FE, C, AND O ISOTOPE COMPOSITIONS OF BANDED IRON FORMATION CARBONATES DEMONSTRATE THE ROLE OF BACTERIAL DISSIMILATORY IRON REDUCTION IN ~ 2.5 B.Y. OLD MARINE ENVIRONMENTS

Combined Fe, C, and O isotope compositions 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 (Campbellrand platform), South Africa, constrain their formation pathways. Vertical intervals of up to 100 m were selectively sampled millimetrically in three cores that cover a lateral extent of ~ 250 km. BIF Fe carbonates have significant Fe isotope variability (δ⁵⁶Fe_IgR = -1 to +1‰) and relatively low δ¹³C (avg. ~ -6‰) and δ¹⁸O values (avg. ~ +21‰ VSMOW). In contrast, Ca-Mg carbonates have δ¹³C values near-zero and δ¹⁸O up to +25‰. The C isotope difference between BIF ankerite and siderite (δ¹³C = -2 to -12‰) and platform calcite/dolomite (δ¹³C ~ 0‰) cannot be explained by equilibrium C isotope fractionation. Likewise, the O isotope variability between these carbonates is the opposite to that expected from equilibrium fractionation factors and from comparisons with modern enclosed basins such as the Black Sea. In the Black Sea, shallow waters with riverine and meteoric influence have the lowest δ¹⁸O values, and these values increase with depth. All this information, along with the small-scale Fe isotope variability measured in BIF Fe carbonates, indicates that Fe carbonates did not precipitate in equilibrium with seawater Fe, C, or O isotope compositions. Instead, isotope mass-balance calculations suggest that these isotope compositions likely reflect authigenic carbonate formation in the sediment, with Fe, C, and O contributions from organic carbon oxidation coupled to bacterial dissimilatory Fe(III) reduction (DIR) of Fe oxides. BIF carbonates that have positive δ⁵⁶Fe values tend to have micron-size hematite inclusions, whereas those that do not contain hematite inclusions tend to have lower δ⁵⁶Fe values. High-δ⁵⁶Fe, low-δ¹³C BIF carbonates reflect formation by in-situ nearly-complete DIR of high-δ⁵⁶Fe Fe oxides residual from prior partial DIR. Low-δ⁵⁶Fe, low-δ¹³C BIF carbonates formed by remobilization of low-δ⁵⁶Fe aqueous Fe²⁺ produced by partial bacterial DIR of ferric oxides in the sediment, followed by siderite precipitation. Our results indicate that the negative to positive δ⁵⁶Fe values in Kuruman BIF carbonates may record biological cycling of Fe that proceeded with various extents of iron reduction.