculate that 8–42% of the original Ca remains in granitic relics, while only 15% of the authigenic Ca released by weathering is reincorporated in the calcite. This suggests that 373 kg/m² of calcium (i.e., three times the original amount) is imported into the calcrete from allochtonous sources, probably due to aeolian transport from distant limestone formations.     

Ab initio quantum-mechanical modeling of pyrophyllite [Al2Si4O10(OH)2] and talc [Mg3Si4O10(OH)2] surfaces

Bulk and slab geometry optimizations and calculations of the electrostatic potential at the surface of both pyrophyllite [Al₂Si₄O₁₀(OH)₂] and talc [Mg₃Si₄O₁₀(OH)₂] were performed at Hartree–Fock and DFT level. In both pyrophyllite and talc cases, a modest (001) surface relaxation was observed, and the surface preserves the structural features of the crystal: in the case of pyrophyllite the tetrahedral and octahedral sheets are strongly distorted with respect to the ideal hexagonal symmetry (and basal oxygen are located at different heights along the direction normal to the basal plane), whereas the structure of talc deviates slightly from the ideal hexagonal symmetry (almost co-planar basal oxygen). The calculated distortions are fully consistent with those experimentally observed. Although the potentials at the surface of pyrophyllite and talc are of the same order of magnitude, large topological differences were observed, which could possibly be ascribed to the differences between the surface structures of the two minerals. Negative values of the potential are located above the basal oxygen and at the center of the tetrahedral ring; above silicon the potential is always positive. The value of the potential minimum above the center of the tetrahedral ring of pyrophyllite is ?0.05 V (at 2 Å from the surface), whereas in the case of talc the minimum is ?0.01 V, at 2.7 Å. In the case of pyrophyllite the minimum of potential above the higher basal oxygen is located at 1.1 Å and it has a value of ?1.25 V, whereas above the lower oxygen the value of the potential at the minimum is ?0.2 V, at 1.25 Å; the talc exhibits a minimum of ?0.75 V at 1.2 Å, above the basal oxygen.     

Experimental determination of coexisting iron–titanium oxides in the systems FeTiAlO, FeTiAlMgO, FeTiAlMnO, and FeTiAlMgMnO at 800 and 900°C, 1–4 kbar, and relatively high oxygen fugacity

A synthetic, low-melting rhyolite composition containing TiO₂ and iron oxide, with further separate additions of MgO, MnO, and MgO + MnO, was used in hydrothermal experiments to crystallize Ilm-Hem and Usp-Mt solid solutions at 800 and 900°C under redox conditions slightly below nickel–nickel oxide (NNO) to $\approx 3\,\log_{10} f_{{{\text{O}}_{2}}}A synthetic, low-melting rhyolite composition containing TiO₂ and iron oxide, with further separate additions of MgO, MnO, and MgO + MnO, was used in hydrothermal experiments to crystallize Ilm-Hem and Usp-Mt solid solutions at 800 and 900°C under redox conditions slightly below nickel–nickel oxide (NNO) to units above the NNO oxygen buffer. These experiments provide calibration of the FeTi-oxide thermometer + oxygen barometer at conditions of temperature and oxygen fugacity poorly covered by previous equilibrium experiments. Isotherms for our data in Roozeboom diagrams of projected %usp vs. %ilm show a change in slope at ≈ 60% ilm, consistent with the second-order transition from FeTi-ordered Ilm to FeTi-disordered Ilm-Hem. This feature of the system accounts for some, but not all, of the differences from earlier thermodynamic calibrations of the thermobarometer. In rhyolite containing 1.0 wt.% MgO, 0.8 wt.% MnO, or MgO + MnO, Usp-Mt crystallized with up to 14% of aluminate components, and Ilm-Hem crystallized with up to 13% geikielite component and 17% pyrophanite component. Relative to the FeTiAlO system, these components displace the ferrite components in Usp-Mt, and the hematite component in Ilm-Hem. As a result, projected contents of ulv?spinel and ilmenite are increased. These changes are attributed to increased non-ideality along joins from end-member hematite and magnetite to their respective Mg- and Mn-bearing titanate and aluminate end-members. The compositional shifts are most pronounced in Ilm-Hem in the range Ilm_50–80, a solvus region where the chemical potentials of the hematite and ilmenite components are nearly independent of composition. The solvus gap widens with addition of Mg and even further with Mn. The Bacon–Hirschmann correlation of Mg/Mn in Usp-Mt and coexisting Ilm-Hem is displaced toward increasing Mg/Mn in ilmenite with passage from ordered ilmenite to disordered hematite. Orthopyroxene and biotite crystallized in experiments with added MgO and MgO + MnO; their X _Fe varies with and T consistent with equilibria among ferrosilite, annite, and ferrite components, and the chemical potentials of SiO₂ and orthoclase in the liquid. Experimental equilibration rates increased in the order: Opx < Bt < Ilm-Hem < Usp-Mag.     

粤北晚泥盆世天子岭组遗迹组构及其环境解释

遗迹组构是由生物成因改造的沉积岩组构,它是物理过程和生物过程相互作用的产物。本文在广东韶关晚泥盆世天子岭组碳酸盐岩沉积中建立了4个遗迹组构,即反映局限台地或封闭海湾环境的Planolies montanus遗迹组构,反映台地潮下低能环境的Thalassinoides—Planolites遗迹组构,反映潮上-潮间环境的．Skolithos遗迹组构和受固底底质控制的Thalassinoides遗迹组构。对遗迹化石和生物扰动构造进行阶层分析,从中识别出受固底底质控制的遗迹化石Thalassinoides.sp,这类遗迹化石常常代表层序地层学中的重要界面。