Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics
Abstract
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 ( P) 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 ( M R), a mass ratio of paleosol to parent rock, of redox-sensitive elements. The CF and Fe R values were evaluated for factors such as homogeneity of immobile elements, erosion, and formation time of weathering. Fe R increased gradually within the time window of ∼2.5-2.1 Ga and remained close to 1.0 since ∼2.1 Ga onwards. Mn R also increased gradually similar to Fe R 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 P 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 P during this time window and their effects on resulting models of P evolution have been also considered. We assumed four cases for the calculations of P variations between 2.5 and 2.0 Ga: no change in either temperature or P, long-term change in only P, long-term changes in both temperature and P, and short-term fluctuations of both temperature and P during the possible, multiple global-scale glaciations. The calculations indicate that P 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 P 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.
- Publication:
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Geochimica et Cosmochimica Acta
- Pub Date:
- July 2011
- DOI:
- 10.1016/j.gca.2011.04.023
- Bibcode:
- 2011GeCoA..75.3982M