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

前寒武纪大气氧浓度与地球表面环境，特别是生命演化密切相关，因此近十年来一直是研究的热点。通过对古土壤、古风化作用形成的土壤、硫同位素等的理化分析，提出了大气氧的演化模式，但至今还没有对地球历史上首次出现大气氧上升的28 ~ 18亿年前的大气氧水平进行定量估算。我们研制了一个可以控制氧浓度的手套箱，使我们能够进行矿物溶解实验。我们在手套箱中通过橄榄石与水反应来模拟风化，并研究了Fe的再分布。在氧分压（PO 2）为0.2大气压时，体系中的Fe氧化和残留的比率为0.92，在10（-4）大气压时为0.15。这一结果与古土壤中的观察结果非常一致。在此基础上，综合考虑矿物溶解速率、铁氧化速率和地下水流量等因素，建立了风化过程中铁的行为模型。根据该模型推导出微分方程，定量估算了风化剖面中Fe（III）/Fe（II）浓度比与PO 2的关系。估算结果表明，pH值和地下水流速对这种关系有影响，但矿物溶解速率和风化剖面的形成时间对这种关系没有影响。模型在古土壤中的应用表明，在25亿~ 120亿年前，大气中的氧含量逐渐上升。

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.