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

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 Mossbauer spectroscopy has been used to characterize newly generated iron(II) carbonates [solid solution FexCa1-xCO3 and 'green rusts', (FeFeII)-Fe-III(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.