Oxidation rate of Fe and dissolution of biotite : Relationship between atmospheric oxygen evolution and mineral weathering under low oxygen conditions

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Atmospheric oxygen concentration in the Precambrian has been intensively studied for the past decade mainly because it is closely related to the environment of the Earth's surface, especially life evolution. Evolution models of atmospheric oxygen have been proposed based on physical and chemical analysis of paleosols, soils formed by ancient weathering, sulfur isotopes and so on. However, quantitative estimation of atmospheric oxygen levels have not been made between 2.8 and 1.8 billion years ago when atmospheric oxygen is considered to rise for the first time in the Earth's history. We developed a glove box which can control oxygen concentration and make us carry out mineral dissolution experiment We mimicked weathering in the glove box by reacting olivine with water, and examined Fe redistribution. The ratio of Fe oxidized and remaining in the system was 0.92 at 0.2 atm of partial pressure of oxygen(PO2) and 0.15 at 10(-4) atm. The results were well agreed with those observed in the paleosols. We, then made a model of Fe behavior during weathering considering the factors, dissolution rate of a mineral, Fe oxidation rate and groundwater flow rate. We derived a differential equation from the model and estimated the relationships between the concentration ratio of Fe(III) to Fe(II) in a weathering profile and PO 2 quantitatively. The estimation has indicated that pH and groundwater flow rate affect the relationships but mineral dissolution rate or formation time of a weathering profile do not The application of the model to paleosols has revealed that atmospheric oxygen rose gradually between 2.5 and 12.0 billion years ago.