Freshwater and Evaporite Brine Compositions on Hadean Earth: Priming the Origins of Life.

Abstract

The chemical composition of aqueous solutions during the Hadean era determined the availability of essential elements for prebiotic synthesis of the molecular building blocks of life. Here we conducted quantitative reaction path modeling of atmosphere-water-rock interactions over a range of environmental conditions to estimate freshwater and evaporite brine compositions. We then evaluated the solution chemistries for their potential to influence ribonucleotide synthesis and polymerization as well as protocell membrane stability. Specifically, solutions formed by komatiite and tonalite (primitive crustal rocks) weathering and evaporation-rehydration (drying-wetting) cycles were studied assuming neutral atmospheric composition over a wide range of values of atmospheric partial pressure of CO2 (PCO2) and temperatures (T). Solution pH decreased and total dissolved concentrations of inorganic P, Mg, Ca, Fe, and C (PT, MgT, CaT, FeT, and CT) increased with increasing PCO2. The PCO2 and T dictated how the solution evolved with regard to minerals precipitated and ions left in solution. At T = 75°C and PCO2 < 0.05 atm, the concentration ratio of magnesium to calcium ion concentrations (Mg2+/Ca2+) was < 1 and predominantly metal aluminosilicates (including clays), dolomite, gibbsite, and pyrite (FeS2) precipitated, whereas at PCO2 > 0.05 atm, Mg2+/Ca2+ was > 1 and mainly magnesite, dolomite, pyrite, chalcedony (SiO2), and kaolinite (Al2Si2O5) precipitated. At T = 75°C and PCO2 > 0.05 atm, hydroxyapatite (HAP) precipitated during weathering but not during evaporation, and so, PT increased with each evaporation-rehydration cycle, while MgT, CaT, and FeT decreased as other minerals precipitated. At T = 75°C and PCO2 ∼5 atm, reactions with komatiite provided end-of-weathering solutions with high enough Mg2+ concentrations to promote RNA-template directed and montmorillonite-promoted nonenzymatic RNA polymerization, but incompatible with protocell membranes; however, montmorillonite-promoted RNA polymerization could proceed with little or no Mg2+ present. Cyclically evaporating/rehydrating brines from komatiite weathering at T = 75°C and PCO2 ∼5 atm yielded the following: (1) high PT values that could promote ribonucleotide synthesis, and (2) low divalent cation concentrations compatible with amino acid-promoted, montmorillonite-catalyzed RNA polymerization and with protocell membranes, but too low for template-directed nonenzymatic RNA polymerization. For all PCO2 values, Mg2+ and PT concentrations decreased, whereas the HCO3- concentration increased within increasing temperature, due to the retrograde solubility of the minerals controlling these ions' concentrations; Fe2+ concentration increased because of prograde pyrite solubility. Tonalite weathering and cyclical wetting-drying reactions did not produce solution compositions favorable for promoting prebiotic RNA formation. Conversely, the ion concentrations compatible with protocell emergence, placed constraints on PCO2 of early Earth's atmosphere. In summary: (1) prebiotic RNA synthesis and membrane self-assembly could have been achieved even under neutral atmosphere conditions by atmosphere-water-komatiite rock interactions; and (2) constraints on element availability for the origins of life and early PCO2 were addressed by a single, globally operating mechanism of atmosphere-water-rock interactions without invoking special microenvironments. The present results support a facile origins-of-life hypothesis even under a neutral atmosphere as long as other favorable geophysical and planetary conditions are also met.