The composition of the precambrian ocean waters

The composition of the Precambrian oceans was different from the present composition; this is a direct consequence of a CO₂-atmosphere depleted in oxygen. At the beginning the mineralization was lower. The oceans waters were chloro-dicarbonated with a lower tenor of chlorine, richer in phosphates and they were depleted in sulphates; sulphur was in the reduced state of sulphides. The abundance of phosphates supported the expansion of life. The cations Na⁺ and Mg²⁺ coexisted with Ca²⁺, Fe²⁺ and Ba²⁺ whose levels were higher than they are today. The main trends of the evolution are schematized in the figure.     

Spectral analysis of earthquake migration in South America

Earthquake migration along linear seismic belts is investigated by analyzing spacetime diagrams using spectral analysis. In order to sample the earthquakes in the space-time domain, they must first be convolved with a (sinx sint)/xt surface to obtain an unbiased and alias free twodimensional Fourier spectrum. Further enhancements are provided by selectively stacking patterns (a pattern is defined as the distribution of earthquakes in space and time before a particular earthquake), thereby reinforcing the similarities within the various patterns. With these techniques, it is possible to quantitatively estimate the migration rates (from theirspatial frequencies) and recurrence intervals (from theirtemporal frequencies) of large earthquakes in South America.Preliminary examination of the spectra for South America indicates that a low frequency peak occurs at approximately 2500 km and 27 years for earthquakes with magnitudes greater than 7.7. The results suggest a migration rate of approximately 95 km/yr from south to north and a recurrence interval of 27 years.     

Ries impact crater,southern Germany: search for meteoritic material

Twenty-three samples from the Ries crater, representing a wide range of shock metamorphism, were analyzed for seven siderophile elements (Au, Ge, Ir, Ni, Os, Pd, Re) and five volatile elements (Ag, Cd, Sb, Se, Zn). Taking Ir as an example, we found siderophile enrichments over the indigenous level of 0.015 ppb Ir occur in only eight samples. The excess is very modest; even the most enriched samples (a weakly shocked biotite gneiss and a metal-impregnated amphibolite) have Ir, Os corresponding to ~4 × 10^?4 C1 chondrite abundances. Of five flädle glasses analyzed only one shows excess Ir. Suevite matrix and vesicular glass have slight enrichment, but homogenous glass from the same rock does not. In flädle glasses, Ni and Se are strongly correlated and apparently reside in Ir, Os-poor Sulfides [pyrrhotite, chalcopyrite, pentlandite(?)]of terrestrial, probably sedimentary, origin. The Ir, Os and Ni enrichments of the metal-bearing amphibolite are compatible with chondritic ratios, but these are ill-defined because of uncertainty in Ni. In the other samples enriched in siderophiles Ir(Os), Ni and Se are mutually correlated; $\text{Ni}\text{Ir}$ and $\text{Ni}\text{Os}$ ~ 11 × C1 and are much higher than any chondritic ratios; $\text{Se}\text{Ni}\text{~ 2 × C1}$ and suggests a sulfide phase, rather than metal may be the host of the correlated elements. Lacking a plausible local source, this material is apparently meteoritic in origin. The unusual elemental ratios, coupled with the very low enrichments, tend to exclude chondrites and most irons as likely projectile material. Of the achondrites, aubrites seem slightly preferable. Ratios of excess siderophiles in Ries materiel match tolerably those of an aubrite (possibly atypical) occurring as an inclusion in the Bencubbin meteorite, Australia. The Hungaria group of Mars-crossing asteroids may be a source of aubritic projectiles.     

Alkali feldspar liquid equilibrium relationships in peralkaline oversaturated systems and volcanic rocks

A critical review of both experimental data and natural rock sequences allows a comparison of liquid lines of descent in peralkaline oversaturated systems. Available experimental data have been used to draw the liquidus surfaces and the fractionation curves in various planes including the alkali feldspar join. The thermal valley becomes steeper, the fractionation curves being channeled faster towards the thermal valley with increasing alkali-silica ratio. The slope of the thermal valley varies so that the higher the peralkalinity, the more sodic the most fractionated liquids. Rock series from Easter Island, Gran Canaria, Pantellaria Island, Boina (Afar), Fantale (Ethiopia) and Menengai (Kenya) in which there is good evidence for alkali feldspar fractionation, show that such variations do occur in nature. Factors other than the peralkalinity of the series, such as additional components and changes in the confining pressure are shown to affect the orientation and location of the thermal valley.     

