Dating multistage paleofluid percolations: A K-Ar and O/O study of fracture illites from altered Hercynian plutonites at the basement/cover interface (Poitou High, France)

Isotopic age determination of fluid events in basement fractures is of prime importance for the reconstruction of paleo-hydraulic regimes. However, material from fracture or wall-rock is not generally easily dated. In the case of fractures filled by minerals such as phyllosilicates, it can be expected that subsequent fluid circulation may cause partial alteration or recrystallization of the previous phyllosilicates, disturbing the isotope signature. Using a specific methodology, including the extraction process without contamination of the finest (<0.2 μm) authigenic fractions, identification of clay fractions by X-ray diffraction (XRD), scanning electron microscopy, and electron microprobe, analysis of stable isotope composition (D/H, δ¹⁸O) and K-Ar dating, fluid event ages can be estimated. This methodology was applied to the northwestern margin of the French Massif Central, where basinal brines have interacted with the sedimentary cover (infra-Toarcian formations) and the fractured Hercynian crystalline basement. During periods of unknown age, precipitates from brines sealed most basement fractures as carbonates, and a recrystallization of early Hercynian phengite (+chlorite) into illite and illite/smectite minerals occurred in the earlier fractures.XRD patterns show that the clay fraction is dominated by illite and ordered mixed-layer minerals having a relatively high amount of illite (I-S R3). In the shallowest samples beneath the paleo-surface, a late assemblage is identified as a mixture of I-S minerals, a part of them being characterized by significant amounts of swelling layers (I-S R0).The δD values of the clay fractions are fairly constant around −50 ± 10‰, but δ¹⁸O values of the same clay fractions display a wide range from 8‰ to 18‰ (SMOW). The study of the size fractions between 2 and 0.2 μm reveals a correlation between K-Ar ages and δ¹⁸O values, which cannot be related to a change in the relative amounts of clay minerals. Deep fractures (at depths of 570 and 923 m) are characterized by δ¹⁸O values around 10.3 ± 0.4‰ SMOW and old ages ranging from 253 to 272 Ma. The finest fractions (<0.2 μm) of clays located at shallow level (<300 m) beneath the paleo-surface yield the highest δ¹⁸O values (15.7-18.2‰) and the youngest K-Ar dates (188 to 198 Ma). These dates are likely to record the extensional episode and thermal anomaly related to the rifting of central Atlantic Ocean (c. 190 Ma). The other important thermal/extensional episode which affected the Aquitaine Basin, namely the opening of the Gascogne gulf at c. 120 Ma, is not detected by the K-Ar method on the clay fractions. We believe that resetting of clay-type K-Ar ages linked to recrystallization was favoured in the upper part of the basement where the ancient (Hercynian) clay minerals were destabilized by subaerial low-temperature alteration during Permo-Triassic times.From a methodologic point of view, the analytical characterization of several size fractions of clays from reactivated fractures, using both stable isotope geochemistry and K-Ar isotope geochemistry, can be considered as a powerful tool for dating fluid events when no suitable material for conventional methods is available.     

Micro‐inertia origin of instabilities in granular materials

Within the framework of the second‐order work theory, the onset of instabilities is explored numerically in loose granular materials through three‐dimensional DEM simulations. Stress controlled directional analysis are performed in Rendulic's plane, and a particular attention is paid to transient evolutions at the microscale. Thanks to a micromechanical analysis, the onset and development of transient mechanical instabilities is explored. It is shown that these instabilities result from the unjamming and bending of a few force chains associated with a local burst of kinetic energy. This burst of kinetic energy propagates to the whole sample and provokes a generalized unjamming of force chains. As force chains buckle, a phase transition from a quasi‐static to an inertial regime is observed. At the macroscopic scale, this results in a transient softening and a loss of controllability. After the collapse of existing force chains, the development of plastic strain is eventually stopped as new stable force chains are built.     

