Anoxic photogeochemical oxidation of manganese carbonate yields manganese oxide

Significance When oxygenic photosynthesis evolved is debated with an uncertainty of approximately 1 Gy. It is generally assumed that the oxidation of manganese minerals requires biological catalysis or molecular oxygen and therefore is often used as a proxy for the presence of oxygenic photosynthetic organisms. We show that anoxic, abiotic oxidation of the mineral rhodochrosite (MnCO3) by UV light forms H2 and manganite (γ-MnOOH). Our results reveal an alternative mechanism for producing manganese oxides from rhodochrosite in the absence of molecular oxygen. These results demonstrate the potential impact of photogeochemical processes on the redox state of transition metals and hence question the interpretation of the rise of atmospheric oxygen based on the oxidation of transition metals, such as Cr isotopes. The oxidation states of manganese minerals in the geological record have been interpreted as proxies for the evolution of molecular oxygen in the Archean eon. Here we report that an Archean manganese mineral, rhodochrosite (MnCO3), can be photochemically oxidized by light under anoxic, abiotic conditions. Rhodochrosite has a calculated bandgap of about 5.4 eV, corresponding to light energy centering around 230 nm. Light at that wavelength would have been present on Earth’s surface in the Archean, prior to the formation of stratospheric ozone. We show experimentally that the photooxidation of rhodochrosite in suspension with light centered at 230 nm produced H2 gas and manganite (γ-MnOOH) with an apparent quantum yield of 1.37 × 10−3 moles hydrogen per moles incident photons. Our results suggest that manganese oxides could have formed abiotically on the surface in shallow waters and on continents during the Archean eon in the absence of molecular oxygen.