Anoxic photogeochemical oxidation of manganese carbonate yields manganese oxide

Abstract

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.

Microbial helpers allow cyanobacteria to thrive in ferruginous waters

“…In monocultures, the rescued growth of Synechococcus PCC 7002 in two out of four of high Fe 2+ treatments with Mn 2+ was likely related to the antioxidant properties of Mn 2+ , although the details of the protective mechanism, chemical or enzymatic, await further study. Our findings generally support the hypothesis that elevated seawater Mn 2+ in early Earth environments (~5-120 µM; Holland, 1984;Johnson et al, 2016;Komiya et al, 2008;Liu et al, 2020) played a role in protecting marine cyanobacteria from Fe 2+ toxicity (Fischer et al, 2016).…”

“…(Table S1). Although further experimentation at more environmentally relevant Mn 2+ concentrations is needed, our initial findings generally support the hypothesis that elevated seawater Mn 2+ in early Earth environments (~5-120 µM; Holland, 1984;Johnson et al, 2016;Komiya et al, 2008;Liu et al, 2020) played a role in protecting marine cyanobacteria from ROS (Fischer et al, 2016;Lingappa et al, 2019).…”

Shedding light on manganese cycling in the early oceans

“…The latter is perhaps the least appreciated among these three, even though abiotic reactions likely had elevated importance in early oceans as life was finding its first footholds. Liu et al (1) refocus attention on the photochemical possibility by exploring the details of photooxidation of Mn(II) in the mineral rhodochrosite (MnCO 3 ; Fig. 1) in a series of elegant experiments and by examining the implications of their results in terms of oceanic and atmospheric chemistry during the Archean Eon (4.0 to 2.5 Ga).…”

“…Liu et al (1), to their credit, turn the light back on for photooxidation. In doing so, they focus on rhodochrosite rather than dissolved (reduced) Mn 2+ in seawater, citing interactions involving the Fe 2+ abounding in the same waters that limit Mn 2+ oxidation.…”

“…Not surprisingly, its secrets are not exposed easily, and the plot thickens with each step forward. Liu et al (1), for example, remind us that Mn oxides could form abiotically through photooxidation in the absence of O 2 , which could in theory drive the Cr, Mo, and Tl isotope trends seen in ancient sediments that are interpreted to reflect the presence of O 2 . What's more, Daye et al ( 13) recently argued for an anoxygenic photosynthetic explanation for Mn(II) oxidation, also calling into question novel metal isotope data as definitive windows to the presence and abundance of ancient O 2 (see also ref.…”