Mn oxides occurrence in the Archean sedimentary records indicates the early emergence of oxygenic photosynthesis on Earth

It has been suggested that the oxidation of manganese in the water column requires substantial dissolved oxygen to prevent Mn oxides from reduction in a pervasive anoxic (ferruginous) environment.

Mn oxides occurrence in the Archean sedimentary records does not imply the early emergence of oxygenic photosynthesis on Earth

Communities of anoxygenic microorganisms are able to drive manganese oxidation in the photic zones (light-driven) of the water column.
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Manganese enrichment in the Mesoarchean sediments


Mn enrichment in Archean sedimentary records implies Mn oxidation

eThe light molybdenum isotope signatures (δ98Mo) of the Fe-Mn carbonates of the Sinqeni Iron Formation (Pongola Supergroup) suggest the existence of Mn oxides at 2.95 Ga and the accompanying preferential adsorption of light Mo isotopes (95Mo) onto Mn oxides

eThe carbon (δ13C) and oxygen (δ18O) isotope of the Fe-Mn carbonates from the Sinqeni Formation, Pongola Supergroup indicate the precipitation of carbonates was not in equilibrium with seawater (in terms of C and O isotopes), implying the reductive Mn cycling in the sediments and subsequent authigenic carbonate formation via early diagenesis

The mechanism of Mn oxidation


Mn enrichment in Archean sedimentary records is primarily ascribed to Mn oxidation by free oxygen

eThe significant manganese enrichment in the Mesoarchean (2.95-2.98 Ga) sedimentary sequences of the Pongola Supergroup, South Africa, was caused by oxidation of Mn(II) via molecular oxygen


Mn oxidation in Archean was achieved by anoxygenic photosynthetic microorganisms under anaerobic conditions

eAnoxygenic photosynthetic microorganisms can produce manganese oxides in the light under the anaerobic conditions

eManganese enrichment in 2.415 Ga iron formation of the Koegas Subgroup, South Africa has been ascribed to Mn oxidation via manganese- oxidizing photosynthesis with a high- potential oxidant other than molecular O2

eMnII-bicarbonate clusters can act as electron donors for primitive anoxygenic phototrophs in the presence of excess bicarbonate, leading to the formation of MnIII

eLaboratory experiments show that manganese oxides precipitate from aqueous MnIICl2 solutions under UV light (λ < 240 nm), suggesting the photo-oxidation of manganese might be a pathway to form manganese oxides in banded iron formations

Mn oxide settling and burial


Mn oxide rarely survive during settling in an anoxic water column

eBoth field observations and modeling studies show that Mn oxide reduction in a modern ferruginous water column is fast enough to eliminate the possibility of Mn oxide settling on to the sediment-water interface


Some oxidants may prevent Mn oxide from reductive dissolution during its settling in an anoxic water column

eHydrogen peroxide (H2O2) generated by photolytic processes may function as an oxidant to oxidize manganese under the pervasive anoxic conditions of the Archean and early Paleoproterozoic

Mesoarchean atmospheric oxygen level


Redox proxies suggest an atmospheric oxygenation event occurred at ~3 Ga

eThe light chromium isotope (δ53Cr) signatures of Nsuze paleosol indicate the redox mobilization of Cr from its igneous mineral hosts occurred ~3 Ga driven by atmospheric oxygenation