光化学実験を反映した太古代大気中の硫黄非質量依存分別のフォワードモデル

抄録

Mass-independent fractionation of sulfur isotopes (S-MIF; Δ33S, Δ36S ≠ 0) will provide a hint of the Archean atmospheric composition. The most basic characteristic of Neoarchean S-MIF is a Δ36S/Δ33S slope of ~-1, which can be explained by a combination of S-MIF produced during SO2 photolysis and photo-deexcitation, as suggested by photochemical experiments (Endo et al., 2016). To quantify Δ33S and Δ36S, we updated the 1D photochemical sulfur isotope model of Claire et al. (2014) to incorporate S-MIF patterns observed in SO2 photochemical experiments. To incorporate experimentally determined isotope effects, we used SO2 absorption cross sections that describes self-shielding in SO2 photolysis (Endo et al., 2019). Our results show that significant Δ33S anomalies (>+1‰ in S8 aerosols) due to self-shielding appear when the SO2 flux exceeds 1.4×1011 mol/yr (~100 times the modern volcanic flux). The propagation of photo-deexcitation-derived S-MIF to reduced sulfur is less efficient, even under high CO levels (1.2% CO). This model represents the first quantitative attempt to reproduce Δ36S/Δ33S slopes using laboratory-constrained mechanisms.