Formation of magnesium‐smectite during lacustrine carbonates early diagenesis: Study case of the volcanic crater lake Dziani Dzaha (Mayotte – Indian Ocean)

The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l^?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO₂‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.     

Long-term tendencies in the MLT prevailing winds and tides over Antarctica as observed by radars at Molodezhnaya,Mawson and Davis

Long-term tendencies in horizontal neutral wind parameters in the southern polar mesosphere/lower thermosphere are presented. The wind data analyzed were obtained from meteor and MF radars situated at Molodezhnaya (45.9°E, 67.7°S), Mawson (62.9°E, 67.6°S) and Davis (78.0°E, 68.6°S). The composite dataset covers years from 1970 to 2006. A Bayesian approach in the form proposed by Wang and Zivot [2000. A Bayesian time series model of multiple structural changes in level, trend and variance. Journal of Business and Economic Statistics 8, 374–386] is used for the trend assessment. This approach allows structural breaks to be identified in the trend parameters (slope, mean or variance of residuals) or demonstrates their absence. The results of our analysis have shown persistence through such structural breaks in trends of the winter and summer prevailing winds and in meridional tidal components. It is demonstrated that the wind parameters exhibit different stable states with transitions between the states. Correlations between the southern polar MLT wind and indices of atmospheric variability (Northern annular mode, Southern annular mode, Multivariate El-Niño/Southern Oscillation Index) were then considered. The results show that statistically significant correlations exist during some periods of observations that do not exist during others.     

Geospace imaging using Thomson scattering

We evaluate the potential of imaging for the first time, the near-earth space plasma environment seamlessly from the ionosphere through the magnetosphere by remotely sensing Thomson scattering of solar visible light by geospace electrons. Using state of the art first principles models of the magnetosphere/ionosphere system, we show that the column emission rates are weak, generally less than 10 Rayleighs, but detectable with currently available instrument technology recently deployed for heliospheric imaging. We demonstrate that distinct features such as the bow shock, magnetosheath and magnetopause are detectable in synthetic images simulated using modified solar coronagraphs and white light imagers, providing that the large background signals are properly quantified. The availability of global geospace images of the electron concentration will enable major advances in our understanding of how Earth's near-space environment responds as a coupled system to changing solar forcings. Such images are expected to play a central role in space weather assessment and forecasting, from which significant capabilities will accrue, much as the imaging of the Earth's surface and lower atmosphere has advanced understanding and forecasting of tropospheric weather.     

Modeling the timing of spring phytoplankton bloom and biological production of the Gulf of St. Lawrence (Canada): Effects of colored dissolved organic matter and temperature

The effects of colored dissolved organic matter (CDOM) from freshwater runoff and seasonal cycle of temperature on the dynamic of phytoplankton and zooplankton biomass and production in the Gulf of St. Lawrence (GSL) are studied using a 3-D coupled physical-plankton ecosystem model. Three simulations are conducted: (1) the reference simulation based on Le Fouest et al. (2005), in which light attenuation by CDOM is not considered and maximum growth rate (_μmax${\mu }_{\max }$) of phytoplankton and zooplankton are not temperature-dependent (REF simulation); (2) light attenuation by CDOM is added to REF simulation (CDOM simulation); and (3) in addition to CDOM, the _μmax${\mu }_{\max }$ of phytoplankton and zooplankton are regulated by temperature (CDOM+TEMP simulation). CDOM simulation shows that CDOM substantially reduces phytoplankton biomass and production in the Lower St. Lawrence Estuary (LSLE), but slightly reduces overall primary production in the GSL. In the LSLE, the spring phytoplankton bloom is delayed from mid-March to mid-April, resulted from light attenuation by CDOM. The CDOM+TEMP simulation shows that the spring phytoplankton bloom in the LSLE is further delayed to July, which is more consistent with observations. Annual primary production is reduced by 33% in CDOM+TEMP simulation from REF and CDOM simulations. Zooplankton production is the same in all three simulations, and export of organic matter to depth is reduced in CDOM+TEMP simulation, suggesting that temperature controlled growth of phytoplankton and zooplankton enhances the coupling between primary production and zooplankton production under the seasonal temperature cycle of the GSL.     

