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

Abstract

Sulfur is important to planetary habitability, but the early sulfur cycle is poorly understood. In particular, S[IV] species (HSO, SO), derived from volcanogenic SO, are critically invoked in recent proposals for origins-of-life chemistry and also influence atmospheric sulfur haze formation, but their abundance in early natural waters is unclear. Here, we combine new laboratory constraints on the kinetics of S[IV] disproportionation 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] limits [S[IV]]< 100 μM in global-mean steady state. Marine S[IV] was sub-saturation with respect to atmospheric SO, meaning that climate-altering, UV-attenuating sulfur hazes did not persist on prebiotic Earth. [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 S[IV]-dependent prebiotic chemistries discovered in the lab could have realistically functioned in nature. [S[IV]]≥1 μM in terrestrial waters for: (1) SO outgassing ≥20× modern, (2) pond depths <10 cm, or (3) UV-attenuating agents present in early waters or the prebiotic atmosphere. Our work illustrates the synergy between planetary science, geochemistry and synthetic organic chemistry experiments in understanding the emergence and maintenance of life on early Earth.

Favorable Environments for the Formation of Ferrocyanide, a Potentially Critical Reagent for Origins of Life

“…The seepage rate S of lakes is variable: we explore a range of S = 0−2 m/year, following Ranjan et al (2023). 34 Closed basin lakes, meaning lakes without outlet streams, have no loss due to seepage (S = 0), and water loss is only due to evaporation in these cases, which allows for increasing concentrations of dissolved chemical species (e.g., Toner and Catling 26 ). The upper limit for seepage rate considered here, S = 2 m/yr corresponds to the estimates for Lake Castor, located in Washington 35 and fishponds near Auburn, Alabama.…”

The Search for Slow Sulfur Sinks

“…Part of that missing map is due to the limits in our understanding of sulfur, as is discussed by Ranjan et al., 2023. Sulfur is thought to play critical roles in prebiotic chemistry, volatile cycling, and climate in large part due to its ability to gain and lose electrons and participate in chemistry as a gas, dissolved in liquids, or as a solid.…”

“…The work of Ranjan et al., 2023 helps to draw a more complete map and furnishing a seaworthy vessel in three ways. First, their efforts are advancing our understanding of the earliest part of Earth's history.…”

“…All these effects together mean that sulfur reactions are some of the most poorly constrained (if at all), and given sulfur's chemical flexibility, that's a large fraction of the prebiotic chemical toolbox that is inaccessible or unknown. The experiments of Ranjan et al., 2023 in particular probe slow reactions, which are competing with other processes that are orders of magnitude faster, but are a necessary component if we are to understand the environment in which life arose on the early Earth.…”

“…Lastly, Ranjan et al. (2023) results and their evaluations are presented in a clear, compelling, and forthright fashion, representing a textbook example of the scientific method in action. Their work should be held up as a demonstration for not just enterprising young scientists but to those of us who occupy more than one discipline or that struggle to communicate with a new audience.…”