Abiotic iron oxidation controlled the deposition of Neoproterozoic iron formations

Abstract

Following a billion-year hiatus, iron formations (IFs) briefly re-emerged during Neoproterozoic Snowball Earth glaciations. Unlike their Archean-Paleoproterozoic counterparts, Neoproterozoic IFs (NIFs) are uniquely associated with glaciogenic diamictites and dominated by hematite, yet the drivers of this mineralogical shift remain debated. Here, we present coupled iron-carbon isotope data from Sturtian-aged NIFs in the Fulu Formation (Nanhua Basin, South China). The Fulu NIFs feature alternating bands of euhedral hematite, quartz, feldspar, Fe chlorite, as well as some siderite. The siderite exhibits exceptionally negative δ13Ccarb values (down to −19 ‰). Isotopic equilibrium modeling supports two hypotheses for this fractionation: (i) a diminished dissolved inorganic carbon (DIC) reservoir influenced by dissimilatory iron reduction (DIR), and (ii) Fe-mediated anaerobic oxidation of methane (AOM) driven by methane diffusion. Both scenarios align with suppressed primary productivity under ice-covered oceans, where microbial activity and organic burial were limited. This low-productivity regime explains the hematite-dominated mineralogy of NIFs, contrasting sharply with the diverse diagenetic phases (e.g., siderite, magnetite) in Archean-Paleoproterozoic IFs. Heavy δ56Fe values (up to 2.03 ‰) further indicate abiotic Fe2+ oxidation via meltwater-derived oxygen, rather than biological pathways like photoferrotrophy. Overall, these results support a model of an anoxic Cryogenian ocean with minimal productivity during NIFs deposition, where Fe cycling was governed by glacially mediated redox dynamics.