Géochimie, spéciation et mobilité des éléments traces métalliques (Fe, Ni, Cr et Mn) au sein des sédiments du lagon de Nouvelle-Calédonie

New Caledonia is covered on 33% of ultramafic rocks enriched in trace metals elements (Fe, Ni, Cr, Mn and Co). The erosion of these massifs represents an important source of these trace metals towards the lagoon, partially registered on the World List Heritage by UNESCO for its biodiversity. Understanding the geochemical cycle of these toxic trace metals is essential to evaluate the potential impacts on the biodiversity of this ecosystem. X-ray absorption spectroscopy (XAS) has shown that the contribution of iron sulfides is low, Ni and Fe are bearing by goethite and clays. These latter minerals representing the major phase for the Fe and Ni speciation were identified as chrysotile and green clays: a smectite (nontronite), a mica (glauconite) and a serpentine like a greenalite/berthierine. The clay minerals cycle and the green clay plays a major role in the Fe and Ni cycle, but also in Mn and to a lesser extent in the Cr cycles. The Mn speciation is shared by the clay minerals and the carbonates species whereas Cr is bearing by goethite and chromite inherited of laterite and in lesser extent by clay minerals. Cr is totally present under the reduced form which is the less toxic form. This absence of Cr(VI) is linked to the absence of Mn oxides in sediments avoiding the oxidation of Cr(III) to Cr(VI) and thus limits the impact on the environment. Finally, the chemical extractions of these metals show a relatively low bioavailability of trace metals except for Mn implying an effective long-term trapping for trace metals and thus limiting the environmental impacts even if the concentrations extracted are not negligible and can be threaten the biodiversity of the lagoon.La Nouvelle-Calédonie est formée sur 33% de son territoire de roches ultrabasiques enrichies en éléments traces métalliques (ETM) (Fe, Ni, Cr, Co et Mn). L’érosion de ces massifs représente une source importante de ces ETM vers le lagon, partiellement inscrit au patrimoine mondial par l’UNESCO pour sa biodiversité. La compréhension des cycles géochimiques de ces éléments toxiques apparaît donc essentielle pour évaluer les impacts possibles sur la biodiversité de cet écosystème. La spectroscopie d’absorption des rayons X a montré que la contribution des sulfures de fer est très faible, Ni et Fe sont portés par la goethite et les argiles. Ces derniers étant la phase majeure de ces deux éléments ont été identifiés comme du chrysotile, et des greens clays : une smectite (nontronite), un mica (glauconite) et une serpentine de type greenalite/berthierine. Le cycle des argiles joue donc un rôle majeur dans le cycle de Fe et de Ni mais aussi de Mn et dans une moindre mesure de Cr. La spéciation de Mn se partage entre les argiles et les carbonates alors que Cr est surtout porté par la goethite et la chromite hérité des massifs et en dernier par les argiles. Il est important de noter que Cr se trouve sous sa forme réduite correspondant à sa forme la moins toxique. L’absence de Cr(VI) est liée à l’absence des oxydes de Mn dans les sédiments, permettant de ne pas oxyder Cr(III) en Cr(VI). Enfin, les extractions chimiques montrent une biodisponibilité relativement faible de ces métaux à l’exception de Mn impliquant un piégeage efficace, limitant ainsi les impacts sur l’environnement même si les concentrations extraites sont loin d’être négligeables pour la biodiversité du lagon

Geochemistry, speciation and mobility of traces metals (Fe, Ni, Cr and Mn) in lagoon sediments from New Caledonia

