Geological history of oxygen

Although oxygen is the most abundant element in Earth's crust, due to its high reactivity it mostly exists in compound (oxide) forms such as water, carbon dioxide, iron oxides and silicates. Before photosynthesis evolved, Earth's atmosphere and ancient superocean had little free elemental diatomic oxygen (O2). Small quantities of oxygen were released by geological processes, but did not build up in the prebiotic atmosphere due to reactions with then-abundant reducing gases such as atmospheric methane and hydrogen sulfide and surface reductants such as ferrous iron and sulfur.

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