Anti-plane foundationless soil–structure interaction

This paper presents a closed-form wave function analytic solution of two-dimensional scattering and diffraction of anti-plane SH-waves by a two-dimensional foundationless structure that corresponds to a shear wall on an elastic half-space. A wave-function expansion method is used to solve this model by first prescribing a set of wave functions with undetermined coefficients and then assembling them together based on the stress and displacement boundary conditions on the surface between the structure and half space. This results in a set of infinite equations to be solved by truncating to a finite set. The amplitudes and residuals of the displacement and stress distributions around the structure and nearby ground surface will be discussed carefully. While the solution is analytical, the computation of the numerical results involves the evaluation of complicated integrals. This analytic solution will be helpful to the understanding of propagation of seismic or other stress waves within the superstructure(s) undergoing earthquakes or other blast loads.     

A solar pattern in the longest temperature series from three stations in Europe

We analyze the longest temperature series from Prague, Bologna and Uccle. We partition daily minimum and maximum temperatures and their differences in two subsets as a function of high vs low solar activity, using the superimposed epochs method. Differences display patterns with significant amplitudes and time constants ～3 months. These are recognized in all stations and are stable against a change in the analyzed period. Amplitude of variations is ～1 °C. Differences between average annual values corresponding to high vs low activity periods are also ～1 °C. Solar activity may account for these long-term temperature variations. These variations also present local characteristics, which may render identification of a global correlation delicate. We discuss possible physical mechanisms by which solar variation could force climate changes (e.g. through solar activity itself, the EUV part of the solar flux, cosmic rays, the downward ionosphere-earth current density, etc.).     

Fluid–rock interactions related to metamorphic reducing fluid flow in meta-sediments: example of the Pic-de-Port-Vieux thrust (Pyrenees,Spain)

In orogens, shortening is mainly accommodated by thrusts, which constitute preferential zones for fluid–rock interactions. Fluid flow, mass transfer, and mineralogical reactions taking place along thrusts have been intensely investigated, especially in sedimentary basins for petroleum and uranium research. This study combines petrological investigations, mineralogical quantifications, and geochemical characterizations with a wide range of analytical tools with the aim of defining the fluid properties (nature, origin, temperature, and redox) and fluid–host rock interactions (mass transfers, recrystallization mechanisms, and newly formed synkinematic mineralization) in the Pic-de-Port-Vieux thrust fault zone (Pyrenees, Spain). We demonstrate that two geochemically contrasted rocks have been transformed by fluid flow under low-grade metamorphism conditions during thrusting. The hanging-wall Triassic red pelite was locally bleached, while the footwall Cretaceous dolomitic limestone was mylonitized. The results suggest that thrusting was accompanied by a dynamic calcite recrystallization in the dolomitic limestone as well as by leaching of iron via destabilization of iron oxides and phyllosilicate crystallization in the pelite. Geochemical and physical changes highlighted in this study have strong implications on the understanding of the thrust behavior (tectonic and hydraulic), and improve our knowledge of fluid–rock interactions in open fluid systems in the crust.     

A simple and effective method for preserving the sediment–water interface of sediment cores during transport

We describe a method for preserving the upper sediments of fragile sediment cores during transport from field sites and assess potential effects on subsequent laboratory analyses. This method addresses the need to minimize disturbance to the surfaces of unfrozen sediment cores used for paleoenvironmental or other high-resolution sedimentological analyses during transport. A polymer gel (sodium polyacrylate) applied above the sediment surface acts as a barrier to movement while also preserving surface undulations. The gel seal can preserve even exceptionally fine sedimentary structures (<0.2 mm) in the upper sediments of lacustrine and fiord sediment cores, but may react with organic material (e.g. algal mats) present on some sediment surfaces. This reaction creates an adhesive layer at the gel’s base but it can be handled effectively during sampling. The gel seal minimizes surface deformation and preserves surficial sediments better than traditional seals made of water-absorbent floral foam, wax or paper towel. In addition to permitting detailed sedimentary and subfossil investigations of the sediment–water interface, this method shows no detectable effects on measurements of total organic carbon or total nitrogen values in the sediment. This method is inexpensive, non-hazardous and applicable to many coring systems and sediment types.     

A neural network model for predicting aquifer water level elevations

Artificial neural networks (ANNs) were developed for accurately predicting potentiometric surface elevations (monitoring well water level elevations) in a semiconfined glacial sand and gravel aquifer under variable state, pumping extraction, and climate conditions. ANNs "learn" the system behavior of interest by processing representative data patterns through a mathematical structure analogous to the human brain. In this study, the ANNs used the initial water level measurements, production well extractions, and climate conditions to predict the final water level elevations 30 d into the future at two monitoring wells. A sensitivity analysis was conducted with the ANNs that quantified the importance of the various input predictor variables on final water level elevations. Unlike traditional physical-based models, ANNs do not require explicit characterization of the physical system and related physical data. Accordingly, ANN predictions were made on the basis of more easily quantifiable, measured variables, rather than physical model input parameters and conditions. This study demonstrates that ANNs can provide both excellent prediction capability and valuable sensitivity analyses, which can result in more appropriate ground water management strategies.     

