Geochemical and Photochemical Constraints on SIV Concentrations in Natural Waters on Prebiotic Earth

Abstract Aqueous SIV species (, ) derived from volcanogenic atmospheric SO 2 are important to planetary habitability through their roles in proposed origins‐of‐life chemistry and influence on atmospheric sulfur haze formation, but the early cycling of SIV is poorly understood. Here, we combine new laboratory constraints on SIV disproportionation kinetics with a novel aqueous photochemistry model to estimate the concentrations of SIV in natural waters on prebiotic Earth. We show that SIV disproportionation is slow in pH ≥ 7 waters, with timescale T ≥ 1 year at room temperature, meaning that SIV was present in prebiotic natural waters. However, we also show that photolysis of SIV by UV light on prebiotic Earth limited S[IV] < 100 µM in global‐mean steady‐state. Because of photolysis, S[IV] was much lower in natural waters compared to the concentrations generally invoked in laboratory simulations of origins‐of‐life chemistry (≥10 mM), meaning further work is needed to confirm whether laboratory SIV‐dependent prebiotic chemistries could have functioned in nature. S[IV] ≥ 1 µM in terrestrial waters for: (a) SO 2 outgassing ≥20× modern, (b) pond depths <10 cm, or (c) UV‐attenuating agents present in early waters or the prebiotic atmosphere. Marine SIV was sub‐saturated with respect to atmospheric SO 2 , meaning that atmospheric SO 2 deposition was efficient and that, within the constraints of present knowledge, UV‐attenuating sulfur hazes could only have persisted on prebiotic Earth if sulfur emission rates were very high (≳100× modern). Our work illustrates the synergy between planetary science, geochemistry and synthetic organic chemistry toward understanding the emergence and maintenance of life on early Earth.