Simultaneous fractionation of sulfur dioxide explains mass independent fractionation of sulfur isotopes in Archean sedimentary pyrites

The relationship between ∆³⁶S and ∆³³S in Archean sedimentary pyrites has been used to evaluate early geologic processes, including photochemical reactions in the anoxic atmosphere, biological activity and thermochemical alteration during sediment deposition. We have applied statistical methods to quadruple S isotope analyses of Archean sedimentary pyrites, using data compiled from the literature. Most of the best-fit lines, on plots of ∆³⁶S against ∆³³S, have Archean reference array-like ∆³⁶S/∆³³S slopes that vary between −1.5 and − 0.9. Rigorous statistical tests were conducted to calculate the probability of the best-fit lines passing through the origin. Seventeen of 23 ∆³⁶S-∆³³S regression lines, which pass our reliability filter of R² ≥ 75% and ∆³³S range ≥ 2‰, have positive intercepts on the ∆³⁶S axis, and 13 of these have a probability of < 5% of a zero intercept on the ∆³⁶S axis. The observed ∆³⁶S/∆³³S slopes and the non-negative intercepts, which requires at least two mass-independent fractionation source reactions to operate simultaneously, can be produced by UV radiation in the atmosphere at low SO₂ partial pressures by combining collision-induced intersystem crossing in the SO₂ photoexcitation band (240–340 nm), with the self-shielding effect in the SO₂ photolysis band (190–220 nm). The two SO₂ photochemical processes must occur simultaneously in a single atmospheric reservoir in order that the fraction contributed by the end-member process remains constant across the full range of ∆³³S values. We call this process simultaneous fractionation. We applied a two-end-member model to calculate the fraction of S contributed by the SO₂ photoexcitation end-member (f) needed to produce the observed ∆³⁶S/∆³³S gradients and variable intercepts on the ∆³⁶S axis in the Archean sedimentary pyrites, when the other end-member is SO₂ photolysis with the self-shielding. The simplest explanation for variations in f, and therefore variations in ∆³⁶S/∆³⁶S gradients, is that it is controlled by changes in the partial pressure of SO₂ in the atmosphere.