The [formula omitted] ratios of siderite and organic matter of a modern metalliferous hydrothermal sediment and their implications for banded iron formations

Abstract

A hydrothermal ferruginous sediment precipitated by the activity of the microaerophilic chemolithotrophic bacterium Galionella ferruginea has been sampled in a bay of Nea Kameni Island, Santorini, Greece. Due to high fugacity of carbon dioxide, precipitation of siderite also occurred. The δ 13C-value (PDB) was determined to be +6.5‰ in the siderite and −26.8‰ in the organic matter of mainly microbiological origin. The enrichment of isotopically light carbon in carbonates of banded iron formations ( δ 13 C = −10‰ ) has recently been discussed in literature. The dilution of these marine carbonates by isotopically light carbon is suggested to have resulted from bacterial oxidation of reduced organic carbon during early diagenesis. It is further suggested that Fe(III) in the newly precipitated sediment served as electron acceptor during oxidation. In a hydrothermal model for the origin of banded iron formations this is consistent with the data presented here. It is therefore postulated from the results presented that the reduced organic carbon in banded iron formations originates from microaerophilic chemolithotrophic bacteria.