Sulfidic Anion Concentrations on Early Earth for Surficial Origins-of-Life Chemistry

Abstract

A key challenge in origin-of-life studies is understanding the environmental conditions on early Earth under which abiogenesis occurred. While some constraints do exist (e.g., zircon evidence for surface liquid water), relatively few constraints exist on the abundances of trace chemical species, which are relevant to assessing the plausibility and guiding the development of postulated prebiotic chemical pathways which depend on these species. In this work, we combine literature photochemistry models with simple equilibrium chemistry calculations to place constraints on the plausible range of concentrations of sulfidic anions (HS^-, HSO₃^-, SO₃^2-) available in surficial aquatic reservoirs on early Earth due to outgassing of SO₂ and H₂S and their dissolution into small shallow surface water reservoirs like lakes. We find that this mechanism could have supplied prebiotically relevant levels of SO₂-derived anions, but not H₂S-derived anions. Radiative transfer modeling suggests UV light would have remained abundant on the planet surface for all but the largest volcanic explosions. We apply our results to the case study of the proposed prebiotic reaction network of Patel et al. ( 2015 ) and discuss the implications for improving its prebiotic plausibility. In general, epochs of moderately high volcanism could have been especially conducive to cyanosulfidic prebiotic chemistry. Our work can be similarly applied to assess and improve the prebiotic plausibility of other postulated surficial prebiotic chemistries that are sensitive to sulfidic anions, and our methods adapted to study other atmospherically derived trace species.

Keywords: Early Earth; Origin of life; Planetary environments; Prebiotic chemistry; UV radiation; Volcanism.

FIG. 1.

Concentrations of sulfur-bearing compounds and pH as a function of pH₂S for a well-mixed aqueous reservoir. [H₂S] is calculated from Henry's law; the concentrations of HS⁻ and S²⁻ are calculated from equilibrium chemistry for (1) solutions buffered to various pH values and (2) unbuffered solutions with varying ionic strengths. The vertical dotted line demarcates the expected pH₂S for an abiotic Earth with a weakly reducing CO₂-N₂ atmosphere with modern levels of sulfur outgassing, from Hu et al. (2013). The vertical dashed line demarcates the expected pH₂S for the same model but with outgassing levels of sulfur corresponding to the upper limit of the estimate for the emplacement of the terrestrial flood basalts. In the red shaded area, pH₂S is so high it blocks UV light from the planet surface, meaning UV-dependent prebiotic pathways, e.g., those of Patel et al. (2015), cannot function (Ranjan and Sasselov, 2017). The red curve largely overplots the orange, demonstrating the minimal impact of ionic strength on the calculation for I ≤ 0.1. The horizontal dashed and dotted lines demarcate micromolar and millimolar concentrations, respectively. The cyanosulfidic chemistry of Patel et al. (2015) has been demonstrated at millimolar S-bearing photoreductant concentrations, and at least high micromolar levels of these compounds are thought to be required for high-yield prebiotic chemistry.

FIG. 2.

Concentrations of sulfur-bearing compounds and pH as a function of pSO₂ for a well-mixed aqueous reservoir. [SO₂] is calculated from Henry's law; the concentrations of HSO₃⁻, SO₃²⁻, and HS₂O₅⁻ are calculated from equilibrium chemistry for (1) solutions buffered to various pH values and (2) unbuffered solutions with varying ionic strengths. The vertical dotted line demarcates the expected pSO₂ for an abiotic Earth with a weakly reducing CO₂-N₂ atmosphere with modern levels of sulfur outgassing, from Hu et al. (2013). The vertical dashed line demarcates the expected pSO₂ for the same model, but with outgassing levels of sulfur corresponding to the upper limit of the estimate for the emplacement of the terrestrial flood basalts. In the red shaded area, pSO₂ is so high it blocks UV light from the planet surface, meaning UV-dependent prebiotic pathways, e.g., those of Patel et al. (2015), cannot function (Ranjan and Sasselov, 2017). The red curve largely overplots the orange, demonstrating the minimal impact of ionic strength on the calculation for I ≤ 0.1. The horizontal dashed and dotted lines demarcate micromolar and millimolar concentrations, respectively. The cyanosulfidic chemistry of Patel et al. (2015) has been demonstrated at millimolar S-bearing photoreductant concentrations, and at least high micromolar levels of these compounds are thought to be required for high-yield prebiotic chemistry.

FIG. 3.

Speciation of sulfur-bearing molecules in an aqueous reservoir buffered to pH = 7 as a function of total sulfur emission flux Φ_S. The range of Φ_S highlighted by Halevy and Head (2014) for emplacement of basaltic plains on Earth is shaded in gray. Horizontal dashed and dotted lines demarcate micromolar and millimolar concentrations, respectively.

FIG. 4.

UV surface radiance for early Earth as a function of Φ_S, using the models of Hu et al. (2013). The black solid line indicates the top-of-atmosphere (TOA) flux, i.e., the irradiation incident at the top of the atmosphere from the young Sun. The vertical dashed line demarcates 254 nm, the wavelength at which the low-pressure mercury lamps commonly used in prebiotic chemistry experiments emit.

FIG. 5.

Speciation of sulfur-bearing molecules in a shallow lake buffered to pH = 7 as a function of total sulfur emission flux Φ_S, using a dynamic calculation with source atmospheric deposition and sink redox reactions. The range of Φ_S highlighted by Halevy and Head (2014) for emplacement of basaltic plains on Earth is shaded in gray. Horizontal dashed and dotted lines demarcate micromolar and millimolar concentrations, respectively. [HS⁻] would not be able to achieve the high concentrations calculated here for the slow disproportionation (low k₁₇) case due to solubility constraints.

FIG. C1.

Dependence of Henry's law constants for H₂S and SO₂ on [NaCl], calculated using the formalism from Burkholder et al. (2015). and are insensitive to [NaCl] for [NaCl] <1 M.

FIG. D1.

Temperature dependence of Henry's law constants for H₂S and SO₂, calculated using the formalism from Burkholder et al. (2015). Varying the temperature by 25 K relative to the reference temperature of 298.15 K (blue line) affects the value of and by less than a factor of 2.5.