Photochemistry of Sulfur Dioxide and the Origin of Mass-Independent Isotope Fractionation in Earth's Atmosphere

Archean sulfide and sulfate minerals commonly exhibit anomalous ratios among four stable sulfur isotopes, 32 S, 33 S, 34 S, and 36 S. These anomalous relationships, referred to as sulfur mass-independent fractionation (S-MIF), provide strong evidence for an early anoxic atmosphere. Correlated variations among three isotope ratios (δ 33 S, δ 34 S, and δ 36 S) can be observed in rocks throughout the Archean and are a key clue toward identifying the source reaction of S-MIF. Studies to investigate the origin of Archean S-MIF so far have primarily focused on the photochemistry of sulfur dioxide (SO 2 ). Photolysis of SO 2 at wavelengths <220 nm and photoexcitation at 240–340 nm both yield large-magnitude S-MIF. Proposed mechanisms of S-MIF include isotopologue-dependent self-shielding, cross-sectional amplitudes, and vibronic coupling during intersystem crossing. This review discusses the emerging picture of the physical origins of S-MIF and their implications for the chemistry of the early Earth's atmosphere.