[B-CG09] Decoding the history of Earth: From Hadean to the present

[BCG09-08] Total pressure dependence of sulfur mass-independent fractionation by SO₂ photolysis

*Yoshiaki Endo¹, Yuichiro Ueno^1,2,3, Miho Oinuma⁴, Sebastian Oscar Danielache^2,4 (1.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2.Earth-Life Science Institute, 3.Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC, 4.Faculty of Science and Technology, Sophia University)

Keywords:Archean atmosphere, Sulfur isotopes, Mass-independent fractionation, Self-shielding

Sulfur mass-independent fractionation (S-MIF) in Archean sedimentary rocks is a useful proxy for atmospheric chemistry. Archean S-MIF signatures typically show that Δ³⁶S/Δ³³S values are close to −1. We have succeeded to reproduce the Δ³⁶S/Δ³³S slope experimentally (Endo et al., 2016). Based on the experimental results, we suggested that Archean S-MIF can be explained by a combination of two effects: (1) self-shielding of SO₂ photolysis (Δ³⁶S/Δ³³S ~ −2.4) and (2) intersystem-crossing in excited SO₂ (Δ³⁶S/Δ³³S ~ +0.7). However, several results of previous photochemical experiments cannot be explained only by the two effects (Ono et al. 2013, Masterson et al. 2011). Total pressure may cause an additional effect on the S-MIF, though the mechanism is poorly understood. Here, we conducted SO₂ photolysis experiments under various total pressures from 0.1 bar to 1.7 bar and measured sulfur isotope fractionations. Measured Δ³⁶S/Δ³³S values changed depending on total pressure. We discuss the mechanism of total pressure dependence of S-MIF by SO₂ photolysis in terms of pressure broadening effect (Lyons et al., 2018) and present an implication for the total pressure of the Archean atmosphere.