Why did the early atmosphere not have much oxygen?

The oxygen did not build up in the atmosphere for a long time, since it was absorbed by rocks that could be easily oxidized (rusted). To this day, most of the oxygen produced over time is locked up in the ancient "banded rock" and "red bed" rock formations found in ancient sedimentary rock.

i think the miocene epoch spans the time from 23 million to 12 million years ago.

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