Abstract EANA2026-4

After a planet formed a crust of silicate minerals (feldspars, pyroxenes, and olivines) and cooled enough to condense water, separating it from an early atmosphere, chemical reactions start. The process can be seen as a kind of planetary titration. Silicate rocks, volcanic and impact-generated glassy clasts, react by hydration and the consumption of acids like H₂CO₃, HCl and H₂SO₃. In the process (→ Urey-reaction), silicates suffer disproportion into insolubles such as clays, and cations in solution, charge-balanced by acid-derived anions. Vice versa, if large amounts of clay minerals are found, then minerals composed of acid anhydrites and soluble cations must exist elsewhere in stoichiometric amounts. The ensuing chemistry of early planetary ocean and/or lakes depends on amounts and proportion of available acids or, more specific, their gaseous anhydrites CO₂, Cl₂ and SO₂. For Earth, their amounts are about 65.5*10²¹g C (equivalent to 5.5*10²¹ mol of H₂CO₃), 52*10²¹g Cl (equivalent to 1.6*10²¹ mol of HCl), and 5.2*10²¹g S (equivalent to 0.16*10²¹ mol of H₂SO₃) which were consumed throughout Earth history (Kempe & Degens, 1985). Thus, H₂CO₃ out-factored HCl 3.7-, and H₂SO₃ 34-times in weathering. A primordial komatiitic crust had a [Ca²⁺+Mg²⁺]/[Na⁺+K⁺] molar ratio of 1.6, with Cl^- and SO₃^2- insufficient to balance Na⁺ + K⁺, leaving much of it to be balanced by total alkalinity (TA = the charge-sum of [HCO₃^- +2CO₃^2-] plus several minor ions). Terrestrial extrusive igneous rocks changed in chemistry towards basaltic composition with lower Mg-concentrations compared to komatiites, thus favouring Na- and K-carbonates even more. Therefore, weathering solutions on early Earth, must have had a predominance of carbonate ions and not of chloride or even sulphate ions. The ensuing chemistry of terminal water bodies must therefore have been alkaline with evermore increasing concentrations forming a “soda ocean” similar in composition as modern “soda lakes”. Calcite supersaturation would be surpassed first, it precipitates inorganically at a SI of 0.8 to 1.2 (Saturation Index (SI_calcite) = log([Ca²⁺]*[CO₃^2-]/K_calcite). With the ongoing depletion of Ca and the increase in the Mg/Ca ratio other minerals precipitate, such as aragonite, high-Mg calcite, magnesite and protodolomit, allowing also for very high dissolved silica concentrations. This scenario provides favourable conditions for biogenesis early on in the Hadean (e.g., Westall, 2025) since it lowers the concentration of free [Ca²⁺] to values of cytosolic levels of 10^-6 molar for proteins to function. It also allows for high free phosphate concentrations, a prerequisite to form cell-walls and RNA. On the other hand, Urey-reactions would quickly lower the PCO₂ of primordial atmospheres which are geologically instable in the presence of water and silicates. The soda ocean on Earth was lost by a combination of carbonate deposition on continents, subduction of seawater and formation of granodioritic continental crust but they may persist on Europa and Enceladus and its salts may have been buried on Mars below a blanket of clay-minerals.

Kempe, S. & Degens, E.T, 1985: An early soda ocean? - Chem. Geol. 53: 95-108.

Westall, F., 2025: What the earliest evidence for life tells us about the early evolution of the biosphere. – Phil. Trans. R. Soc. B 380: 20240106.