Photochemical Reactivity of Mn-bearing Carbonate Minerals Under Early Earth and Mars Conditions

Mn-oxides (MnO_x) usually requires highly oxidizing conditions to form under ambient conditions, and thus its occurrance has been widely interpreted as a strong sign for oxidizing atmosphere sustained by oxygenic photosynthesis. However, lack of ozone would allow direct penetration of UV radiation through the early anoxic atmospheres on the Mars and Earth, which could drive multiple photo-oxidation reactions. Here, we test the photo-reactivity of Mn(II)-bearing carbonate minerals using first principle calculations. We found that trace incorporation of Mn(II) would susbtantially lower the band gap of common carbonate minerals, enabling the anoxic photo-oxidation of Mn(II)-bearing carbonates upon 200-400 nm light. Our results imply that photo-oxidation of Mn(II) could be prevalent on the early Mars and Earth, complicating its applicability as a oxygen-barometer and biosignature for oxygenic photosynthesis.