1 item with the tag “reduction

  • Project 4E: Preliminary Studies of Fe Isotope Biogeochemistry in Fe-Rich Yellowstone National Park Hot Springs
    NAI 2012 University of Wisconsin Annual Report

    This preliminary project provided background information for future studies of the structure, function, and signatures (living and non-living) of Fe redox-based microbial life in the volcanic terrain of Yellowstone National Park (YNP). The focus on Fe redox-based systems stems from our expanding knowledge of the wide range of microbial energy metabolisms that are known to be associated with Fe redox transformations on Earth and potentially on other planets. Moreover, Fe redox transformations provide the potential for generation of mineralogical, isotopic, and organic biosignatures of past and present microbial life, which represent premier targets for testing the hypothesis that life currently exists or existed in the past on Mars. Preliminary data on Fe geochemistry and isotopic composition, and microbial community composition, was obtained for two contrasting Fe-rich springs in YNP: Chocolate Pots (CP), a warm, circumneutral environment that has formed on top of the Pleistocene-age Lava Creek Tuff, where a mixture of Fe-rich acid-sulfate geothermal fluids and neutral-pH groundwater from the Gibbon River catchment emerge to the surface; and The Gap site, a hot, acid-sulfate spring in the Norris Basin which supports active chemolithotrophic Fe(II) oxidation, analogous to other hot spring environments in YNP. The geochemical data demonstrated significant changes in aqueous Fe abundance and/or speciation along the flow paths at both sites, leading to accumulation of abundant Fe(III) oxides as well as aqueous Fe(III) at the acidic Gap site. A distinct separation in Fe isotope composition between aqueous Fe and deposited Fe(III) oxides (mainly amorphous Fe-Si coprecipitates) was also detected, with the oxide enriched in 56Fe relative to 54Fe as expected for redox-driven Fe isotope fractionation. However, the degree of fractionation was less the value of ca. 3 ‰ expected in closed system at isotope equilibrium. We suggest that internal regeneration of Fe(II) via dissimilatory Fe(III) reduction could enrich the aqueous Fe(II) pool in the heavy isotope, leading a much lower degree of Fe isotope fractionation – and hence a fundamentally different pattern of Fe isotope fractionation – than would occur in a strictly Fe(II) oxidation-driven reaction system. In support of this argument, an initial set of culturing experiments designed to recovery thermophilic Fe(III)-reducing organisms from CP and Gap materials resulted in the recovery of active Fe(III) reducers from both sites. In addition, preliminary pyrosequencing of 16S rRNA genes recovered from Gap solids provide evidence for Fe(III) reduction potential by the resident microflora. Particularly in the case of the Gap, sequences related to known Archaeal fermenters and elemental S/Fe(III) oxide reducers were abundant.

    ROADMAP OBJECTIVES: 2.1 5.1 5.3