The molybdenum cycle in the oxygenated Neoproterozoic ocean was coupled to manganese carbonate mineralization

Abstract

The Neoproterozoic oxygenation event is a milestone in Earth’s history, yet the redox structure and elemental cycling of the Neoproterozoic ocean remain debated. Here, we investigated iron speciation, molybdenum, and nitrogen isotopes in black shales and manganese carbonates from the upper Ediacaran Doushantuo Formation, South China, to examine the links between redox state, manganese mineralization, and molybdenum cycling. In both lithologies, high pyritic iron ratios (> 0.8) indicate a localized euxinic zone, while estimated seawater molybdenum (1.9‰) and sediment nitrogen isotopic compositions (4.19 ± 1.96‰) resemble modern values, suggesting oxygenated surface waters over euxinic depths. Negative molybdenum isotope in manganese(II) carbonates points to manganese(IV) oxide reduction, acting as a molybdenum shuttle from oxygenated surface to euxinic deep waters. Periodic euxinic contractions drove manganese(II) oxidation and mineralization, shaping molybdenum cycling. This study highlights essential manganese(II) oxidation for manganese carbonate mineralization and offers new insights into molybdenum geochemistry and ancient ocean oxygenation events. The periodic contraction of euxinic zones in the Neoproterozoic Ocean shaped the mineralization of manganese and the oceanic molybdenum cycle, according to iron, molybdenum and nitrogen geochemical data from back shales and carbonates in the upper Doushantuo Formation, South China.