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

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.