Parallel mineralogical, geochemical, and lipid biomarker analyses of sedimentary siderite concretions and their host sediments were undertaken to give detailed information about 1 he microbially mediated diagenetic processes involved. Concretions were collected from two contrasting emironments at Warham, north Norfolk-typical brownish silty marsh sediment and black anaerobic mud from a creek bank-but no major differences in their mineralogy or geochemistry were found. Samples consisted of siderite, amorphous iron oxides, and iron sulfides with more carbonate present in the outer than in the inner parts. Subsamples from the concretions were found to be more similar to each other in their sulfide and organic carbon contents than to the host sediments. The microbiological 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 concretions. The predominant sulfate-reducing bacteria were more numerous in the concretions than in host sediments, but varied greatly within the cc,ncretions. The biomarker i 17: lw7c for Desulfovibrio was enriched within the concretions, suggesting that these bacteria, which are capable of reducing iron directly, played an important role in concretion formation. The lipid biomarker data indicated that the bacterial community changed during concretion formation and that different conditions for preservation existed in the concretions compared with the host sediments.

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