How is molecular oxygen proposed to have first appeared in Earth's atmosphere?

How is molecular oxygen proposed to have first appeared in Earth's atmosphere?

• A. Oxygen was produced by volcanic outgassing and released into the atmosphere.
• B. Oxygen appeared in the atmosphere immediately after Earth's formation.
• C. Oxygen first bound with iron in oceans to form banded iron formations, then gas out of the oceans slowly.
• D. Oxygen was never present in Earth's early history.

Answer: (C)

Oxygen in Earth's early atmosphere came from photosynthesis by early microbes that produced O2 as a byproduct. When this oxygen began to build up, it was rapidly consumed by reactions with dissolved iron in the oceans, forming iron oxides and creating banded iron formations. This process kept atmospheric oxygen very low for a long time. Only after those iron sinks became saturated did oxygen start to accumulate in the atmosphere, gradually increasing over billions of years. That sequence—oxygen produced by photosynthesis, first sequestered by iron in oceans as banded iron formations, then released into the atmosphere—is the best explanation for how oxygen first appeared in Earth's air. Volcanic outgassing contributed gases but not sustained free oxygen; oxygen did not appear immediately after Earth's formation, and while some oxygen was present earlier, it was largely tied up in sinks.

Oxygen in Earth's early atmosphere came from photosynthesis by early microbes that produced O2 as a byproduct. When this oxygen began to build up, it was rapidly consumed by reactions with dissolved iron in the oceans, forming iron oxides and creating banded iron formations. This process kept atmospheric oxygen very low for a long time. Only after those iron sinks became saturated did oxygen start to accumulate in the atmosphere, gradually increasing over billions of years. That sequence—oxygen produced by photosynthesis, first sequestered by iron in oceans as banded iron formations, then released into the atmosphere—is the best explanation for how oxygen first appeared in Earth's air. Volcanic outgassing contributed gases but not sustained free oxygen; oxygen did not appear immediately after Earth's formation, and while some oxygen was present earlier, it was largely tied up in sinks.