Formation of green rust and elemental sulfur in an analogue for oxygenated ferro-euxinic transition zones of Precambrian oceans

For much of the Precambrian era, the bulk ocean was anoxic and Fe(II) rich (ferruginous), except for the first development of shallow ocean oxygenation and temporally/spatially restricted sulfide-rich waters (euxinia) along productive continental margins in the late Archean, which prevailed throughout much of the remaining Precambrian. There is little detail pertaining to transition zones between ferruginous, euxinic, and oxic seawater over the continental shelf that may have played an important role in shaping the composition of the underlying sediment. Here we present spectroscopic data on the Fe and sulfur mineralogy in the Arvadi Spring (Switzerland), a proposed analogue for such conditions. Our study reveals green rust, ferrihydrite, and lepidocrocite as the main Fe minerals. Because the reactivity of green rust differs from that of ferric hydroxides and Fe(II) sulfides, it is important to understand its role in the transfer of metals and nutrients from seawater to underlying sediments, if those sediments are to be used as chemical archives of paleo-seawater. We observed elemental sulfur (S-0) as the dominant sulfur precipitate and found indications for its role in pyrite formation, implying that S-0 could have had a similar role in Precambrian deposition of pyrite-poor or pyrite-rich sediments.