Long term high frequency sediment observatory in an alpine catchment: The Arc-Isère rivers,France

We present a dataset on to the Arc-Isère long-term environmental research observatory, which is part of the Rhône Basin Long Term Environmental Research Observatory. This alpine catchment located in the French Alps is characterized by high Suspended Particulate Matter (SPM) in anthropogenized valleys. Suspended Sediment Concentrations (SSC) naturally observed in the river are very high, ranging from a few tens of milligrams per litre at low flow to tens of grams per litre during major natural hydrological events (floods, debris flows) or river dam hydraulic flushes. One research objective related to this site is to better understanding the SSC dynamics along the river using a system of nested catchments (Arvan, Arc, and Isère) in order to assess both temporal and spatial dynamics. The data allow the quantification of fine sediment yields and also the evaluation of possible morphological changes due to fine sediment deposition or resuspension. Additionally, the observatory database support studies on contaminants (either dissolved or particulate contaminants). Our monitoring includes six stations with high frequency (2–30 min) streamflow, SSC measurement using turbidity sensors, and associated automatic sampling. Discharge is measured via water level measurements and a rating curve. The oldest station (Grenoble-campus) started recording discharge and concentration data from April 2006 while others stations were built between 2009 and 2011. Data are available in an online data website called ‘Base de Données des Observatoires en Hydrologie’ (Hydrological observatory database, https://bdoh.irstea.fr/ARC-ISERE/ ) with a DOI reference for the dataset. The hydrological and sediment transport time series are stored, managed and made available to a wide community with unfettered access in order to be used at their full extent. This database is used as a data exchange tool for both scientists and operational end-users and there is an associated online tool to compute integrated fluxes.     

Methylmercury in tailings ponds of Amazonian gold mines (French Guiana): Field observations and an experimental flocculation method for in situ remediation

Sites of monomethylmercury (MMHg) production in Amazonian regions have been identified in hydraulic reservoirs, lake sediments and wetlands, but tailings ponds have not yet received sufficient attention for this purpose. This work evidenced high MMHg production within the water column and the interstitial water of two tailings ponds of French Guiana Au mines located; (i) in a small scale exploitation (Combat) where Hg was used for Au amalgamation, and (ii) in an industrial on-going Au mine (Yaoni) processing without Hg. The (MMHg)_D maximum (2.5 ng L⁻¹) occurred in the oxic water column above the sediment-water interface (SWI) of the most recent tailings pond (Combat), where the substrate was fresh, the redox transition was sharp and the pool of total Hg was large. In the Yaoni pond, the (MMHg)_D maximum concentration (1.4 ng L⁻¹) was located at the SWI where suboxic conditions prevailed. Using the (MMHg)_D concentration as a proxy for Hg methylation rates, the present results show that Hg methylation may occur in various redox conditions in tailings ponds, and are favored in areas where the organic matter regeneration is more active.A 3-month long laboratory experiment was performed in oxic and anoxic boxes filled with high turbidity waters from the Combat Au mine to simulate tailings ponds. Slaked lime was added in an experimental set (2 mg L⁻¹) and appeared to be very efficient for the reduction of suspended particulate matter (SPM) to environmentally acceptable concentrations. However, at the end of the experiment, large (MMHg)_D concentrations were monitored under treated anoxic conditions with the (MMHg)_D maximum located at the SWI above the Fe-reducing zones. No (MMHg)_D was detected in oxic experiments. The use of slaked lime for SPM decantation appears to be an efficient and non-onerous process for Au miners to avoid Hg methylation in tailings ponds when it is combined with rapid drainage of the mine waters. A subsequent human intervention is however necessary for the recovery of soil structure through the cover of dried ponds with organic rich materials and reforestation to avoid the stagnation of rain waters and the occurrence of anoxia.     

A search for starlight reflected from HD 75289b

We have used a Doppler tomographic analysis to conduct a deep search for the starlight reflected from the planetary companion to HD 75289. In four nights on VLT(UT2)/UVES in 2003 January, we obtained 684 high-resolution echelle spectra with a total integration time of 26 h. We establish an upper limit on the geometric albedo of the planet   p < 0.12  (to the 99.9 per cent significance level) at the most probable orbital inclination   i ≃ 60°  , assuming a grey albedo, a Venus-like phase function and a planetary radius   R _p= 1.6 R_Jup  . We are able to rule out some combinations of the predicted planetary radius and atmospheric albedo models with high, reflective cloud decks.     

