Transfer and Biogeochemistry of Organic Matter at Sediment-Water Interface on the Northwestern Mediterranean Margin

Abstract

Abstract The northwestern margin of the Gulf of Lions is cut by numerous canyons, areas of advective fluxes transferring matter from the adjacent continental shelf towards the Mediterranean basin. With reference to the biogeochemical cycle of organic carbon in the marine environment, the deposit of organic matter was studied from the coastal zone near a river mouth (Tet), down to the base of the slope. The importance of continental or marine sources was stressed by biogeochemical contents of surficial deposits. The physico-chemical parameters and various organic compounds (total org C, hydrolyzable org C, N, amino acids, carbohydrates, phenolic compounds and humic substances) were studied for the different environments. The input of organic compounds at the sediment-water interface was simulated by injection of 14 C labelled organic molecules in sea water overlying surficial sediments. An amino-acid ( 14C (U) glutamic acid) was used to simulate a biodegradable marine organic molecule reaching the sea floor, whereas continental degraded compounds were represented by a lignin derived ( 14 C(U) catechol). After 6-day laboratory incubations under in situ simulated conditions, in a dark thermostat-regulated chamber (13°C, oxidizing conditions), the dynamic of organic matter exchanges between overlying water and sediment was established. Global budget is based on the following: (1) 14 CO 2 released during 6-day kinetics (7% with glutamic acid, 0.8% with catechol); (2) 14 C remaining in the water: dissolved organic 14 C which averages 15% of the initial and mineral 14 C fraction (0.1–3%); (3) 14 C activity incorporated in sediment: a direct relationship can be observed between injected 14 C catechol and the oxidation stage of the sediment. As far as glutamic acid is concerned, a higher fraction is recovered as 14 CO 2 which decreases the active 14 C available for migrating in the sediment. Moreover, the 14 C activity in sediment was counted for different geochemical fractions for evaluating the relative part of biochemical and geochemical processes. This evaluation shows that distribution varies depending on the nature of injected organic molecules, and is principally influenced by oxido-reduction potential, water content and geochemical characteristics of the natural organic matter. Site differentiation seems to be independent from the quantity of benthic microflora and corresponds better to the degree of heterotrophic activity.