Signatures of sulfur isotope mass-independent fractionations (S-MIF) in Archean sulfate and sulfide minerals have been considered as the most convincing evidence for early anoxic atmosphere, and constrain the atmospheric oxygen level to less than a few ppm until before 2.3 Ga  . Despite the critical importance in understanding early evolutionary history of atmospheric oxygen and biosphere, the source reaction(s) and physical mechanism of S-MIF are poorly constrained. Detailed mechanistic understanding of physical origin of MIF is expected to provide critical constraints on the atmospheric chemistry and ultraviolet light transparency of Earth’s early atmosphere beyond the absence of molecular oxygen, and thus, provide environmental contexts for the origin of life.
Since the pioneering study by Farquhar et al.,  SO2 photolysis with <220 nm UV radiation has been the main candidate for the source of S-MIF. Later laboratory photochemistry experiments, however, yield much smaller magnitude S-MIF  . Theoretical ab initio calculations suggest only weak perturbations in the C state SO2 . Furthermore, the latest spectroscopy measurements showed small isotopologue dependent cross section amplitude difference . What is the S-MIF source reaction if it is not SO2 photolysis? SO2 photoexcitation at 240 to 340 nm produces large MIF . The long life time of the low lying 3B excited state allows expression of isotopologue specific perturbation in the photochemical products (organic-S or sulfate). In addition, photochemistry of SO and S2 could be the source of S-MIF. Photochemical model suggests the preservation of S-MIF signal from SO (and S2) would be more favored than that from SO2 photolysis . I will discuss implications for the early sulfur cycle and pO2 constraints if SO2 photolysis is not the S-MIF source reaction.
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