No abstract

The effect of dissolved O 2 (DO) concentration or partial pressure of atmospheric oxygen (P O 2 ) on the Fe(II) oxidation rate was investigated at room temperature, by changing pH from 6.89 to 8.03 and P O 2 from 7.5 × 10 −5 to 0.20 atm. The oxidation experiments were conducted in a glove box into which a gas mixture of Ar and Ar + O 2 (1%) was introduced continuously to maintain a given DO concentration. The DO concentration was changed by varying the mixing ratio of Ar and Ar + O 2 (1%). The decay constant of the oxidation (λ) was estimated from the experimental data, by assuming that the reaction shows pseudo -first -order behavior: ) and P O 2 . The relationships revealed that the oxidation rate deviated from the linear dependence (x = 1) at low P O 2 (<10 −2 atm) and that the oxidation proceeded faster than in the case where x was assumed to be 1. Our results implied that the atmospheric O 2 levels (between 2.5 and 2.0 Ga) estimated from the Fe(II) oxidation in paleosols, ancient soils formed by weathering, are overestimated if the x dependence of the oxidation rate is not taken into account.