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 delta Fe-56 values of 210 pyrite grains in these samples show a large variation from -4.2 parts per thousand to +3.0 parts per thousand. We also analyzed 128 and 40 carbon isotope compositions of organic (delta C-13(org): -51.8 to -10.3 parts per thousand) and inorganic (delta C-13(carb): -6.1 to 0.6 parts per thousand) carbons, respectively. Microscopic observations show obvious relationships between pyrite grain morphology and iron isotope ratio. Most pyrite grains with positive delta Fe-56 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 delta Fe-56 values show irregular forms. The correlation allows the possibility to solve the origin and the formation process of each grain of pyrite. The positive delta Fe-56 values suggest the partial oxidation of iron in an oxygen-limited environment. Some pyrites show very lower delta Fe-56 values below -2.2 parts per thousand suggesting a biological origin, probably due to microbial iron reduction. On the other hand, the pyrite is accompanied by isotopically very light organic carbon (delta C-13(org): -51.8 parts per thousand to -40 parts per thousand), which indicates aerobic or anaerobic methanotrophy. The coexistence of the low delta Fe-56 values and low delta C-13 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. (C) 2012 Elsevier B.V. All rights reserved.