A heavy molybdenum reservoir in Neoarchean seawater tracks extensive iron oxidation

Progressively heavier molybdenum isotope compositions (δ ⁹⁸ Mo) in Neoarchean marine sediments have been interpreted as evidence for widespread surface oxygenation prior to the Great Oxidation Event. Here, we assess whether the deposition of banded iron formations (BIFs) — iron-rich sedimentary rocks formed predominantly in the Neoarchean — can account for this isotopic signal through processes operating under pervasively anoxic conditions. BIF samples analyzed here possess a wide range of δ ⁹⁸ Mo values, which are attributed to the combined effects of Mo adsorption onto primary ferric iron (Fe) oxyhydroxides and subsequent diagenetic incorporation into Fe-Mo-sulfides. Given the isotopic fractionation associated with Mo adsorption, we estimate that Neoarchean seawater δ ⁹⁸ Mo ranged from 1.5‰ and 1.6‰, and was more stable than previously suggested. If Mo in Neoarchean rivers had an average isotopic composition like today, a mass balance model predicts that only a modest manganese oxide sink is required to generate these heavy δ ⁹⁸ Mo values. Instead, the dominant control may have been the removal of isotopically light Mo via adsorption to abundant ferric oxyhydroxide particles setting through a ferruginous water column. These findings imply that the Neoarchean Mo isotope record may track extensive photosynthetic iron oxidation rather than pervasive oxygen accumulation in the surface ocean.