[U-05_30AM2] 生命-水-鉱物-大気相互作用

★ [U05-06] 初期大気進化の解読：D36S/D33S化学層序の実験的復元

*上野 雄一郎¹、遠藤 美朗¹、三島 郁¹、ダニエラチェ セバスチアン² (1.東京工業大学地球惑星科学専攻、2.上智大学理工学研究科)

キーワード：太古代, 大気化学, 非質量依存分別

Sulfur Mass-Independent Fractionation (S-MIF) has potential to monitor chemistry of the Earth's early atmosphere (Farquhar et al., 2000). Nonetheless, detailed mechanism of the S-MIF occurred in the Archean atmosphere is still poorly understood. Previous laboratory experiments indicate the anomalous isotopic fractionation depends largely on (1) wavelength or spectrum of the incident light source and (2) partial pressure of SO2, though none of these experiments have not yet succeeded to fully reproduce the S-MIF recorded in the Archean sedimentary rocks (e.g., Danielache et al., 2008; Masterson et al., 2011; Whitehill & Ono, 2012). We have developed a new photochemical chamber for determining isotopic effect of the SO2 photolysis under optically thin condition. Also, a new direct fluorination technique of carbonyl sulfide allowed us precise isotopic analysis down to 50 nmolS of photolysis product. The results indicate that the basic character of the S-MIF observed in the Archean record can be reproduced when SO2 column density is reasonably low (i.e. 10 to 50 times higher than preindustrial atmosphere). The results with a numerical modeling of the atmospheric reaction network suggest that the observed change in D36S/D33S ratio can be adequately explained by the two factors: (1) SO2 partial pressure and (2) amount of reducing gas (H2, CH4 and CO). In light of the new perspective, we have re-evaluated the geological record of the D36S/D33S ratio with additional analyses of Archean sedimentary sulfides from South Africa and India. Based on the magnitude of the S-MIF and the D36S/D33S ratio, the Archean period can be subdivided into four stages (i.e. ›3.0 Ga, 3.0-2.7 Ga, 2.7-2.5 Ga and 2.5-2.4 Ga). These changes probably reflect both intensity of volcanic SO2 emission and concentration of reducing gasses under the O2-free atmosphere. Particularly, the maximum scatter of D33S values observed in the stage 3 (2.7-2.5 Ga) requires high volcanic emission as well as very reducing atmospheric condition in the atmosphere at that time.