[BCG06-05] Mantle data imply that secular oxidation can drive oxygenation of the atmosphere
★Invited Papers
キーワード：GOE
One of the major components of the modern Earth’s atmosphere is O2, which is produced by photosynthesis and is necessary for aerobic lifeforms, including humans. However, according to geological evidence, such as mass-independent sulfur isotope fractionation, O2 levels in the atmosphere were low during the Archean, even after the emergence of oxygenic photosynthesis around 3.2 - 2.8 Ga. It took several hundreds of millions of years before the O2 level increased around 2.4 Ga, which is the start of the Great Oxidation Event (GOE) from ~2.4 to ~2.1 Ga.
In a majority of the literature, a delay of atmospheric oxygenation has been attributed to a secular decrease in O2 sinks rather than a secular increase in O2 sources. However, the actual mechanism for the decrease in O2 sinks is still uncertain. One possibility is the secular oxidation of the mantle, which decreases the proportion of reducing gases in volcanic emissions and therefore, lowers the O2 sinks relative to CO2 fluxes, where CO2 is a substrate for O2 production. Although secular mantle oxidation has been long dismissed, two new studies reveal that the oxygen fugacity of the mantle increased by ~ 1.3 log10 units from the Archean onward. These studies qualitatively suggest that the new trend of the mantle redox state would cause the oxygenation of the atmosphere.
Here, we quantitatively examine the possibility of atmospheric oxygenation caused by mantle oxidation using the data from the new studies. We calculate a ratio of O2 sources to rapid O2 sinks: the oxygenation parameter, Koxy. As explained in the previous studies, Koxy less than unity indicates a reducing atmosphere, while Koxy larger than unity indicates an oxic atmosphere. The effect of secular mantle oxidation increases Koxy with time: Koxy shifts from < 1 to > 1 at ~ 2.5 Ga. Hence, mantle oxidation would trigger the GOE around 2.5 Ga. On the other hand, if we model the mantle with a constant oxidation state, a shift of Koxy from < 1 to > 1 never occurs.
These results do not exclude the possibility that other processes proposed previously contributed the GOE, such as a shift from the submarine to subaerial volcanoes, a secular increase in the degassing ratio of carbon and/or sulfur to hydrogen, and/or a decrease in hydrogen flux via serpentinization. The exact timing of the GOE (~2.4 Ga) is probably explained by combining these processes with the effect of mantle oxidation on the atmosphere. However, our results show that mantle oxidation alone could drive the oxygenation of the atmosphere.
If secular oxidation of the mantle ultimately controlled the timing of the GOE, then the cause of mantle oxidation is ultimately important for setting the tempo of biological evolution. Possible drivers of mantle oxidation are the mixing of a redox heterogeneous mantle and/or the escape of hydrogen to space. Such processes could also apply to other Earth-like planets and would determine whether such planets could be habitats for aerobic lifeforms.
In a majority of the literature, a delay of atmospheric oxygenation has been attributed to a secular decrease in O2 sinks rather than a secular increase in O2 sources. However, the actual mechanism for the decrease in O2 sinks is still uncertain. One possibility is the secular oxidation of the mantle, which decreases the proportion of reducing gases in volcanic emissions and therefore, lowers the O2 sinks relative to CO2 fluxes, where CO2 is a substrate for O2 production. Although secular mantle oxidation has been long dismissed, two new studies reveal that the oxygen fugacity of the mantle increased by ~ 1.3 log10 units from the Archean onward. These studies qualitatively suggest that the new trend of the mantle redox state would cause the oxygenation of the atmosphere.
Here, we quantitatively examine the possibility of atmospheric oxygenation caused by mantle oxidation using the data from the new studies. We calculate a ratio of O2 sources to rapid O2 sinks: the oxygenation parameter, Koxy. As explained in the previous studies, Koxy less than unity indicates a reducing atmosphere, while Koxy larger than unity indicates an oxic atmosphere. The effect of secular mantle oxidation increases Koxy with time: Koxy shifts from < 1 to > 1 at ~ 2.5 Ga. Hence, mantle oxidation would trigger the GOE around 2.5 Ga. On the other hand, if we model the mantle with a constant oxidation state, a shift of Koxy from < 1 to > 1 never occurs.
These results do not exclude the possibility that other processes proposed previously contributed the GOE, such as a shift from the submarine to subaerial volcanoes, a secular increase in the degassing ratio of carbon and/or sulfur to hydrogen, and/or a decrease in hydrogen flux via serpentinization. The exact timing of the GOE (~2.4 Ga) is probably explained by combining these processes with the effect of mantle oxidation on the atmosphere. However, our results show that mantle oxidation alone could drive the oxygenation of the atmosphere.
If secular oxidation of the mantle ultimately controlled the timing of the GOE, then the cause of mantle oxidation is ultimately important for setting the tempo of biological evolution. Possible drivers of mantle oxidation are the mixing of a redox heterogeneous mantle and/or the escape of hydrogen to space. Such processes could also apply to other Earth-like planets and would determine whether such planets could be habitats for aerobic lifeforms.