In situ iron isotope analyses of pyrite and organic carbon isotope ratios in the Fortescue Group: Metabolic variations of a Late Archean ecosystem

Kazumi Yoshiya, Manabu Nishizawa, Yusuke Sawaki, Yuichiro Ueno, Tsuyoshi Komiya, Keita Yamada, Naohiro Yoshida, Takafumi Hirata, Hideki Wada, Shigenori Maruyama

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    Abstract

    The biogeochemical cycle of the Late Archean ocean is important for understanding the relationships between biological activity and oxygenation of the atmosphere and ocean. Based on the detailed geological survey of the Fortescue Group in the Redmont area in Pilbara Craton, Western Australia, we carefully selected 44 samples for iron isotope analyses, which consist of sandstones, stromatolitic carbonate rocks, alternating mudstone/sandstone rocks, mudstones and cherts. Our in situ analyses of δ 56Fe values of 210 pyrite grains in these samples show a large variation from -4.2‰ to +3.0‰. We also analyzed 128 and 40 carbon isotope compositions of organic (δ 13C org: -51.8 to -10.3‰) and inorganic (δ 13C carb: -6.1 to 0.6‰) carbons, respectively. Microscopic observations show obvious relationships between pyrite grain morphology and iron isotope ratio. Most pyrite grains with positive δ 56Fe values show hexagonal, rectangular, and parallelogram shapes, which may replace former iron-oxide crystal systems: hematite, magnetite, and goethite, respectively. In contrast, more than half the pyrite grains with negative δ 56Fe values show irregular forms. The correlation allows the possibility to solve the origin and the formation process of each grain of pyrite. The positive δ 56Fe values suggest the partial oxidation of iron in an oxygen-limited environment. Some pyrites show very lower δ 56Fe values below -2.2‰ suggesting a biological origin, probably due to microbial iron reduction. On the other hand, the pyrite is accompanied by isotopically very light organic carbon (δ 13C org: -51.8‰ to -40‰), which indicates aerobic or anaerobic methanotrophy. The coexistence of the low δ 56Fe values and low δ 13C values in the some rocks suggests anoxic oxidation of methane by iron-reduction (AOM/IR). The iron and carbon isotopes demonstrate the metabolic variations of microorganisms in a Late Archean shallow marine environment.

    Original languageEnglish
    Pages (from-to)169-193
    Number of pages25
    JournalPrecambrian Research
    Volume212-213
    DOIs
    Publication statusPublished - Aug 1 2012

    Keywords

    • Anaerobic methanotrophy
    • Archean
    • Carbon isotope
    • Iron isotope
    • Microbial iron reduction

    ASJC Scopus subject areas

    • Geology
    • Geochemistry and Petrology

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    Yoshiya, K., Nishizawa, M., Sawaki, Y., Ueno, Y., Komiya, T., Yamada, K., Yoshida, N., Hirata, T., Wada, H., & Maruyama, S. (2012). In situ iron isotope analyses of pyrite and organic carbon isotope ratios in the Fortescue Group: Metabolic variations of a Late Archean ecosystem. Precambrian Research, 212-213, 169-193. https://doi.org/10.1016/j.precamres.2012.05.003