Paleoproterozoic manganese oxide precipitation in oxic seawater surface and reductive enrichment in anoxic seafloor

Accelerated precipitation of Mn-rich rocks in the early Paleoproterozoic ocean is considered to reflect the irreversible rise of oxygen in the atmosphere during the Great Oxidation Event (GOE), 2.501–2.220 billion years ago. But the precipitation conditions, pathways, mechanisms, and linkages to ocean redox, broadly remain unresolved. The Lagoa do Riacho Mn deposit in Borborema Province, northeastern Brazil, Ceará state, consists mainly of manganese-rich and manganese-poor silicates deposited during the GOE epoch. Negative Ce anomalies in the manganese-poor silicates point to the scavenging of Ce³⁺ onto Mn-oxyhydroxide reactive surfaces in the oxygenated surface waters. In contrast, samples with positive Ce anomalies, predominantly associated with the manganese-rich silicates, indicate a reductive dissolution of Mn-oxyhydroxides across a redoxcline, enriching the anoxic bottom water with Ce and Mn²⁺. A paleoredox reconstruction based on couple Mn, Mo, and U systematics supports the existence of the proposed Mn-oxide redox shuttle that enriched the anoxic sediment pile with Mn precipitated from the oxygenated shallow surface waters. This study uncovers a unique pathway in Paleoproterozoic Mn mineralization involving the transfer of Mn oxides from an oxygenated upper ocean reservoir to a predominantly deep anoxic silicate reservoir that was subsequently metamorphosed. The proposed Mn oxide sink switch mechanism is different from the commonly reported reductive transfer of Mn to a diagenetic carbonate sink during the Paleoproterozoic.