The oxygen level of Earth’s past atmosphere has been directly measured from inclusion gases trapped in halite. A direct glimpse of the composition of the mid- to late Neoproterozoic atmosphere, ~815 million years ago registers an average oxygen content of 10.9%. (open access - pdf)

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The oxygen level of Earth’s past atmosphere has been directly measured from inclusion gases trapped in halite. A direct glimpse of the composition of the mid- to late Neoproterozoic atmosphere, ~815 million years ago registers an average oxygen content of 10.9%. (open access - pdf)