[M-IS14] Aqua planetology

[MIS14-P09] Prediction of the composition of the primordial ocean just after its formation

*Hisahiro Ueda¹, Takazo Shibuya¹ (1.JAMSTEC)

Keywords:primordial ocean composition, ocean formation, water-rock reactions, Hadean, experiment

Hadean is an enigmatic period in the Earth’s history when ocean formation and the emergence of life occurred. However, minimal geological evidence is left from this period. Regarding the ocean in the early Earth, previous research indicates that seawater had weak acidity (pH 4–7) because the carbon dioxide concentration in the atmosphere was high compared to that today (e.g., Walker, 1985; Grotzinger and Kasting, 1993; Macleod et al., 1994), or it was strongly acidic (pH 1–2) because a large amount of halogen substance in the high-temperature vapor atmosphere eventually dissolved into seawater (Maruyama et al., 2013). Previously, pH, CO₂ concentrations, and other elements were considered (Morse and Mackenzie, 1988) but these estimations did not provide information about the quantitative chemical compositions of seawater. To understand the primordial ocean’s composition, we focused on the ocean’s formation processes from CO₂- and HCl-bearing water vapor in the high-temperature atmosphere. When the temperature of the lower atmosphere fell below the critical point, high-temperature rain reached the ground surface. Then, hydrothermal reactions between the subcritical fluid and primordial crust started. Eventually, a liquid ocean emerged on the completely altered crust as the temperature decreased to approximately 25 °C until the ocean’s temperature stabilized. Here, we conducted two experiments and modeling simulating the reactions between hypothetical primordial crustal rock (basalt or komatiite). The results indicate that the primordial ocean was mildly acidic and rich in CO₂, Mg, and Ca relative to Na, irrespective of the rock type, which is different from the modern equivalents. Therefore, unlike the present seawater, the primordial seawater could have been carbonic, bitter, and harsh rather than salty. The ocean’s composition started to evolve once plate tectonics began on Earth.