Sedimentary record of redox-sensitive elements (U, Mn, Mo) in a transitory anoxic basin (the Thau lagoon, France)

In both cores, sedimentary Mn is relatively depleted through out the whole sedimentary column and varies with the proportion of clay minerals. After an initial release into solution at the sediment-water interface in relation to Mn-oxide reductive dissolution, authigenic U is immobilized when sulphides appear. Despite the occurrence of anoxic conditions at the sediment-water interface at the site influenced by shellfish farming, the burial of U is reduced by bioturbation, which raises reducing sediments to the surface. In the centre of the lagoon, Mo profiles reflect continuous diffusion into pore water and immobilization at 15 cm probably in anoxic microenvironments. At shellfish farms, dissolved Mo undergoes removal with sulphides but contrary to U, sedimentary Mo does not appear to be strongly affected by bioturbation. The profile indicates an increase in the frequency of anoxia crises during the second half of the 20th century.