Much of the original free oxygen in the atmosphere combined with iron dissolved in water to become iron oxide.

Question

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This process is known as the formation of banded iron formations (BIFs) and occurred during the Great Oxygenation Event approximately 2.4 billion years ago. The iron oxide formed layers at the bottom of the ocean, which can still be found and extracted today as a valuable source of iron ore.

Much of the Original Free Oxygen in the Atmosphere Combined