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 MnO 2. Owing to the preferential incorporation of 233U in MnO 2, an isotopic effect is observed during the deposition of U. It is linked to the oxidation of U(IV) on MnO 2 and possible substitution of Mn 2+ by U 6+ in poorly cristallized MnO 2. 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 FeMn deposits.