2024-03-13 アリゾナ大学
<関連情報>
- https://news.arizona.edu/story/loathed-scientists-loved-nature-sulfur-and-origin-life
- https://news.arizona.edu/story/loathed-scientists-loved-nature-sulfur-and-origin-life
前生物学的地球の自然水中のS[IV]濃度に関する地球化学的および光化学的制約 Geochemical and Photochemical Constraints on S[IV] Concentrations in Natural Waters on Prebiotic Earth
Sukrit Ranjan, Khaled Abdelazim, Gabriella G. Lozano, Sangita Mandal, Cindy Y. Zhou, Corinna L. Kufner, Zoe R. Todd, Nita Sahai, Dimitar D. Sasselov
AGU Advances Published: 15 December 2023
DOI:https://doi.org/10.1029/2023AV000926
Abstract
Aqueous S[IV] species (HSO3–, SO32-) 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.
Key Points
- We use experiments and modeling to constrain S[IV] (sulfite) concentrations in marine and terrestrial waters on prebiotic Earth (∼3.9 Ga)
- We show that S[IV] was a prebiotic reagent, but its concentration were limited to <100 μM by photolysis in early natural waters
- Our work shows the need to characterize the sensitivity of proposed chemical pathways for the origin of life to S[IV] abundance
Plain Language Summary
Sulfur cycling on early Earth is not well understood because its chemical reactions in water in the absence of biology are poorly constrained. Here, we build a new model to estimate the concentrations of a key family of sulfur molecules, S[IV] (“sulfite”), in oceans and ponds on early Earth. We use new measurements of S[IV] reactions to calibrate our model and include the effects of UV light. We show that S[IV] was present on early Earth, but that UV light limited its concentrations to <100 μM. This finding has significant implications for efforts to understand the origin and maintenance of early life. First, it means that early natural environments featured S[IV], but at much lower concentrations than considered in laboratory simulations of origin-of-life chemistry. It is necessary to confirm whether proposed S[IV]-dependent origins-of-life chemistry can function at the generally lower S[IV] concentrations characteristic of early Earth environments. Second, it means that sulfur hazes could only have persisted on early Earth if volcanic sulfur emission was very high, favoring a generally UV-rich surface environment for nascent life. Our work illustrates the critical role planetary science and geochemistry play in guiding and testing chemical theories of the origin and endurance of life.
