The increase in atmospheric oxygen during the Precambrian is a key to understand the co-evolution of life and environment and has remained as a debatable topic. Among various proxies for the estimation of atmospheric oxygen levels, paleosols, ancient weathering profiles, can provide a quantitative pattern of atmospheric oxygen increase during the Precambrian period of Earth history. We have re-evaluated the chemical compositions of paleosols, and presented a new method of applying Fe
2+ oxidation kinetics to the Fe
2+ and Fe
3+ concentrations in paleosols to decipher the quantitative partial pressure of atmospheric oxygen (
PO2) between 2.5 and 2.0 Ga. We first estimated the compaction factor (
CF, the fraction of original thickness) using the immobile elements such as Ti, Al and Zr on equal volume basis, which was then used to calculate retention fractions (
MR), a mass ratio of paleosol to parent rock, of redox-sensitive elements. The
CF and
FeR values were evaluated for factors such as homogeneity of immobile elements, erosion, and formation time of weathering.
FeR increased gradually within the time window of ∼2.5-2.1 Ga and remained close to 1.0 since ∼2.1 Ga onwards.
MnR also increased gradually similar to
FeR but at a slower rate and near complete retention was observed ∼1.85 Ga, suggesting an almost continuous increase in the oxidation of Fe
2+ and Mn
2+ in paleosols ranging in age between ∼2.5 and 1.9 Ga.We have modeled
PO2 variations during the Paleoproterozoic by applying Fe
2+ oxidation kinetics to the Fe
2+ and Fe
3+ concentrations in paleosols, which enabled us to derive an Fe
2+ oxidation term referred to as
ψ. Possible changes in temperature and
PCO2 during this time window and their effects on resulting models of
PO2 evolution have been also considered. We assumed four cases for the calculations of
PO2 variations between 2.5 and 2.0 Ga: no change in either temperature or
PCO2, long-term change in only
PCO2, long-term changes in both temperature and
PCO2, and short-term fluctuations of both temperature and
PCO2 during the possible, multiple global-scale glaciations. The calculations indicate that
PO2 increased gradually, linearly on the logarithmic scale, from <∼10
−6 to >∼10
−3 atm between 2.5 and 2.0 Ga. Our calculations show that the
PO2 levels would have fluctuated significantly, if intense, global glaciation(s) followed by period(s) of high temperature occurred during the Paleoproterozoic. This gradual rise model proposes a distinct, quantitative pattern for the first atmospheric oxygen rise with important implications for the evolution of life.
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