Behaviour of suspended and sedimentary organic matter in the deltaic areas of the Gulf of Lions (Mediterranean Sea)

The behaviour of organic matter has been approached in two fluvio-marine areas of the Gulf of Lions in the Northwestern Mediterranean Sea: deltas of the Rhône and Têt rivers. Elemental analysis, pyrolysis-gas chromatography-mass spectrometry (main classes of organic compounds) and high performance liquid chromatography (phenolic compounds) enabled to show the fractioning processes and the differentiation of water layers from the river mouth to the open sea. Transitory horizontal and vertical geochemical gradients appeared in the water column. They were more marked in surficial sediments. They underlined the effects of the river inputs which were characterized by high phenol contents. The increase of the marine character towards the open sea was shown by increasing nitrogen-containing compounds. In the Rhône delta, the distribution of the suspended material and organic compounds emphasized the occurrence of a multi-layered water system. In the surficial layers the suspended organic matter was in a little degraded state. On the contrary, the bottom nepheloid contained altered organo-mineral complexes enriched in resistant organic compounds. In the deeper areas of the Lacaze-Duthiers Canyon, in front of the Têt prodelta, the abundance of phytoplanktonic material in the euphotic zone was emphasized by the increase of both nitrogen-containing compounds and aminosugars. In the deeper layers, the suspended material transported by advective currents showed a detrital characteri.e. enriched in clays and in degraded organic matter. The observations carried out in the Rhône delta, the Têt prodelta and the Lacaze-Duthiers Canyon underlined the importance of the general transfer system of suspended material from East to West enhanced by the Liguro-Provençal current.