[B-CG06] 地球史解読：冥王代から現代まで

[BCG06-07] Possibility of atmospheric deoxygenation events caused by intense volcanisms during the Proterozoic

*渡辺 泰士¹、尾崎 和海²、門屋 辰太郎²、原田 真理子³、松本 廣直⁴、田近 英一¹ (1.東京大学、2.東京科学大学、3.海洋研究開発機構、4.筑波大学)

キーワード：原生代、酸素

The dynamics and stability of the atmospheric pO₂ is fundamental in understanding the long-term maintenance of the Earth’s habitability and evolutions of life and environment. One of the most dramatic changes in the atmospheric evolution of the Earth occurred during the Paleoproterozoic, which is the pervasive oxygenation of the atmosphere known as the Great Oxidation Event at ~2.4–2.2 billion years ago. Previously, the onset of the Great Oxidation Event has been constrained based on the disappearance of the mass-independent fractionation of sulfur (S-MIF), which is estimated to be around 2.43 Ga (Gumsley et al., 2017; Warke et al., 2020). However, the estimated timings of the complete disappearance of the S-MIF signal from geological records, which corresponds to the end of the Great Oxidation Event, disperses from 2.32 to 2.22 Ga (Bekker et al., 2004; Luo et al., 2016; Poulton et al., 2021; Uveges et al., 2023). Specifically, it has been shown that the post-2.3-Ga S-MIF signals are stratigraphically restricted (Uveges et al., 2023). This compilation indicates the potential occurrence of repeated very short-term fluctuations of the atmospheric pO₂, whose origin remains unresolved. In this study, we show that these atmospheric deoxygenation events can be explained by the supply of reducing materials from intense volcanism associated with emplacements of large igneous provinces using an equilibrium thermodynamic model (Kadoya et al., 2021). We further employ a model of C–P–O₂–Fe–S biogeochemical cycles developed based on the previous study (Watanabe et al., 2023) and demonstrate that intense volcanisms associated with emplacements of large igneous provinces can cause the transient atmospheric deoxygenation events during the Paleoproterozoic. We show that the atmospheric pO₂ would drop to reducing conditions that can produce S-MIF signals during an intense volcanism and recover within ~0.5 million years supported by the oceanic eutrophication caused by the intense continental weatherings. During these transient atmospheric deoxygenation events, the deposition site of iron hydroxides would have shifted from deep oceans to the surface oceans. This would have caused the massive deposition of iron formations onto the continental crusts, which can explain the remaining Paleoproterozoic iron formations. We further discuss that the atmospheric deoxygenation events are prone to occur in the Proterozoic condition when the reservoir size of atmospheric oxygen is small compared with the present-day value. These results imply that the Earth has experienced repeated atmospheric deoxygenation in the Paleoproterozoic.