14 / 2025-03-22 19:04:44

Abiotic iron oxidation controlled the deposition of Neoproterozoic iron formations

Cryogenian; DIR; Fe-AOM; Fe isotopes; C isotopes

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

Neoproterozoic iron formations (NIFs) represent a rare resurgence of IF deposition following a billion-year gap, uniquely linked to Snowball Earth glaciations. Unlike their Archean–Paleoproterozoic counterparts, NIFs are intimately associated with glaciogenic diamictites and exhibit a distinct hematite-dominated mineralogy. However, the origin of this mineralogical shift remains unresolved. In this study, we present new coupled Fe–C isotope data from the Sturtian-aged Fulu Formation (Nanhua Basin, South China), offering fresh constraints on redox processes during the Cryogenian.



The Fulu NIFs comprise alternating bands of authigenic hematite, quartz, feldspar, Fe-rich chlorite, and locally abundant siderite. Remarkably, siderite displays highly negative δ¹³C values (as low as -19‰). Isotope equilibrium modeling suggests two plausible pathways for this signal: (i) a DIC pool modified by dissimilatory iron reduction (DIR), and (ii) Fe-driven anaerobic oxidation of methane (AOM) under restricted methane diffusion. Both scenarios point to a low-productivity, ice-covered ocean system where microbial activity and organic carbon burial were limited. Such a setting would favor early hematite precipitation and suppress the formation of diverse diagenetic minerals typical of earlier IFs. In addition, elevated δ⁵⁶Fe values (up to +2.03‰) imply abiotic Fe²⁺ oxidation by meltwater-derived oxygen, contrasting with microbial oxidation pathways like photoferrotrophy. Together, these findings support a revised model of Cryogenian marine iron cycling: one dominated by glacial redox stratification, limited productivity, and non-biological Fe oxidation. Our study highlights the potential of Fe–C isotope systems to reconstruct complex environmental dynamics during pivotal intervals of Earth’s climate evolution.