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Sulfur Mass-Independent Fractionation (S-MIF) may provide a clue to understanding Earth's early atmosphere. We examined total pressure dependence of the S-MIF produced by SO2 photolysis. Isotopic self-shielding is known to produce S-MIF, which could be changed by both the partial pressure of SO2 (pSO(2)) and by the total pressure (pTotal). Our experimental results show that both S-33 and S-36 values are constant when total pressure is below 10kPa at constant pSO(2), whereas they decrease as total pressure increases. The result suggests that pressure broadening of the SO2 absorption line is responsible for the S-MIF. The modeled high-resolution isotopologue cross sections can reproduce our experimental results and its changes depending on both pSO(2) and pTotal. Consequently, we conclude that the Archean S-33 and S-36 correlation can only be achieved when the total pressure of the Archean atmosphere was below 100kPa, or it was produced in the upper atmosphere.

Plain Language Summary Sulfur Mass-Independent Fractionation (S-MIF) in Archean sedimentary rocks could be useful to constrain the chemical and physical states of Earth's early atmosphere. SO2 photolysis is known to produce large MIF. However, experimental results have yet to model the observed mechanisms of S-MIF in SO2 photolysis. We examined the total pressure effect of S-MIF by SO2 photolysis and succeeded in modeling the observed S-MIF and its total pressure dependence. Comparing the results of this study with geological records, large Archean S-MIF could only be achieved when total pressure is below 100kPa, suggesting that the Archean atmosphere was not thicker than the modern atmosphere or that the S-MIF-yielding photolysis occurred mainly at a high altitude.