Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics |
| |
| Authors: | Takashi Murakami Bulusu Sreenivas Subrata Das Sharma |
| |
| Institution: | a Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-0033, Japan
b National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad 500 007, India |
| |
| 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 Fe2+ oxidation kinetics to the Fe2+ and Fe3+ 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 Fe2+ and Mn2+ in paleosols ranging in age between ∼2.5 and 1.9 Ga.We have modeled PO2 variations during the Paleoproterozoic by applying Fe2+ oxidation kinetics to the Fe2+ and Fe3+ concentrations in paleosols, which enabled us to derive an Fe2+ 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. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
