Interactions between trace elements and dissolved organic matter in the stagnant anoxic deep layer of a meromictic lake

About 80% and more than 90% of the dissolved organic carbon (DOC) in the bottom water of Lake Pavin were isolated, respectively, on Amberlite XAD and Spherodex DEAE resins. Trace element concentrations in the fractions isolated were analyzed by using ICP-MS. Uranium, molybdenum, and antimony were found to be about 40% associated with fulvic and hydrophilic acids adsorbed on XAD resins at pH 2, the metal-organic association being not dissociated. Much higher percentages of the same elements, plus vanadium, were adsorbed on the DEAE resin at the pH of natural water, possibly because of the better preservation of acid labile organic complexes or of the supplementary adsorption of inorganic compounds. Ultrafiltration and dialysis made it possible to exclude colloidal fractions together with much U (78%), Mo (80%), V (55%), and DOC (65%). These elements and many others (including barium) were previously found to be quite reactive at the bottom of the lake, (apparently being scavenged by settling particles, which are mostly diatoms, then dissolved at the sediment-water interface). Conversely, trace elements with a conservative behavior in the bottom layer (such as lithium and cesium) were not found associated with dissolved organic mater (DOM). Barium was not strongly associated with the extractable DOM, which may argue for a direct interaction with inorganic particles or the existence of very labile complexes. Our work suggests the existence of relatively stable (nonacid labile) U and Mo-DOM colloidal associations in the anoxic bottom waters of the lake and their importance in the scavenging of those metals

Biogeochemical modelling of anaerobic vs. aerobic methane oxidation in a meromictic crater lake (Lake Pavin, France)

International audienceMethane is a powerful greenhouse gas and its concentration in the atmosphere has increased over the past decades. Methane produced by methanogenic Archae can be consumed through aerobic and anaerobic oxidation pathways. In anoxic conditions found in freshwater environments such as meromictic lakes, CH4 oxidation pathways involving different terminal electron acceptors such as NO 3 , SO2 4 , and oxides of Fe and Mn are thermodynamically possible. In this study, a reactive transport model was developed to assess the relative significance of the different pathways of CH4 consumption in the water column of Lake Pavin. In most cases, the model reproduced experimental data collected from the field from June 2006 to June 2007. Although the model and the field measurements suggest that anaerobic CH4 oxidation may contribute to CH4 consumption in the water column of Lake Pavin, aerobic oxidation remains the major sink of CH4 in this lake

The Outer Disks of Early-Type Galaxies. II. Surface-Brightness Profiles of Unbarred Galaxies and Trends with Hubble Type

We present azimuthally averaged radial profiles of R-band surface brightness for a complete sample of 47 early-type, unbarred galaxies, as a complement to our previous study of early-type barred galaxies. Following very careful sky subtraction, the profiles can typically be determined down to brightness levels well below 27 mag arcsec^{-2} and in the best cases below 28 mag arcsec^{-2}. We classified the profiles according to the scheme used previously for the barred sample: Type I profiles are single unbroken exponential radial declines in brightness; Type II profiles ("truncations") have an inner shallow slope (usually exponential) which changes at a well defined break radius to a steeper exponential; and Type III profiles ("antitruncations") have an inner exponential that is steeper, giving way to a shallower outer (usually exponential) decline. By combining these profiles with previous studies, we can make the first clear statements about the trends of outer-disk profile types along the Hubble sequence (including both barred and unbarred galaxies), and their global frequencies. We find that Type I profiles are most frequent in early-type disks, decreasing from one-third of all S0--Sa disks to barely 10% of the latest type spirals. Conversely, Type II profiles (truncations) increase in frequency with Hubble type, from only ~25% of S0 galaxies to ~80% of Sd--Sm spirals. Overall, the fractions of Type I, II, and III profiles for all disk galaxies (Hubble types S0--Sm) are: 21%, 50%, and 38%, respectively; this includes galaxies (~8% of the total) with composite Type II+III profiles (counted twice). Finally, we note the presence of bars in ten galaxies previously classified (optically) as "unbarred". This suggests that ~20% of optically unbarred galaxies are actually barred; the bars in such cases can be weak, obscured by dust, or so large as to be mistaken for the main diskComment: Accepted for publication in AJ. 35 pages, 14 figures, 7 table

Emplacement and deformation of mesozoic Gabbros of the High Atlas (Morocco): paleomagnetism and magnetic fabrics

