Trace metals distribution from a serpentinite weathering at the scales of the weathering profile and its related weathering microsystems and clay minerals.

International audienceThe distribution of four trace metals - nickel (Ni), chromium (Cr), manganese (Mn) and cobalt (Co) - and some major elements is studied in a soil profile derived from a natural trace metals-bearing rock (serpentinite), at the scales of (1) the weathering profile (C and B horizons) and (2) its related weathering microsystems and clay minerals. All trace elements are relatively enriched in weathered horizons compared to the rock, in the same way as iron and aluminium whereas magnesium is largely leaching. An evolutive sequence of trace metals host phases is formed along the weathering profile from magnesian at the bottom to iron rich clay minerals, and then aluminous clay minerals and oxy-hydroxides at the top. Different chemical forms of trace metals in weathering profile have been quantified by chemical extractions. Ni and Cr remain mainly in the silicates. In contrast, Mn and Co are mainly distributed into amorphous oxy-hydroxides. In addition, trace metals contents are measured (using electron microprobe) in individual rock-forming minerals and in their specific weathering microsystem each with its specific clay mineral. Each rock-forming mineral serves as the host phase for a specific trace metal. Silicates (serpentine and clinochlore) contain Cr, Ni and Mn whereas oxides concentrate Co and Cr. In the first weathering stages, Ni and Cr are concentrated in secondary clays, especially in Fe-montmorillonite (derived from mesh serpentine minerals) and in trioctahedral vermiculite (derived from chlorite) respectively, whereas Mn and Co are mainly used to constitute secondary oxides. At the top of the profile, Mn, Co and Ni host secondary oxy-hydroxides have developed due to iron segregation

Étude préliminaire des retombées de poussières industrielles sur les sols de Gravelines (Hauts-de-France)

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Diagnostic d’eutrophisation des zones humides chassées des Hauts de France

La Fédération Régionale des Chasseurs (FRC) des Hauts de France a contacté l’Université duLittoral Côte d’Opale et son Centre Commun de Mesures, ainsi que le Laboratoired’Océanologie et de Géosciences pour établir un diagnostic d’eutrophisation des zoneshumides chassées de la région, et trouver des solutions pour endiguer le problème, le caséchéant. En effet, les résultats à un questionnaire de la FRC envoyé à ses membres indiquentque 58% des propriétaires de zones humides chassées ayant répondu (>800) rencontrent desproblèmes grandissants de gestion à cause de proliférations d’algues filamenteuses, delentilles d’eau, d’eaux vertes, et/ou d’envasement important. Le diagnostic, mené d’avril àseptembre 2018 sur 26 zones humides chassées réparties équitablement sur les 4départements, montre que la majorité d’entres-elles sont dans un état physico-chimique etbiologique médiocre voire mauvais. Celles du Nord souffrent principalement d’un excès dephosphore et de proliférations de cyanobactéries potentiellement toxiques, rendant les eauxvertes et fortement turbides, tandis que celles de la Somme et de l’Aisne souffrent plutôt d’unexcès de nitrates+nitrites engendrant des proliférations d’algues filamenteuses. Pour le Pasde-Calais, les raisons varient d’une ZH à l’autre. De manière générale, il apparaît que (1) lesdynamiques du phosphore et de l’azote Kjeldahl sont indépendantes de celle desnitrates+nitrites, (2) plus l’eau de la zone humide est riche en nitrates+nitrites plus il y ad’algues filamenteuses, et (3) une pollution au phosphore favorise les proliférationsphytoplanctoniques dominées par les cyanobactéries potentiellement toxiques, engendrant deseaux vertes et turbides. D’autre part, cette étude montre que l’indice IPLAC, développé en2016 par l’IRSTEA en collaboration avec l’ONEMA pour évaluer l’état de santé écologiquedes lacs français d’intérêt public, est applicable aux zones humides. Comme constaté par lesauteurs de cet indice, l’indice IPLAC est inversement et significativement lié aux phosphates,au phosphore total, à l’azote Kjeldahl, à la biomasse chlorophyllienne, à la turbidité, et auxproliférations de cyanobactéries. Il est donc un bon indicateur d’une pollution par lephosphore principalement, qui engendre des proliférations de cyanobactéries, une coloration« verte » de l’eau, et une certaine turbidité de l’eau. Par contre, l’indice IPLAC est inefficacepour tracer les pollutions par les nitrates et l’accumulation conséquente d’alguesfilamenteuses. Un bio-indicateur complémentaire doit donc être développé pour évaluerpleinement l’état de santé écologique d’une zone humide. Des solutions pour mitigerl’eutrophisation des zones humides échantillonnées, lorsque c’est le cas, sont proposées àchaque propriétaire sous forme de fiche individuelle. Cependant, l’idéal serait de traiter lescauses plutôt que les symptômes, c’est à dire réduire les sources de pollution à l’échelle dubassin versant. C’est particulièrement urgent pour mitiger les blooms de cyanobactériespotentiellement toxiques, dont les spécialistes prédisent une augmentation importante de leurfréquence, durée, et ampleur, en raison du réchauffement climatique

