Fe, C, and O isotope compositions of banded iron formation carbonates demonstrate a major role for dissimilatory iron reduction in ∼2.5 Ga marine environments

Combined Fe, C, and O isotope measurements of similar to 2.5 Ga banded iron formation (BIF) carbonates from the Kuruman Iron Formation and underlying BIF and platform Ca-Mg carbonates of the Gamohaan Formation, South Africa, constrain the biologic and abiologic formation pathways in these extensive BIF deposits. Vertical intervals of up to 100 m were sampled in three cores that cover a lateral extent of similar to 250 km. BIF Fe carbonates have significant Fe isotope variability (delta Fe-56 = +1 to 1 parts per thousand) and relatively low delta C-13 (down to - 12 parts per thousand) and 6180 values (delta O-18 -+ 21 parts per thousand). In contrast, Gamohaan and stratigraphically-equivalent Campbellrand Ca-Mg carbonates have near-zero delta C-13 values and higher delta O-18 values. These findings argue against siderite precipitation from seawater as the origin of BIF Fe-rich carbonates. Instead, the C, O, and Fe isotope compositions of BIF Fe carbonates reflect authigenic pathways of formation in the sedimentary pile prior to lithification, where microbial dissimilatory iron reduction (DIR) was the major process that controlled the C, O, and Fe isotope compositions of siderite. Isotope mass-balance reactions indicate that the low-delta C-13 and low-delta O-18 values of BIF siderite, relative to those expected for precipitation from seawater, reflect inheritance of C and O isotope compositions of precursor organic carbon and ferric hydroxide that were generated in the photic zone and deposited on the seafloor. Carbon-Fe isotope relations suggest that BIF Fe carbonates formed through two end-member pathways: low-delta C-13, low-delta Fe-56 Fe carbonates formed from remobilized, low-delta Fe-56 aqueous Fe2+ produced by partial DIR of iron oxide, whereas low-delta C-13, high-delta Fe-56 Fe carbonates formed by near-complete DIR of high-delta Fe-56 iron oxides that were residual from prior partial DIR. An important observation is the common occurrence of iron oxide inclusions in the high-delta Fe-56 siderite, supporting a model where such compositions reflect DIR "in place" in the soft sediment In contrast, the isotopic composition of low-Fe carbonates in limestone/dolomite may constitute a record of seawater environments, although our petrographic studies indicate that the presence of pyrite in most low-Fe carbonates may influence the Fe isotope compositions. The combined Fe, C, and O isotope data from Kuruman BIF carbonates indicate that BIF siderites that have negative, near-zero, or positive delta Fe-56 values may all record biological Fe cycling, where the range in delta Fe-56 values records differential Fe mobilization via DIR in the sediment prior to lithification. Our results demonstrate that the inventory of low-delta Fe-56 marine sedimentary rocks of Neoarchean to Paleoproterozoic age, although impressive in volume, may represent only a minimum of the total inventory of Fe that was cycled by bacteria. (C) 2010 Elsevier B.V. All rights reserved.