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

dos Santos, Felipe Holanda, da Silva Amaral, Wagner, Chi Fru, Ernest ORCID:, de Souza, Ana Clara Braga and Bosco-Santos, Alice 2022. Paleoproterozoic manganese oxide precipitation in oxic seawater surface and reductive enrichment in anoxic seafloor. Chemical Geology 588 , 120655. 10.1016/j.chemgeo.2021.120655

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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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Publisher: Elsevier
ISSN: 0009-2541
Date of First Compliant Deposit: 11 January 2022
Date of Acceptance: 27 November 2021
Last Modified: 08 Nov 2023 00:31

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