Diagnostic d’eutrophisation des zones humides chassées des Hauts de France

La Fédération Régionale des Chasseurs (FRC) des Hauts de France a contacté l’Université duLittoral Côte d’Opale et son Centre Commun de Mesures, ainsi que le Laboratoired’Océanologie et de Géosciences pour établir un diagnostic d’eutrophisation des zoneshumides chassées de la région, et trouver des solutions pour endiguer le problème, le caséchéant. En effet, les résultats à un questionnaire de la FRC envoyé à ses membres indiquentque 58% des propriétaires de zones humides chassées ayant répondu (>800) rencontrent desproblèmes grandissants de gestion à cause de proliférations d’algues filamenteuses, delentilles d’eau, d’eaux vertes, et/ou d’envasement important. Le diagnostic, mené d’avril àseptembre 2018 sur 26 zones humides chassées réparties équitablement sur les 4départements, montre que la majorité d’entres-elles sont dans un état physico-chimique etbiologique médiocre voire mauvais. Celles du Nord souffrent principalement d’un excès dephosphore et de proliférations de cyanobactéries potentiellement toxiques, rendant les eauxvertes et fortement turbides, tandis que celles de la Somme et de l’Aisne souffrent plutôt d’unexcès de nitrates+nitrites engendrant des proliférations d’algues filamenteuses. Pour le Pasde-Calais, les raisons varient d’une ZH à l’autre. De manière générale, il apparaît que (1) lesdynamiques du phosphore et de l’azote Kjeldahl sont indépendantes de celle desnitrates+nitrites, (2) plus l’eau de la zone humide est riche en nitrates+nitrites plus il y ad’algues filamenteuses, et (3) une pollution au phosphore favorise les proliférationsphytoplanctoniques dominées par les cyanobactéries potentiellement toxiques, engendrant deseaux vertes et turbides. D’autre part, cette étude montre que l’indice IPLAC, développé en2016 par l’IRSTEA en collaboration avec l’ONEMA pour évaluer l’état de santé écologiquedes lacs français d’intérêt public, est applicable aux zones humides. Comme constaté par lesauteurs de cet indice, l’indice IPLAC est inversement et significativement lié aux phosphates,au phosphore total, à l’azote Kjeldahl, à la biomasse chlorophyllienne, à la turbidité, et auxproliférations de cyanobactéries. Il est donc un bon indicateur d’une pollution par lephosphore principalement, qui engendre des proliférations de cyanobactéries, une coloration« verte » de l’eau, et une certaine turbidité de l’eau. Par contre, l’indice IPLAC est inefficacepour tracer les pollutions par les nitrates et l’accumulation conséquente d’alguesfilamenteuses. Un bio-indicateur complémentaire doit donc être développé pour évaluerpleinement l’état de santé écologique d’une zone humide. Des solutions pour mitigerl’eutrophisation des zones humides échantillonnées, lorsque c’est le cas, sont proposées àchaque propriétaire sous forme de fiche individuelle. Cependant, l’idéal serait de traiter lescauses plutôt que les symptômes, c’est à dire réduire les sources de pollution à l’échelle dubassin versant. C’est particulièrement urgent pour mitiger les blooms de cyanobactériespotentiellement toxiques, dont les spécialistes prédisent une augmentation importante de leurfréquence, durée, et ampleur, en raison du réchauffement climatique

Bioavailability of Sediment-borne Lead for ragworms (Hediste diversicolor) Investigated by Lead Isotopes.

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Physical properties of muddy sediments from French Guiana

International audienceThe North West migration of long and discontinuous mud banks along the French Guiana coast has been extensively studied during the past years, in particular with a large-scale vision, which consequently has integrated morpho and hydrodynamic data. The aims of the present paper were to use intrinsic sediment properties (grain-size, mineralogy, concentration, and cohesion) to (1) highlight the sedimentary conditions during the consolidation processes from fluid deposit to vegetation development, and (2) verify the apparent homogeneously derived sedimentary facies.Two intertidal transects, Macouria and Cayenne, were compared from the coast to offshore. Their altitude averages of 1 m and 2.8 m above mean sea level, respectively, were different enough to compare the influence of the hydrodynamic impact and emersion time on their sediment properties. The latter, i.e. grain size distribution, mineralogical content, mud concentration, and shear strength (cohesion), were determined from sampled surface sediments (first cm) and along sediment cores (20–30 cm depth) from each transect. A specific X-ray technique was applied to the whole core to differentiate clearly its thin layers.On both intertidal sites, the grain size dominated by the fine silt fraction (2–20 μm) and the bulk mineralogy characterized by five major minerals (quartz, feldspars, chlorite, illite, and kaolinite) appeared homogeneous along both transects and cores. In spite of this apparent uniformity of particle size and mineralogical parameters, as well as for visual observation along the core, high precision X-rays still showed a cyclic sedimentation at a micro-scale level. This cyclicity with intercalation of fine layers was related to distinct dynamic deposits marked by both tidal processes and hydrodynamic factors (swell propagation). The cohesion and concentration results were dependent on the topography, where high topography was characterized by sediments with high cohesion and concentration values, and vice versa. A comparison between these two parameters was done to define critical limits between soft and stiff muds, as well as unvegetated and colonized muds. The favorable intrinsic sedimentary properties for consolidation and colonization were also discussed according to the field observations and bibliographic dat

