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

Abstract

Combined Fe, C, and O isotope measurements of ~ 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 ~ 250 km. BIF Fe carbonates have significant Fe isotope variability ( δ 56Fe = + 1 to − 1‰) and relatively low δ 13C (down to − 12‰) and δ 18O values ( δ 18O ~ + 21‰). In contrast, Gamohaan and stratigraphically-equivalent Campbellrand Ca–Mg carbonates have near-zero δ 13C values and higher δ 18O 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- δ 13C and low- δ 18O 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- δ 13C, low- δ 56Fe Fe carbonates formed from remobilized, low- δ 56Fe aqueous Fe 2+ produced by partial DIR of iron oxide, whereas low- δ 13C, high- δ 56Fe Fe carbonates formed by near-complete DIR of high- δ 56Fe iron oxides that were residual from prior partial DIR. An important observation is the common occurrence of iron oxide inclusions in the high- δ 56Fe 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 δ 56Fe values may all record biological Fe cycling, where the range in δ 56Fe values records differential Fe mobilization via DIR in the sediment prior to lithification. Our results demonstrate that the inventory of low- δ 56Fe 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.