Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea.

Highlights • Redox-stratified water column in a Mesoarchean epicontinental sea, Pongola basin. • Mesoarchean aerobic iron and manganese cycling in the Pongola epicontinental sea. • Aerobic biological catalysis enhanced manganese oxidation in a Mesoarchean sea. • Evidence for oxygenated surface seawater ca. 3 billion years ago. Abstract Redox conditions in the marine realm prior to the Great Oxidation Event (GOE; ∼2.46–2.32 Ga ago), during which the atmospheric oxygen level rose dramatically for the first time, are still debated. Here, we present C, O, Fe, and Mo stable isotope systematics of Fe-, Mn-, and carbonate-rich shales, deposited at different water depths in association with iron formations (IFs) of the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. δ 13 C values between −22.3 and −13.5‰ VPDB, and δ 18 O values between −21.1 and −8.6‰ VPDB for Fe–Mn-rich carbonate minerals indicate their precipitation out of equilibrium with seawater. Instead, early diagenetic reduction of Fe–Mn-oxyhydroxide precursor minerals, along with microbially induced oxidation of organic matter (OM), formed these carbonates. δ 56 Fe IRMM - 014 values between −1.27 and 0.14‰ and δ 98 Mo NIST 3134 + 0.25 values between −0.46 and 0.56‰ co-vary with Mn concentrations and inferred water depth of deposition. This suggests that, despite the diagenetic origin of the Fe–Mn carbonates, the primary light Fe and Mo isotopic signature of Fe–Mn-oxyhydroxides that originally precipitated from seawater is still preserved. While isotopically light Mo implies that Mn(II) was oxidized to Mn(IV) due to the availability of free, photosynthetically produced O 2 , Mn enrichment suggests that the water column was redox stratified with a Mn-redoxcline situated at a depth below the storm wave base. A trend to highly negative δ 56 Fe values with increasing Mn/Fe ratios and decreasing depositional depth suggests progressive oxidation of Fe(II) as deep-waters upwelled across a redoxcline towards shallow, locally oxygenated waters where Mn(IV) oxyhydroxides precipitated. Combined δ 56 Fe and δ 98 Mo data indicate pervasive oxygenation of seawater with the O 2 content in the photic zone likely reaching levels higher than the maximum value of 10 μM proposed for Archean oxygen oases. Since abiotic Mn(II) oxidation is kinetically very slow in marine environments, it is likely that Mn-oxidizing microorganisms catalyzed Mn-oxidation in the oxygenated Pongola surface waters during deposition of IFs. This implies that aerobic metabolism had evolved before the GOE in shallow, aquatic habitats, where it exerted a first-order control on the deposition of shallow-marine, Mn-rich iron formations. [ABSTRACT FROM AUTHOR]