A heavy molybdenum reservoir in Neoarchean seawater tracks extensive iron oxidation.

Heavy molybdenum isotope signatures in Neoarchean banded iron formations (2.57–2.51 Ga) are best explained by Mo cycling in largely anoxic seawater, not widespread surface oxygenation.

• Why it matters: Uncertainty about when and how much oxygen rose before the Great Oxidation Event limits how we interpret early ocean chemistry from sediment isotope records.
• What they did: The study analyzes δ98Mo in Neoarchean BIFs and uses adsorption plus diagenetic incorporation into Fe(II)-Mo(IV)-sulfides, then applies a Mo isotope mass balance model.
• The result: Modeled Neoarchean seawater δ98Mo stays stable at 1.5‰ to 1.6‰, requiring only a minor MnO2 sink and implying dominant removal of isotopically light Mo via ferrihydrite precipitation driven by photoferrotrophs.

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