Variations of Fatty Acids During the Sulphidization Process in the Authie Bay Sediments

• Published in: Journal of soils and sediments 2007-02, Vol.7 (1), p.17-24
• Main Authors: Billon, Gabriel, Thoumelin, Guy, Barthe, Jean-François, Fischer, Jean-Claude
• Format: Article
• Language: English
• Subjects: Authie estuary (Northern France); Bacillariophyceae; early diagenesis; estuary; fatty acids; Iso; sediments; sulfur
• ISSN: 1439-0108; 1614-7480
• DOI: 10.1065/jss2006.11.197

Background, Aim and Scope The sulphidization process in relatively clean sediments sampled in a mudflat of the Authie estuary (located in Northern France) has been studied by coupling geochemical expertise and the use of fatty acids (FAs) as biochemical markers. Materials and Methods: Three sediment cores have been sampled in September 2003, November 2003 and May 2004, and cut every 2 cm in the field under nitrogen atmosphere so as to prevent any oxidation of reduced species. In the solid phase, reduced sulphur compounds, e.g. AVS (Acid Volatile Sulphides) and CRS (Chromium Reducible Sulphur) [including also the calculation of the degree of sulphidization (DOS) and the degree of pyritization (DOP)], and fatty acids have been carried out. Eh, pH, metal species (mostly iron and manganese), dissolved S(-II) and sulphate have also been determined in the porewaters. Results: The sediment cores display a lot of differences due to the high sedimentation rate and the seasonal evolution as well. The presence of Mn2+, Fe2+, S(-II) and the decrease of the redox potential and the concentration of sulphates clearly indicate early diagenetic transformations promoted by the bacterial activity. Acid Volatile Sulphides are produced in the first cm and are stabilized with depth. A rapid decrease of FAs concentrations in September and May has also been pointed out owing to a rapid consumption of the labile organic matter. Several categories of FAs have been separated and most of them belong here to the saturated and monounsaturated groups. In the saturated group, branched chain FAs, iso and anteiso C15:0 are predominant and represent the bacterial imprint in the sediments. Maximum proportions are observed between 5 and 10 cm in September, and between 13 and 17 cm in November and May. Discussion: As sulphate concentrations remain high in the porewater, the limitation of the sulphidization process in our sediments must be due to a lack of labile organic matter input. The presence of pyrite in our sediment is bound to its formation at the water-sediment interface, where partial reoxidation may take place. However, at deeper depths, pyritization processes does not continue any more. Presence of maximum, dissolved S(-II) concentrations have been observed, simultaneously with maximum proportion relative to total FAs of iso and anteiso C15:0, and, in September, with an increase in proportions of C18:1ω7. This indicates the presence of sulphate-reducing bacterial activity at the tim