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Third atmosphere - free oxygen
About 2 billion years ago, free oxygen began to accumulate in our atmosphere. Organisms with traits of both blue-green algae and bacteria released oxygen as a waste product of their metabolism. In the next 1.5 billion years, atmospheric oxygen levels increased, carbon dioxide levels decreased, and an ozone layer formed.
Without ozone protection, scientists think that the sun’s radiation would have prevented any animals from living on land.
Visitors may touch these specimens.
| How do scientists know free oxygen increased in our
atmosphere? Beginning about 3.5 bya, banded iron ore deposits began to form underwater. Reduced iron (without oxygen) and silica washed into the ocean, combined with oxygen produced by algae, and settled together in banded layers. About 2 bya oceans saturated with oxygen released free oxygen to the atmosphere. Reduced iron no longer existed on the surface; banded iron ore deposits ceased to form. |
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| Is there any evidence to show that atmospheric oxygen levels
increased? Yes. About the same time banded iron ore stopped forming underwater, "rusty" red beds of sedimentary rock began to form on land. Free atmospheric oxygen began to oxidize iron on Earth’s surface. |
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