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The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitised ultramafic watershed (New Caledonia)

Abstract : In New Caledonia, one third of the Island is composed of ultramafic rocks, and lateritic soils enriched in Fe, Ni and Cr. Open-cast mining occurs all around the Island, and processes of erosion and sedimentation, which occur naturally along the coastline, are strongly amplified by mining activities. Due to their position, at the interface between land and sea, mangroves receive extensive amounts of particles emanating from rivers through estuaries. The purpose of this study is to understand the distribution and partitioning of some transitional metals (Fe, Mn, Ni, Cr) in sediments and pore-waters in a mangrove swamp, which is situated downstream of a catchment characterized by lateritic soils that were exploited a century ago. Quantitative analyses on bulk and after selective extraction, were carried out on cores collected along a transect within the intertidal zone, i.e. beneath a Rhizophora stylosa stand, an Avicennia marina stand, and within a salt flat. Mean metal concentrations were (μmol g-1): Fe (1997)>Ni (44.2)>Cr (31.9)>Mn (8.8). Thus, Ni, Cr and Fe concentrations in this study are substantially higher than mangrove world average. In addition, Ni concentrations are 10 to 100 times higher than in other New Caledonian mangrove developing downstream of a catchment not composed of ultramafic rocks. The studied mangrove is characterized by gradients of water and organic contents with depth and along the intertidal zone, which induced different redox conditions, and thus different metals partitioning. Transitional metals are deposited in the mangrove mainly as oxides and/or oxyhydroxydes, that are subsequently dissolved by bacteria for the decomposition of organic matter, and which leads to a strong increase of metals in the dissolved phase. Then, dissolved metals precipitated with organic and sulphide compounds. To conclude, organic diagenesis in mangrove sediments leads to the transfer of transitional metals from oxide form to organic and sulphide forms.
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Cyril Marchand, J.M. Fernandez, B. Moreton, L. Landi, Elisabeth Lallier-Vergès, et al.. The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitised ultramafic watershed (New Caledonia). Chemical Geology, Elsevier, 2012, 300-301, pp.70-80. ⟨10.1016/j.chemgeo.2012.01.018⟩. ⟨insu-00666172⟩

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