Utilisation du radon comme indicateur de l'intensite des echanges pres du sol (0–100 M)

Resumé Les donées moyennes relatives à deux années de mesures continues du radon à 1.5–30 et 100 m au-dessus du sol sont présentées sous forme de variations diurnes standard; ces dernières sont étudiées par une analyse de Fourier (amplitude et phase). On précise l'évolution des variations au cours du cycle annuel.Le but de l'étude des donées moyennes étant d'accéder aux informations relatives à la variation diurne des échanges, on a recherché des relations entre la variation diurne standard des concentrations et celles des échanges; d'autre part on a défini des critères, déduits des paramètres météorologiques, classiques susceptibles de caracteriser l'intensité de la variation diurne des échanges.

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Continuous measurement of radon concentrations at, 1.5, 30 and 100 m has been carried out for 2 years, from October 1966 to October 1968. The mean diurnal variations of the concentrations are characterized in a simple manner using a Fourier analysis, revealing the diurnal and annual waves, and showing the variations with altitude of the radon concentrations. The amplitude of this diurnal wave can be considered as representative of the mean diurnal variation of the intensity of the vertical exchange processes.This method of analysis is applied to the study of influence of standard meteorological elements measured at the site on the diurnal variation of the exchanges. Additional indicators are derived from temperature, humidity and wind (speed and direction) measurements, and their daily cycles (mean winds, mean direction, temperature diurnal variation amplitude...) are shown to be related to the cycles of radon concentrations.The mean diurnal variations of the radon concentrations classified according to several meteorological parameters are used to characterize the intensity of mixing, in terms of a mixing coefficient.This characterization is achieved both from the average gradients (local value) and from the diurnal wave at the soil.

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Equipe de Recherche Associée au CNRS, No. 378.     

L'évolution magmatique du Complexe de Morin et son apport au problème des anorthosites

The Morin Plutonic Complex, in the southern part of the Grenville tectonic province (Canadian Shield) consists of a rock suite ranging from troctolite to farsundite. The centre of the complex is a large domical anorthosite body. Twenty-six chemical analyses have been carried out in order to compare the Morin Complex with similar complexes in the Grenville and Nain provinces. Variation diagrams constructed by plotting the major oxides vs. the differentiation index Al₂O₃+CaO+MgO show a gap between troctolite and anorthosite in the Morin Complex whereas this gap is absent in the anorogenic complexes of Labrador (Nain province). It is inferred that this gap is the result of intrusion, from the same magma chamber, of two distinct pulses separated by an intervening period of crustal uplift.A second gap occurs between anorthosites and farsundites in all complexes as yet investigated. This gap is an argument for an independent, possibly anatectic, origin of the farsundites.Mineral assemblages in the Morin Complex are compatible with cooling in the lower crust, at a depth of approximately 30 km.     

Characterization of water in synthetic rhyolitic glasses and natural melt inclusions by Raman spectroscopy

Raman spectroscopy was used to analyze quantitatively water in silicate glasses and melt inclusions and to monitor H₂O–OH speciation. Calibration is based on synthetic glasses with various water contents (0.02–7.67% H₂O); water determination and OH–H₂O differentiation on the area of the Si–O broad band at 468 cm^–1 and the asymmetric O–H band at 3,550 cm^–1. Each Raman spectrum has been decomposed into four Gaussian + Lorentzian components centered at 3,330, 3,458, 3,560, and 3,626 cm^–1 using the Levenberg–Marquardt algorithm. These components are interpreted to be two different types of H₂O molecule sites. The influence of the temperature on the loss of water is more important for molecular water than for the hydroxyl groups. The H₂O–OH partition confirms the typical evolution of water speciation in rhyolitic glasses as a function of the bulk water content. Method limitations have been studied for the application to natural melt inclusions.Editorial responsibility: T.L Grove     