La Nouvelle-Calédonie est formée sur 33% de son territoire de roches ultrabasiques enrichies en éléments traces métalliques (ETM) (Fe, Ni, Cr, Co et Mn). L’érosion de ces massifs représente une source importante de ces ETM vers le lagon, partiellement inscrit au patrimoine mondial par l’UNESCO pour sa biodiversité. La compréhension des cycles géochimiques de ces éléments toxiques apparaît donc essentielle pour évaluer les impacts possibles sur la biodiversité de cet écosystème. La spectroscopie d’absorption des rayons X a montré que la contribution des sulfures de fer est très faible, Ni et Fe sont portés par la goethite et les argiles. Ces derniers étant la phase majeure de ces deux éléments ont été identifiés comme du chrysotile, et des greens clays : une smectite (nontronite), un mica (glauconite) et une serpentine de type greenalite/berthierine. Le cycle des argiles joue donc un rôle majeur dans le cycle de Fe et de Ni mais aussi de Mn et dans une moindre mesure de Cr. La spéciation de Mn se partage entre les argiles et les carbonates alors que Cr est surtout porté par la goethite et la chromite hérité des massifs et en dernier par les argiles. Il est important de noter que Cr se trouve sous sa forme réduite correspondant à sa forme la moins toxique. L’absence de Cr(VI) est liée à l’absence des oxydes de Mn dans les sédiments, permettant de ne pas oxyder Cr(III) en Cr(VI). Enfin, les extractions chimiques montrent une biodisponibilité relativement faible de ces métaux à l’exception de Mn impliquant un piégeage efficace, limitant ainsi les impacts sur l’environnement même si les concentrations extraites sont loin d’être négligeables pour la biodiversité du lagon.New Caledonia is covered on 33% of ultramafic rocks enriched in trace metals elements (Fe, Ni, Cr, Mn and Co). The erosion of these massifs represents an important source of these trace metals towards the lagoon, partially registered on the World List Heritage by UNESCO for its biodiversity. Understanding the geochemical cycle of these toxic trace metals is essential to evaluate the potential impacts on the biodiversity of this ecosystem. X-ray absorption spectroscopy (XAS) has shown that the contribution of iron sulfides is low, Ni and Fe are bearing by goethite and clays. These latter minerals representing the major phase for the Fe and Ni speciation were identified as chrysotile and green clays: a smectite (nontronite), a mica (glauconite) and a serpentine like a greenalite/berthierine. The clay minerals cycle and the green clay plays a major role in the Fe and Ni cycle, but also in Mn and to a lesser extent in the Cr cycles. The Mn speciation is shared by the clay minerals and the carbonates species whereas Cr is bearing by goethite and chromite inherited of laterite and in lesser extent by clay minerals. Cr is totally present under the reduced form which is the less toxic form. This absence of Cr(VI) is linked to the absence of Mn oxides in sediments avoiding the oxidation of Cr(III) to Cr(VI) and thus limits the impact on the environment. Finally, the chemical extractions of these metals show a relatively low bioavailability of trace metals except for Mn implying an effective long-term trapping for trace metals and thus limiting the environmental impacts even if the concentrations extracted are not negligible and can be threaten the biodiversity of the lagoon

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Production of carbon-containing pyrite spherules induced by hyperthermophilic Thermococcales: a biosignature?

International audienceThermococcales, a major order of hyperthermophilic archaea inhabiting iron- and sulfur-rich anaerobic parts of hydrothermal deep-sea vents, are known to induce the formation of iron phosphates, greigite (Fe 3 S 4 ) and abundant quantities of pyrite (FeS 2 ), including pyrite spherules. In the present study, we report the characterization of the sulfide and phosphate minerals produced in the presence of Thermococcales using X-ray diffraction, synchrotron-based X ray absorption spectroscopy and scanning and transmission electron microscopies. Mixed valence Fe(II)-Fe(III) phosphates are interpreted as resulting from the activity of Thermococcales controlling phosphorus–iron–sulfur dynamics. The pyrite spherules (absent in abiotic control) consist of an assemblage of ultra-small nanocrystals of a few ten nanometers in size, showing coherently diffracting domain sizes of few nanometers. The production of these spherules occurs via a sulfur redox swing from S 0 to S –2 and then to S –1 , involving a comproportionation of (-II) and (0) oxidation states of sulfur, as supported by S-XANES data. Importantly, these pyrite spherules sequester biogenic organic compounds in small but detectable quantities, possibly making them good biosignatures to be searched for in extreme environments

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Experimental redox transformations of uranium phosphate minerals and mononuclear species in a contaminated wetland

International audienceReducing conditions and high organic carbon content make wetlands favorable to uranium (U) sequestration. However, such environments are subjected to water-table fluctuations that could impact the redox behavior of U and its mobility. Our previous study on U speciation in a contaminated wetland has suggested a major role of water-table redox fluctuations in the redistribution of U from U(IV)-phosphate minerals to organic U(VI) and U(IV) mononuclear species. Here, we investigate the mechanisms of these putative processes by mimicking drying or flooding periods via laboratory incubations of wetland samples. LCF-XANES and EXAFS analyses show the total oxidation/reduction of U(IV)/U(VI)-mononuclear species after 20 days of oxic/anoxic incubation, whereas U-phosphate minerals are partly oxidized/reduced. SEM-EDXS combined with μ-XRF and μ-XANES analyses suggest that autunite Ca(UO2)2(PO4)2⋅11H2O is reduced into lermontovite U(PO4)(OH)⋅H2O, whereas oxidized ningyoite CaU(PO4)2⋅2H2O is locally dissolved. The release of U from this latter process is observed to be limited by U(VI) adsorption to the soil matrix and further re-reduction into mononuclear U(IV) upon anoxic cycling. Analysis of incubation waters show, however, that dissolved organic carbon enhances U solubilization even under anoxic conditions. This study brings important information that help to assess the long-term stability of U in seasonally saturated organic-rich contaminated environments

Carbonate Facilitated Mobilization of Uranium from Lacustrine Sediments under Anoxic Conditions

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