Ocean Deoxygenation
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ant (47%) element in the earth crust, almost all of this oxygen is bound in rocks and soils. The amount of oxygen bound in seawater is merely ~1.5 ppm of the total oxygen inventory on our planet, and the amount of elemental oxygen (O2) in the atmosphere is about a thousand times smaller. The elemental oxygen is produced from splitting of water by organisms (autotrophs) that synthesize organic matter using solar energy. This process, called oxygenic photosynthesis, began about 2.7 billion years ago (Ga). Prior to that, for nearly 2 billion years the Earth had little or no free O2, although anoxygenic photoautotrophs had evolved around 3.5 Ga (Canfield, 2014). The oceans were until then fully anoxic. Even after the evolution of oxygenic photoautotrophs, there was initially not much accumulation of O2 in the atmosphere as the O2 being produced was probably being titrated by hydrogen (H2) coming out of the Earth’s mantle; a decrease in the H2 flux may have led to the Great Oxidation Event (GOE) that occurred after 2.4 Ga (Canfield, 2014). A conspicuous aspect of GOE was the banded iron formation (precipitation of insoluble Fe(III) from Fe(II), derived from the anoxic deep sea, within the oxygenated surface layer). Atmospheric O2 content remained relatively low (
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