Biogeochemical Fe-S-cycling in a late Archean and Proterozoic ocean model habitat - the high alpine Arvadi Spring

Abstract

Earth’s atmosphere, hydrosphere and biosphere were subjected to substantial changes during the Archean-Proterozoic redox-transition as a consequence of the evolution of oxygenic photosynthesis. Most of our understanding on this highly complex timeframe in Earth’s history comes from the rock record. However, knowledge gaps on the composition and structure of ferro-euxinic intermixing zones, i.e. transition zones between ferruginous (Fe(II)-rich) and euxinic (sulfide-rich) seawater that emerged across the late Archean – early Proterozoic, prevail due to the incomplete and diagenetically altered state of the rock record. A promising way to assess the nature of ferro-euxinic transition waters is the use of modern model habitats that resemble anticipated seawater conditions. The Fe- and S-rich Arvadi Spring in the eastern Swiss Alps borrows high potential for such an analogue. In this thesis, the suitability of the Arvadi Spring as a model habitat for ferro-euxinic transition zones was evaluated and knowledge that was obtained on its geochemical, mineralogical and microbial composition was transferrred on existing models of ancient Fe-S-cycling. In summary, the major findings of this PhD study are (1) that dissolved Fe(II) and sulfide can coexist at low micromolar concentrations under full oxygen saturation without precipitating abundantly as Fe(II) sulfides, (2) that ferrihydrite, lepidocrocite, green rust and elemental sulfur are the major Fe- and S-minerals that precipitate from the Arvadi water, (3) that microaerophiles are the most abundant type of Fe- and S-metabolizers and (4) that Fe(II) oxidation and Fe(III) reduction processes are spatially separated to oxic and anoxic niches, whereby Fe(III) reduction was shown to be limited by the availability of organic carbon and contemporaneously proceeding sulfate reduction. Overall, the results of this PhD study imply a minor role for Fe(II) sulfide mineral precipitation from ferro-euxinic intermixed waters and a special role for Fe(III) reduction and hence Fe(II) formation coupled to the prevalence of S-species and sulfate-reducers in respective parts of late Archean and Proterozoic oceans.