[B-PT05] 地球史解読：冥王代から現代まで

[BPT05-19] 35 – 34億年前と27 – 22億年前の地球表層の酸素増加

*海保 邦夫¹、片山 悠貴¹、川瀬 智大¹、Altermann Wladyslaw³、Van Kranendonk Martin⁴、小宮 剛²、齊藤 諒介¹ (1.東北大学大学院理学研究科、2.東京大学、3.プレトリア大学、4.ニューサウスウェールズ大学)

キーワード：始生代、酸素、有機分子

Great oxidation event (GOE) on Earth occurred approximately 2.3 billion years ago (Ga) based on iron redox state and sulfur isotopes. However, the age of first oxidation of the Earth’s surface is controversial. Presence of small amount of O₂ in the environment at 2.5 Ga has been suggested by redox sensitive elements and sulfur isotopes and at 2.7 Ga by nitrogen isotopes. We analyzed hydrocarbon-derived geochemical parameters diagnostic for sedimentary redox conditions, i.e., the pristane/phytane ratio (Pr/Ph), from shallow marine sedimentary rocks in Labrador, Canada, Western Australia, and South Africa. Here, we show that anoxic shallow sea at >3.95 Ga was followed by intermediate conditions at 3.5–2.9 Ga, local oxic conditions at 2.7 Ga, and subsequently global oxidation at 2.6–2.2 Ga. These results indicate that the rise of oxygen in the Earth surface occurred at 3.5–3.4 Ga and 2.7–2.2 Ga.