Geochemical and Photochemical Constraints on S[IV] Concentrations in Natural Waters on Prebiotic Earth

Aqueous S[IV] species (HSO₃^-SO₃^2-) derived from volcanogenic atmospheric SO₂ 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 S[IV] is poorly understood. Here, we combine new laboratory constraints on S[IV] disproportionation kinetics with a novel aqueous photochemistry model to estimate the concentrations of S[IV] in natural waters on prebiotic Earth. We show that S[IV] disproportionation is slow in pH ≥ 7 waters, with timescale T ≥ 1 year at room temperature, meaning that S[IV] was present in prebiotic natural waters. However, we also show that photolysis of S[IV] 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 S[IV]-dependent prebiotic chemistries could have functioned in nature. [S[IV]] ≥ 1 µM in terrestrial waters for: (a) SO₂ outgassing ≥20× modern, (b) pond depths <10 cm, or (c) UV-attenuating agents present in early waters or the prebiotic atmosphere. Marine S[IV] was sub-saturated with respect to atmospheric SO₂, meaning that atmospheric SO₂ 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.