Cerium Stable Isotopes Unveil Ferromanganese Encrustation Across the Oxygen Minimum Zone

he interaction between a large, dissolved Mn reservoir in the oxygen minimum zone (OMZ) and the deeper oxygenated water allows for Mn oxidation and precipitation at their interface. The current paradigm posits that the OMZ acts as a Mn²⁺ source necessary for ferromanganese encrustation, while the encrustation itself is not thought to occur within the OMZ, though this remains a subject of ongoing debate. Marine Fe-Mn crusts enrich metals including those with high affinity for Mn oxides (e.g., Ce), which can provide insights into the origin of Mn oxides. In this study, we identify heterogeneous Ce and δ¹⁴²Ce_SW profiles in seawater and Fe-Mn crusts across the OMZ in the Northwest Pacific Ocean, closely linked to the cycle of Mn. Moreover, we quantify a close association of Ce with Mn oxides in Fe-Mn crusts and associated Ce isotope fractionation between the crusts and ambient seawater, bridging the marine Mn and Ce cycles. The outcome reveals that continuous precipitation of Mn oxides could initiate within the OMZ and extend into the deep ocean (5,000–6,000 m seawater depth) in the Northwest Pacific Ocean.