The history of atmospheric oxygen
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- Rutten, M.G. Space Life Sciences (1970) 2: 5. doi:10.1007/BF00928950
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A primeval anoxygenic terrestrial atmosphere having been postulated on astronomical grounds, experiments using simulated conditions have shown that the formation of ‘organic’ molecules by abiogenic processes with proceed freely in such an environment.
Atmospheric oxygen will at first be limited to 0.001 PAL through the Urey mechanism which inhibits further dissociation of water above this level. All atmospheric oxygen exceeding this level must be biogenic and produced by photosynthesis. Molecular fossils prove its existence 2.7 billion years ago. Sedimentary ores, notably pyrite sands of gold-uranium reefs and banded iron formations, attest to the existence of an atmosphere with ‘little’ oxygen up to 1.8 billion years ago. Geochemistry does not, howerver, supply us with data as to the level of oxygen at that time. The Pasteur Point, on the other hand, at which microbes change from fermentation to respiration and vice versa, is a powerful regulating factor situated at 0.01 PAL of free oxygen.
It is postulated that the primeval atmosphere of Lower and Middle Precambrian was limited to this level of free oxygen. At this level pre-life — the formation of ‘organic’ compounds through inorganic processes — still exists. Pre-life and early life therefore were coexistent for two billion years at least, and were able to influence each, other over all this time. The primeval, atmosphere was definitely superseded by an oxygenic one about 1.45 billion years ago, but the level of 0.1 PAL of free oxygen was only reached during the Ordovician, 0.4–0.5 billion years ago.