MICROBIAL INFLUENCE ON THE OXYGEN ISOTOPIC COMPOSITION OF DIAGENETIC SIDERITE

Abstract

Numerous early diagenetic siderite concretions previously described have been interpreted as the results of microbially mediated reactions. Interpretation of oxygen isotope data for such material requires an understanding of the effect of temperature on fractionation processes. However, whilst equilibrium fractionation of oxygen isotopes between siderite and water has been measured down to 33°C, extrapolation to lower temperatures may be invalid. Furthermore, inorganic measurements may not be applicable to microbial systems.The specific iron-reducing microorganism, Geobacter metallireducens, has been cultured anaerobically in the laboratory using acetate as an organic substrate and amorphous FeOOH as an electron acceptor. The acetate is oxidised to CO2, with concurrent iron reduction and extracellular siderite precipitation. Rhombohedral siderite crystals up to 25 μm in size have been precipitated over a range of temperatures (18-40°C). Stable isotopic analysis of these crystals and the solutions from which they precipitate shows that this microbial precipitation of siderite has an associated isotopic fractionation different from the published equilibrium, and which is not simply a function of temperature. In all cases, δ18O values of siderite are lower than predicted by inorganic equilibrium fractionation data. This may explain the numerous anomalously-low δ18O values reported for early diagenetic marine siderites and previously attributed to mechanisms such as mixing with meteoric water, sediment-water interaction, recrystallisation, or variable isotopic fractionation, despite a lack of supporting evidence.