Prévision des crues éclair

Flash-flood events resulting from paroxystic meteorological events concentrated in time and space are insufficiently documented as they produce destructive effects. They are hardly measurable and present single features that are not transposable to another event. In the South of France, the flash flood of November 1999 gives a perfect illustration of these characteristics. The physical complexity of the process and consequently the volume and the variety of the data to take into account are incompatible with the real time constraint allocated to the forecasters confronted to the occurrence of such phenomena. So, we have to make choices to afford acceptable simplifications to the complete mechanical model. MARINE (‘Modélisation de l'Anticipation du Ruissellement et des Inondations pour des évéNements Extrêmes’) is the operational and robust tool we developed for flash-flood forecasting. This model complies with the criterions of real-time simulation. It is a physically based distributed model composed of two parts: first the flood runoff process simulation in the upstream part of the basin modelled from a rainfall–runoff approach, then the flood propagation in the main rivers described by the Saint-Venant equations. It integrates remote sensed data – Digital Elevation Model, land-use map, hydrographic network for the observations from satellites and the rainfall evolution from meteorological radar. The main goal of MARINE is to supply real time pertinent information to the forecasters. Results obtained on the Orbieu River (Aude, France) show that this model is able to supply pertinent flood hydrograph with a sufficient precision for the forecasting service to take the appropriate safety decisions. Furthermore, MARINE has already been tested in the French National Flood Forecasting Service of Haute-Garonne in real conditions. To cite this article: V. Estupina Borrell et al., C. R. Geoscience 337 (2005).     

Including prior information in the estimation of effective soil parameters in unsaturated zone modelling

In this paper we propose a methodology to include prior information in the estimation of effective soil parameters for modelling the soil moisture content in the unsaturated zone. Laboratory measurements on undisturbed soil cores were used to estimate the moisture retention curve and hydraulic conductivity curve parameters. The soil moisture content was measured at 25 locations along three transects and at three different depths (surface, 30 and 60 cm) on an 80×20 m hillslope for the year 2001. Soil cores were collected in 84 locations situated in three profile pits along the hillslope. For the estimation of the effective soil hydraulic parameters the joint probability distribution of measured parameter values was used as prior information. A two-horizon single column 1D MIKE SHE model based on Richards' equation was set-up for nine soil moisture measurement locations along the middle transect of the hillslope. The goal of the model is to simulate the soil moisture profile at each location. The shuffled complex evolution (SCE) algorithm has been applied to estimate effective model parameters using either wide parameter ranges, referred to as the ‘no-prior’ case, or the joint probability distribution of measured parameter values as prior information (‘prior’ case). When the prior information is incorporated in the SCE optimisation the goodness-of-fit of the model predictions is only slightly worse compared to when no-prior information is incorporated. However, the effective parameter estimates are more realistic when the prior information is incorporated. For both the no-prior and prior case the generalised likelihood uncertainty estimation procedure (GLUE) was subsequently used to estimate the uncertainty bounds (UB) on the model predictions. When incorporating the prior information more parameter sets were accepted for the estimation of the predictive uncertainty and the parameter values were more realistic. Moreover, UB better enclosed the observations. Thus, incorporating prior information in GLUE reduces the amount of model evaluations needed to obtain sufficient behavioural parameter sets. The results indicate the importance of prior information in the SCE and GLUE parameter estimation strategies.     

Origin and evolution of fluids from mud volcanoes in the Barbados accretionary complex

A large collection of fluids (54 interstitial fluids and four expelled fluids) were sampled at the Manon site, at the outer edge of the Barbados accretionary complex. These warm fluids (up to 20°C) are expelled by sub-marine (5000 mbsl) mud volcanoes consisting of diapirs (unchanneled flow) and diatremes (channeled).Chlorine stable isotope ratios of these fluids were measured by IRMS with a reproducibility of ± 0.05‰ (1σ) versus SMOC (Standard Mean Ocean Chloride).A large range of δ³⁷Cl between −5.3‰ and +0.1‰ is observed. Data from each volcanic structure describe a mixing between seawater and a low-δ³⁷Cl fluid. The whole set of data is interpreted as the result of a mixing between two deep components and seawater. The two deep fluids are chemically distinct (e.g., in Ca, Mg, K, Li, Sr and Br contents and Br/Cl ratio). They display low and significantly different ⁸⁷Sr/⁸⁶Sr ratios (0.707790 and 0.707892, respectively) and δ³⁷Cl values (−4.51 and −5.24‰, respectively).Physicochemical processes such as mineralogical transformation, diffusion, compaction or ion filtration are known to fractionate chlorine stable isotopes and can produce fluids with negative δ³⁷Cl values. Ion filtration due to sediment compaction appears to be the more likely process to explain the negative δ³⁷Cl values observed at the Manon site. A model for the generation of these signatures is proposed where a residual negative δ³⁷Cl fluid reservoir is created at the bottom of the prism or the sediment pile. Further compaction/fracturing and/or dewatering of the slab may flush out these fluids and focus them towards the décollement zone. Mixing between the fluids and ultimately with seawater and water released during gas hydrate destabilizations may explain the data set within the individual cores and between the different structures.