The post-depositional reactivity of iron and managanese in the sediments of a macrotidal estuarine system |
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| Authors: | B Ouddane D Boust E Martin J C Fischer M Wartel |
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| Institution: | 1. Université des Sciences et Technologies et Lille Laboratoire de Chimie Analytique et Marine (LCAM) UPRESA CRNS 8013 Bat, C8 2ème étage, 59655, Villeneuve d’Ascq Cedex, France
2. Laboratoire d’Etudes Radioécologiques de la Fa?ade Atlantique (LERFA), IPSN B.P. 10, 50130, Octeville, France
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| Abstract: | Four cores of anoxic sediments were collected from the Seine estuary to assess the early diagenesis pathways leading to the
formation of previously reactive phase. Pore waters were analyzed for dissolved iron (Fe) and manganese (Mn) and different
ligands (e.g., sulfate, chloride, total inorganic carbon). The anoxic zone is present up to the first centimeter depth, in
these conditions the reduction of Mn and Fe oxides and SO4
2− was verified. The sulfate reduction was well established with a subsequent carbon mineralization in the NORMAI94 core. The
chemical speciation of Mn and Fe in the dissolved and solid phases was determined. For the dissolved phase, thermodynamic
calculations were used to characterize and illustrate the importance of carbonate and phosphate phases as sinks for Fe and
Mn. The ion activity product (IAP) of Fe and Mn species was compared to the solubility products (Ks) of these species. In
the solid phase, the presence of higher concentration of calcium carbonate in the Seine sediments is an important factor controlling
Mn cycle. The carbonate-bound Mn can reach more than 75% of the total concentration. This result is confirmed by the use of
electron spin resonance (ESR) spectroscopy. The reduction of Fe is closely coupled to the sulfate reduction by the formation
of new solid phases such as FeS and FeS2, which can be regarded as temporal sinks for sulfides. These forms were quantified in all cores as acid volatile sulfide
(AVS: FeS+ free sulfide) and chromium reducible sulfide (CRS: FeS2+elemental sulfur S0). |
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