Structures tectoniques et contraintes de cheminement des eaux dans les aquifères karstiques du barrois (Lorraine/Champagne,France)

Résumé

Entre Lorraine et Champagne, dans l’est du Bassin parisien, au contact des calcaires portlandiens du Barrois et des sables et argiles crétacés du Perthois, se développe un karst couvert. Le paysage de côtes est ici perturbé par une tectonique souple (synclinal de Treveray) et cassante (fossé d’effon-drement de la Marne) déterminant l’organisation spatiale des écoulements. En période d’étiage, les méthodes hydrologiques (profils hydrologiques lissés, traçages), permettent de montrer l’influence de cette tectonique d’ondulation et de basculement sur l’écoulement des rivières barroises (Saulx, Marne). Fractures et microfractures influencent la zone de transit vertical des eaux du karst (zone des puits) tandis que les failles compartimentent les aquifères (système du Rupt-du-Puits, 21 km de conduits, 13 km²). Les méthodes d’études hydrologiques, confirment le basculement de la région vers l’ouest déduit des comparaisons de nivellement. © Elsevier, Paris     

Accretion of Saturn’s mid-sized moons during the viscous spreading of young massive rings: Solving the paradox of silicate-poor rings versus silicate-rich moons

The origin of Saturn’s inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) and Saturn’s rings is debated. Charnoz et al. [Charnoz, S., Salmon J., Crida A., 2010. Nature 465, 752–754] introduced the idea that the smallest inner moons could form from the spreading of the rings’ edge while Salmon et al. [Salmon, J., Charnoz, S., Crida, A., Brahic, A., 2010. Icarus 209, 771–785] showed that the rings could have been initially massive, and so was the ring’s progenitor itself. One may wonder if the mid-sized moons may have formed also from the debris of a massive ring progenitor, as also suggested by Canup [Canup, R., 2010. Nature 468, 943–946]. However, the process driving mid-sized moon accretion from the icy debris disks has not been investigated in details. In particular, Canup’s (2010) model does not seem able to explain the varying silicate contents of the mid-sized moons (from 6% to 57% in mass). Here, we explore the formation of large objects from a massive ice-rich ring (a few times Rhea’s mass) and describe the fundamental properties and implications of this new process. Using a hybrid computer model, we show that accretion within massive icy rings can form all mid-sized moons from Mimas to Rhea. However in order to explain their current locations, intense dissipation within Saturn (with Q_p < 2000) is required. Our results are consistent with a satellite origin tied to the rings formation at least 2.5 Gy ago, both compatible with either a formation concurrent to Saturn or during the Late Heavy Bombardment. Tidal heating related to high-eccentricity post-accretional episodes may induce early geological activity. If some massive irregular chunks of silicates were initially present within the rings, they would be present today inside the satellites’ cores which would have accreted icy shells while being tidally expelled from the rings (via a heterogeneous accretion process). These moons may be either mostly icy, or, if they contain a significant amount of rock, already differentiated from the ice without the need for radiogenic heating. The resulting inner mid-sized moons may be significantly younger than the Solar System and a ∼1 Gyr formation delay is possible between Mimas and Rhea. The rings resulting from this process would evolve to a state compatible with current mass estimates of Saturn’s rings, and nearly devoid of silicates, apart from isolated silicate chunks coated with ice, interpreted as today Saturn’s rings’ propellers and ring-moons (like Pan or Daphnis).     

