始生代の地球表層環境 : 二酸化炭素分圧, 酸素分圧, メタン分圧

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Recently, a large number of geochemical and geological data on Archean have been obtained and the atmospheric gaseous oxygen (O2), carbon dioxide (CO2) and methane (CH4) concentrations were estimated based on these data.PO2 (oxygen partial pressure) of Archean (3.8-2.5 billion age) atmosphere was estimated to be very low (10-5 PAL) from Paleosol compositions (FeO, trace elements, nitrogen isotope), sulfur isotopic composition of sedimentary rocks and presence of major banded iron formation and uranium deposits.PCO2 (carbon dioxide partial pressure) was estimated to be 10-2.5-10 atm and 10-2.5-10-3.5 atm from dynamic equilibrium and chemical equilibrium model, respectively.Recently, it was cited that sulfur isotopic composition of the sedimentary rocks cannot be explained by mass dependent isotopic fractionation but can be by mass independent fractionation.Holland, Kasting, Pavlov and others inferred from these evidence that the Archean atmosphere was characterized by low O2, high CO2 and high CH4 concentrations. Their theory is widely accepted. In contrast Ohmoto and others insist oxygen-rich Archean atmosphere. For example, Ohmoto et al. (2006) and Watanabe and Ohmoto (2007) inferred that mass independent sulfur isotopic fractionation links to thermal and biological evolution of earth (thermal and chemical reduction of sulfate in sedimentary and diagenetic processes). Ohmoto and his associates postulate an essentially constant atmospheric PO2 level since ∼ 4Ga.We need to study the deviation from chemical equilibrium between waters and rocks, mechanism of water-rock interaction, mass independent sulfur isotopic fractionation, mechanism of methane generation and flux to precisely estimate Archean PO2, PCO2 and PCH4.
日本鉱物科学会の論文
2008-05-30

始生代の地球表層環境 : 二酸化炭素分圧, 酸素分圧, メタン分圧

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