Identification of newly generated iron phases in recent anoxic sediments: ⁵⁷Fe Mössbauer and microRaman spectroscopic studies

Abstract

Both biota (primarily bacteria) and conditions of low natural redox potential and pH involved in surface sediments from the Seine estuary (in Northern France) are responsible for the reduction of sedimentary oxidized forms of iron into iron(II). Such processes generate insoluble inorganic compounds. Iron-57 Mössbauer spectroscopy has been used to characterize newly generated iron(II) carbonates [solid solution Fe_xCa_1−xCO₃ and ‘green rusts’, Fe^IIIFe^II(CO₃)(OH)] in these recent anoxic sediments. It is also demonstrated that sedimentary iron reductions occurring in these sediments play an important role in the biogeochemical cycles of sulfur. Thus, using X-ray diffraction, pyrite was detected in sulfide-rich sediments (1.5–1.8 wt.%). Furthermore, laser Raman microprobe spectroscopy has helped us to establish the existence of small quantities of pyrite and amorphous iron sulfide(s), scarcely detectable by conventional macroanalysis techniques, and to visualize/discriminate them on the anoxic sediment surface.