Benthic and hyporheic invertebrate assemblages along a gradient of increasing streambed colmation by fine sediment

Streambed colmation by fine sediment, e.g. the deposition, accumulation and storage of fines in the substrate, is a major environmental concern throughout the world. Nevertheless, the ecological effects of streambed colmation on both benthic and hyporheic invertebrate assemblages have rarely been considered simultaneously. We studied a continuum of a naturally increasing percentage of fine sediment in three temperate rivers and hypothesized that the increasing percentage of fine sediment would decrease both benthic and hyporheic invertebrate densities and diversities, and reduce the similarities between them. To test these hypotheses, we first compared heavily, moderately and lightly clogged reaches located in downwelling areas and sampled invertebrates in the benthic zone and at 3 different depths (10, 30 and 50 cm) in the hyporheic zone. Secondly, we modified the sediment grain size distribution experimentally by increasing the percentage of fine sediment and using artificial substrates. The increasing colmation halved the hyporheic taxonomic richness and reduced benthic and hyporheic densities to a third. Some taxa were found in both zones, mainly in high colmation (e.g. Baetidae) or low colmation contexts (e.g. Orthocladiinae, Cyclopoida and Harpacticoida). The dissimilarity between benthic and hyporheic fauna (only at ?50 cm) was significantly higher in heavily clogged reaches than in moderately and lightly clogged ones, suggesting reduced vertical exchange of invertebrates or differential impacts between zones. The total abundance, taxonomic richness, percentage of EPT taxa and densities of most organisms observed using the artificial substrates decreased linearly with the increasing percentage of fine sediment in the experiment. Only the Ephemeroptera Caenis spp. and Heptageniidae disappeared above 30 and 50 % of fine sediment, respectively, suggesting that the response to increasing colmation are strongly taxon-specific. High amount of fine sediments within the substrate significantly decreased habitat quality for benthic and hyporheic invertebrates and thus limit the production of streams and their capacity to recover after disturbance. Moreover, the use of hyporheic invertebrates seems more relevant than benthic invertebrates to assess the effect of colmation and thus could be tested in future research as indicators.     

Building a VO-compliant Radio Astronomical DAta Model for Single-dish radio telescopes (RADAMS)

The Virtual Observatory (VO) is becoming the de-facto standard for astronomical data publication. However, the number of radio astronomical archives is still low in general, and even lower is the number of radio astronomical data available through the VO. In order to facilitate the building of new radio astronomical archives, easing at the same time their interoperability with VO framework, we have developed a VO-compliant data model which provides interoperable data semantics for radio data. That model, which we call the Radio Astronomical DAta Model for Single-dish (RADAMS) has been built using standards of (and recommendations from) the International Virtual Observatory Alliance (IVOA). This article describes the RADAMS and its components, including archived entities and their relationships to VO metadata. We show that by using IVOA principles and concepts, the effort needed for both the development of the archives and their VO compatibility has been lowered, and the joint development of two radio astronomical archives have been possible. We plan to adapt RADAMS to be able to deal with interferometry data in the future.     

An exceptional rocky shore preserved during Oligocene (Late Rupelian) transgression in the Upper Rhine Graben (Mainz Basin,Germany)

The Early Oligocene (Late Rupelian) Alzey Formation (Mainz Basin, Upper Rhine Graben, Germany) records the development of a rocky coast depositional system during transgression. The formation unconformably overlies Permian bedrock across a composite transgressive ravinement surface. Exposure of the surface shows a succession of subplanar bedrock terraces, separated by near‐vertical risers. Terraces show a broad staircase geometry and display wave‐erosional features (notches, sea stacks, furrows). Detailed sedimentological and palaeoecological investigations reveal prograding beachface and shoreface depositional units that overlie terraces and are adjacent to risers. Terraces are interpreted as wave‐cut platforms, backed by palaeocliffs. The staircase architecture records the episodic landward migration of palaeoshorelines onto palaeotopographic highs during the Early Oligocene. Stacking patterns of gravelly beach and shoreface associated units (facies tracts) indicate successive episodes of terrace cutting, beach development, drowning and shoreline backstepping during an overall relative sea‐level rise. The exceptional preservation of the stair‐cased rocky shore may be attributed to a highly jerky rising relative sea‐level, as the result of the conjugated effects of rift‐controlled tectonic subsidence and eustatic sea‐level oscillations. Copyright © 2012 John Wiley & Sons, Ltd.