Mass-independent fractionation of sulfur isotopes by isotope-selective photodissociation of SO₂

A quantitative understanding the origin of sulfur isotope mass-independent fractionation (MIF) is essential to a full interpretation of the Archean sulfur geochemical record. Laboratory experiments have demonstrated that a MIF signature is present in elemental sulfur produced during SO₂ photolysis, but the underlying mechanism remains unknown. Here, I report the results of atmospheric chemistry modeling of isotope-selective photodissociation of SO₂ in the C¹B₂ - X¹A₁ bands from 190 to 220 nm. This band system is dominated by a bending mode progression that produces shifts in the absorption spectrum upon sulfur isotope substitution. Self-shielding in the rotationally-resolved lines of ³²SO₂ produces MIF signatures in SO and residual SO₂. A self-shielding origin for sulfur MIF implies that the variations observed in Δ³³S in Archean rocks reflect variation in atmospheric SO₂ concentration, and demonstrates that MIF: in terrestrial rocks can be derived from photochemistry independent of molecular symmetry.