MICROBIAL INFLUENCE ON THE OXYGEN ISOTOPIC COMPOSITION OF DIAGENETIC SIDERITE

Numerous early diagenetic siderite concretions previously described ha ve been interpreted as the results of microbially mediated reactions. Interpretation of oxygen isotope data for such material requires an un derstanding of the effect of temperature on fractionation processes. H owever, whilst equilibrium fractionation of oxygen isotopes between si derite and water has been measured down to 33 degrees C, extrapolation to lower temperatures may be invalid. Furthermore, inorganic measurem ents may not be applicable to microbial systems. The specific iron-red ucing microorganism, Geobacter metallireducens, has been cultured anae robically in the laboratory using acetate as an organic substrate and amorphous FeOOH as an electron acceptor. The acetate is oxidised to CO 2, with concurrent iron reduction and extracellular siderite precipita tion. Rhombohedral siderite crystals up to 25 mu m in size have been p recipitated over a range of temperatures (18-40 degrees C). Stable iso topic analysis of these crystals and the solutions from which they pre cipitate shows that this microbial precipitation of siderite has an as sociated isotopic fractionation different from the published equilibri um, and which is not simply a function of temperature. In all cases, d elta(18)O values of siderite are lower than predicted by inorganic equ ilibrium fractionation data. This may explain the numerous anomalously -low delta(18)O values reported for early diagenetic marine siderites and previously attributed to mechanisms such as mixing with meteoric w ater, sediment-water interaction, recrystallisation, or variable isoto pic fractionation, despite a lack of supporting evidence. Copyright (C ) 1997 Elsevier Science Ltd.