Why were Earth’s earliest oceans probably acidic?

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abundance of oxygen — evolution of the atmosphere A “best guess” reconstruction of the abundance of O₂ in Earth's atmosphere as a function of time. The O₂ abundance axis is logarithmic.

Earth’s earliest oceans were likely acidic because of the chemical composition of the gases released from the planet's interior during its early history. As the Earth's crust cooled and water vapour condensed to form the first oceans, various acid gases—most notably hydrochloric acid—dissolved into the water.12

This early acidic environment reacted vigorously with crustal igneous rock minerals, a process that dissolved silica and cations while creating a residue of aluminous clay minerals. Over time, as these acid gases were consumed through continued reactions with crustal minerals, the oceans were neutralized. This chemical evolution was a critical step in the development of the Earth's surface, eventually leading to the modern chemical characteristics of seawater.123