Fissure and mineral weathering impacts on heavy metal distribution in sludge-amended soil

International audienceThe purpose of this study was to follow the distribution and migration of the metallic trace elements (MTE) zinc (Zn), lead (Pb) and cadmium (Cd) in a sludge-amended soil, both at the metric scale of the bulk soil horizons and at the micrometric scale of mineral weathering microsites. In the soil scale approach, the MTE contents determined by ICP-AES and ICP-MS analyses in amended and control soil samples were compared through enrichment factor calculation to assess the extent to which spread MTE may have moved throughout the soil profile. In the mineral scale approach, the MTE were analysed on thin sections in specific weathering microsystems including (1) rock-forming minerals (amphiboles, biotites, plagioclases) and their specific weathering clay minerals; (2) weathering clayey plasma, which obliterates the original rock structure with newly-formed clay minerals; and (3) the fissural network with its clay minerals. The purpose of this mineralogical approach, using X-ray diffraction (XRD) for mineral identification and electron probe microanalyses (EPMA) for MTE analyses, was to determine where and at which concentrations spread MTE can concentrate within the soil and weathered rock. The chemical analyses of MTE in the bulk samples reveal strong Cd and Pb accumulation at the surface of the amended soil due to anthropic contamination. Cadmium undergoes a vertical migration in deeper soil horizons, whereas Zn and Pb do not show significative transfer within the amended soil. Accurate MTE analyses in weathering microsites indicate that, except in plagioclase microsites, (1) Zn and Cd accumulate in clay minerals from surface horizons and migrate downwards through the fissural system, and (2) Pb does not show any significant mobility throughout the amended soil. The MTE migration evidenced through the fissural system gives rise to two main environmental problems. Zn and Cd have the potential to move several meters deep along fissures in the soil profiles and may represent potential contaminants for unconfined aquifer. Secondly, because the plant root system grows preferentially along soil fissural pattern, it may adsorb MTE

Isotopes du plomb et caractérisation de la contamination de sédiments de rivière au voisinage de fonderies historiques: cas du canal de la Deûle (Région Haut de France)

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A better understanding of mud cracking processes gained from in situ measurements on an intertidal mudflat in French Guiana

International audienceRecent work has revealed that desiccation processes can occur over elongated off-shore mud bars that form on Amazon-derived mud banks. Once mud cracks appear there, opportunistic mangroves, i.e. Avicennia germinans, commence the colonization process. Through the advantage of buoyancy of the mangrove propagules which can be trapped by the mud cracks, propagules can establish sometimes 1-2 kilometres offshore of the adult colonies. In a few months, these pioneer colonies become new sources of propagules, promoting rapid ecosystem expansion. Thus, mud cracking may be seen as an important factor in mangrove resilience. To investigate this phenomenon, a field experiment was conducted during the 2008 equinoctial spring tide in French Guiana in a study area located on the landward face of an elongated mud bar. A digital elevation model (DEM) was computed from DGPS and high-resolution laser station data. Meteorological data including air temperature and humidity, wind speed and orientation, rainfall, solar radiation, temperatures of the mud surface and at a depth of 30-cm, were recorded. Additionally, the water content of the upper mud layer was monitored along a topographic transect. An installed camera took instant photographs every half hour. Preliminary results show that climatic conditions are not the main parameter controlling the desiccation process. Strengthening and consolidation of mud are mainly controlled by mud elevation (water loss by draining) and by the local tidal signal (determinant in spatial behaviour). The DEM enabled computation of the tidal frequency emersion signal, an important tool in analyzing the spatio-temporal patterns of mud cracking and mangrove colonization

Initiation and development of tectonic stylolite -Vein system in micritic limestone (Les Matelles, France)

International audienceIn this study, we describe the characteristics of tectonic stylolites and related veins affecting a low-porosity micritic limestone (Jurassic carbonates, Les Matelles, South of France) in order to unravel the conditions of initiation and interaction between pressure-solution and fracturing in such rock. Field description, various petrographic and microstructural investigations (cathodoluminescence, SEM imaging, EBSD analysis), and petrophysical/geochemical analyses (Hg porosimetry, XRD, EPMA) are used. We document that pressure-solution initiates at micropores and propagates along calcite grain contacts, connecting surrounding stylolite microsegments, and progressively concentrates insoluble material such as clays and siliceous particles. The dissolved material is evacuated to the veins where the newly-formed porous space is progressively filled by calcite cement. These deformation processes are strictly restricted to the stylolitic interface and veins, as no modification of porosity or grain deformation is detected in the neighboring host rock. This is due to the low-permeability of the surrounding host rock impeding the evacuation of dissolved material and fluids through interstitial porosity around the pressure-solution zone, leading to overpressure and veins formation. The water release and microporosity caused by diagenesis of the clay fraction (smectite-illite transformation) are discussed as key diagenetic processes instigating the conditions of pressure-solution initiation, then tectonic stylolite formation in low-porosity limestones