Sulfur isotope fractionation by broadband UV radiation to optically thin SO2 under reducing atmosphere

Abstract

Photochemical mechanisms of Sulfur Mass-Independent Fractionation (S-MIF) are still poorly understood. Previous laboratory experiments have indicated that the S-MIF depends largely on the spectrum of the incident light source and the partial pressure of SO2, though the basic character of the Archean S-MIF (ΔS36/ΔS33=∼−1) has never been reproduced. We have conducted new photochemical experiments at low pSO2 (1–10 Pa) conditions under the presence of CO and found a reasonable mechanism to reproduce the Δ36S/Δ33S slope about −1. As previously suggested (Ono et al., 2013), the low pSO2 is key to studying the self-shielding effect within a range of realistic atmospheric conditions. Also, reducing conditions are critical for simulating the O2-poor atmosphere, whereas photolysis of pure SO2 provides excess O atoms that significantly change the overall chemistry. Our experimental results confirmed that significant S-MIF (ΔS36/ΔS33=−2.4) can be produced by self-shielding in the SO2 photolysis band (185–220 nm), even if the SO2 column density is as low as 1016 molecules cm−2. Thus, photolysis within a volcanic plume of ∼0.1 ppm SO2 is capable of producing a large S-MIF signature. The isotopic fractionations originating from the different absorption cross sections of SO2 isotopologues (i.e. wavelength dependent effect; without self-shielding) are only minor (potentially up to +4‰ for Δ33S). Under reducing conditions, however, another S-MIF signal with Δ36S/Δ33S ratio of ∼+0.7 is produced due to collision-induced intersystem crossing (ISC) from singlet to triplet states of SO2 (Whitehill et al., 2013), and should also be transferred into the final product that is responsible for changing the Δ36S/Δ33S slope. Based on a photochemical model of the S–O–C system with the two S-MIF-yielding reactions, the largest S-MIF observed in the late Archean Mt. McRae Fm. (ΔS33=+9.4‰, ΔS36=−7.5‰) can be reproduced by solar UV irradiation of a SO2 column of ∼6.4×1016 molecules cm−2 with a sufficiently high concentration of reducing gasses (∼2% CO or CH4) where the ISC-derived MIF contributes ∼3% through the photoexcitation channel initiated in the 240–340 nm region. Our work shows that a photochemical model considering the two major S-MIF-yielding reactions (SO2+hν→SO+O and SO21+M→SO23+M) can explain the behavior of the S-MIF observed in laboratory experiments. The combination of the two effects is more important under reducing condition and should be considered to interpret the geological record.