Manganese dioxide concretions on granite outcrops in an intertidal area (cove of Belmont, France), II. Rapid manganese dioxide formation during the mixing of freshwater and seawater

Abstract

The mixing of a freshwater spring (Carré Douce) with seawater in the intertidal area of the cove of Belmont leads to the non-conservative behaviour of Mn, Co and U. Mn, Co and U have a larger concentration in Carré Douce water than in seawater. At the contact of sea-water and atmosphere, Mn is oxidized in manganese dioxide. Co and U precipitate with MnO2. Owing to the preferential incorporation of 233U in MnO2, an isotopic effect is observed during the deposition of U. It is linked to the oxidation of U(IV) on MnO2 and possible substitution of Mn2+ by U6+ in poorly cristallized MnO2. These characteristics are also found in the concretions forming on the granite outcrops in the cove of Belmont. These results can be of importance in understanding some processes related to the submarine hydrothermal Fe-Mn deposits.