Sedimentary record of redox-sensitive elements (U, Mn, Mo) in a transitory anoxic basin (the Thau lagoon, France)
Résumé
In order to further document the relation between redox conditions and the sedimentary record of Mn, U and Mo in a transitory anoxic water basin, their distribution has been studied along two profiles in the Thau lagoon (France). Sediments and pore-water have been sampled at two contrasting sites located, respectively, in the shellfish-farming area and in the centre of the lagoon. In the shellfish-fanning area, the particulate organic carbon (POC) data indicate a more rapid organic matter mineralisation compared to the centre of the lagoon. This results in a sharper redox gradient characterized by the appearance of H2S in pore-water a few millimetres below the sediment-water interface. In the centre of the lagoon, H2S appears at a depth of 35 cm. 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 fanning, 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.