A paleomagnetic and magnetic fabric study is performed in Upper Jurassic gabbros of the central High Atlas (Morocco). These gabbros were emplaced in the core of preexisting structures developed during the extensional stage and linked to basement faults. These structures were reactivated as anticlines during the Cenozoic compressional inversion. Gabbros from 19 out of the 33 sampled sites show a stable characteristic magnetization, carried by magnetite, which has been interpreted as a primary component. This component shows an important dispersion due to postemplacement tectonic movements. The absence of paleoposition markers in these igneous rocks precludes direct restorations. A novel approach analyzing the orientation of the primary magnetization is used here to restore the magmatic bodies and to understand the deformational history recorded by these rocks. Paleomagnetic vectors are distributed along small circles with horizontal axes, indicating horizontal axis rotations of the gabbro bodies. These rotations are higher when the ratio between shales and gabbros in the core of the anticlines increases. Due to the uncertainties inherent to this work (the igneous bodies recording strong rotations), interpretations must be qualitative. The magnetic fabric is carried by ferromagnetic (s.s.) minerals mimicking the magmatic fabric. Anisotropy of magnetic susceptibility (AMS) axes, using the rotation routine inferred from paleomagnetic results, result in more tightly clustered magnetic lineations, which also become horizontal and are considered in terms of magma flow trend during its emplacement: NW-SE (parallel to the general extensional direction) in the western sector and NE-SW (parallel to the main faults) in the easternmost structures

Dissolution d'une solution solide : compléments et corrections

The model previously presented by Denis and Michard (1983) for the dissolution of a solid solution is reexamined. The importance of the non ideal solid solutions and kinetic effects (partition coefficients differing from equilibrium) are assessed. Results previously obtained for ideal solutions can be extended to non ideal solid solutions without unmixing. For systems with unmixing, kinetic factors govern the composition of aqueous solution which reach a value close to equilibrium with the two solid phases system. If coprecipitation is controlled by a kinetic partition coefficient, the aqueous solution reaches a steady state which depends strongly on the value of this coefficient and to a lesser extent on the relative kinetics of dissolution and precipitation.Le modèle de dissolution d'une solution solide présenté par Denis et Michard (1983) est complété par la prise en compte : — de solutions solides non idéales, — de coefficients de partage différents du coefficient de partage d'équilibre que nous appellerons coefficients cinétiques. Les résultats obtenus pour les solutions solides idéales peuvent être étendus sans modification majeure aux solutions non idéales sans démixtion. Quand il y a démixtion, l'évolution de la solution aqueuse est contrainte par des facteurs cinétiques qui la conduisent à une composition proche de l'équilibre avec le système solide biphasé. L'introduction de coefficients de partage cinétiques modifie de façon plus importante les conclusions de l'article de 1983. Au lieu d'atteindre un état d'équilibre, on atteint un état stationnaire dont les caractéristiques sont fonction des cinétiques relatives de dissolution et précipitation et surtout de la valeur du coefficient de partage cinétique.Michard Gil. Dissolution d'une solution solide : compléments et corrections. In: Bulletin de Minéralogie, volume 109, 3, 1986. pp. 239-251

Équilibre entre minéraux et solutions géothermales

Activities of elements estimated in geothermal solutions are compared with activities calculated from minerals equilibria. A fair agreement is observed for many elements. The influence of temperature is generally more important than the influence of the nature of mineral association. Redox equilibria were also investigated, showing that geothermal solutions are generally strongly reduced.Les activités des éléments déterminées dans diverses eaux géothermales et sources thermales sont comparées aux activités calculées à partir d'équilibres entre minéraux. En plus d'un accord satisfaisant entre les valeurs déduites de l'analyse et celles calculées à partir des données thermodynamiques, cette étude met en évidence l'influence relativement secondaire de la nature des minéraux sur la composition de l'eau. Le facteur primordial est la température. Nous envisageons également l'ensemble des espèces redox et montrons que les propriétés redox des eaux thermales varient dans une très faible gamme correspondant à des conditions nettement réductrices.Michard Gil. Équilibre entre minéraux et solutions géothermales. In: Bulletin de Minéralogie, volume 108, 1, 1985. pp. 29-44

Discussion de l'article de M. G. Michard: Signification du potentiel redox dans les eaux naturelles - Mineralium Deposita 2, 34-36 (1967)

SCOPUS: le.jinfo:eu-repo/semantics/publishe

Constantes de formation des ions polysulfures S

A partir d'expériences d'équilibration dans le système H2S-S8H2O nous avons montré que les ions polysulfurés S2-n sont particulièrement stables. Les principaux ions polysulfures sont sans doute S52-, S42- et S62-. Les constantes d'équilibre de formation des ions polysulfures KN* selon la réaction ((n-I) /8) S8 X HS- MATHS Sn2- + H+ ont été déterminées : K6* = 10-9,62 K6* = 10-9,41 et K4* = 10-9,52 avec une précision de ± 0.2 unités sur le logarithme de la constante d'équilibre