A PRELIMINARY-STUDY OF THE GEOCHEMICAL AND MICROBIOLOGICAL CHARACTERISTICS OF MODERN SEDIMENTARY CONCRETIONS

Parallel mineralogical, geochemical, and lipid biomarker analyses of s edimentary siderite concretions and their host sediments were undertak en to give detailed information about the microbially mediated diagene tic processes involved. Concretions were collected from two contrastin g environments at Warham, north Norfolk-typical brownish silty marsh s ediment and black anaerobic mud from a creek bank-but no major differe nces in their mineralogy or geochemistry were found. Samples consisted of siderite, amorphous iron oxides, and iron sulfides with more carbo nate present in the outer than in the inner parts. Subsamples from the concretions were found to be more similar to each other in their sulf ide and organic carbon contents than to the host sediments. The microb iological characteristics revealed by lipid biomarker analysis showed considerable variation between concretions and host sediments, between individual concretions, and between inner and outer parts of the conc retions. The predominant sulfate-reducing bacteria were more numerous in the concretions than in host sediments, but varied greatly within t he concretions. The biomarker i17:1 omega 7c for Desulfovibrio was enr iched within the concretions, suggesting that these bacteria, which ar e capable of reducing iron directly, played an important role in concr etion formation. The lipid biomarker data indicated that the bacterial community changed during concretion formation and that different cond itions for preservation existed in the concretions compared with the h ost sediments.