Rates of salinization by free convection in high-permeability sediments: insights from numerical modeling and application to the Dutch coastal area

Numerical modeling and dimensional analysis is used to study the salinization of thick, high-permeability aquifers by free convection from a salt source at the surface. Current understanding of this process mainly concerns the initial stages of salinization only (boundary-layer development, break-up into fingers and initial phase of finger descent). In the modeling, special attention is paid to the role of two processes in the long-term salinization rate: (1) the progressive loss of salt from fingers by lateral diffusion, and (2) the coalescence of fingers during their descent. From the numerical simulations a relationship is derived that describes the development of the horizontally averaged salinity with depth and time as a function of permeability and initial-density contrast for aquifer Rayleigh numbers up to Ra =6,000. This relationship is consistent with and provides an extension to previous generalized relationships of the rate of finger descent. Its applicability to real-world aquifers (Ra >10⁵) that include complexities due to anisotropy, heterogeneity, and mechanical dispersion is discussed. Application to the Pleistocene coastal aquifer of the Netherlands (thickness 200 m, permeability 10^-11 m²) suggests that salinization of the aquifer during historic episodes of inundation by seawater occurred within decades.

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Resumen Se utiliza modelos numéricos y un análisis dimensional para estudiar la salinización de acuíferos potentes de alta permeabilidad por convección libre a partir de una fuente salina superficial. El conocimiento acutal de este proceso se limita a las fases iniciales de la salinización (desarrollo de la capa de contorno, creación de digitaciones y fase inicial de la progresión de éstas). En la modelación, se presta atención especial al papel desempeñado por dos procesos de salinización a largo plazo: (1) la pérdida progresiva de sal por difusión lateral desde las digitaciones, y (2) la coalescencia de las digitaciones durante su avance. A partir de las simulaciones numéricas, se obtiene una relación que describe el desarrollo de la salinidad con la profundidad y el tiempo, promediada horizontalmente, el cual depende de la permeabilidad y del contraste inicial de densidad para números de Rayleigh inferiores a 6.000. Esta relación es coherente con índices previos generalizados del avance de las digitaciones, y representa una extensión a estos. Se discute su aplicabilidad a acuíferos reales (con números de Rayleigh superiores a 105), que tienen complejidades asociadas a la anisotropía, la heterogeneidad y la dispersión mecánica. La aplicación al acuífero costero Pleistoceno de los Países Bajos (20 m de potencia y 10-11 m2 de permeabilidad) sugiere que la salinización tuvo lugar en décadas, debido a episodios históricos de inundación por aguas marinas.

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Résumé Une modélisation numérique et une analyse dimensionnelle ont été mises en oeuvre pour étudier la salinisation d'aquifères épais et à forte perméabilité par convection libre d'une source de sel en surface. La compréhension habituelle de ce processus concerne principalement les étapes initiales de la salinisation seule (développement d'une couche limite, partition en digitations et phase initiale de développement des digitation). Dans la modélisation, une attention particulière a été portée au rôle de deux processus du taux de salinisation à long terme: (1) la perte progressive de sel dans les digitations par diffusion latérale et (2) la coalescence des digitations au cours de leur développement. À partir de simulations numériques, une relation a été obtenue qui permet de décrire l'extension de la salinité horizontalement en profondeur et au cours du temps en fonction de la perméabilité et du contraste initial de densité pour des nombres de Rayleigh de l'aquifère jusqu'à Ra =6,000. Cette relation est compatible avec ces résultats et fournit une extension des relations précédemment généralisées du taux de développement des digitations. On discute son applicabilité à des aquifères réels (Ra >105) incluant des complexités liées à l'anisotropie, l'hétérogénéité et la dispersion mécanique. L'application à l'aquifère côtier du Pléistocène des Pays-Bas (épaisseur environ 200 m, perméabilité environ10–11 m²) laisse penser que la salinisation de cet aquifère au cours d'épisodes historiques d'inondation par la mer s'est produit durant des décennies.

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