Oxygen levels early atmosphere

The actual quantity of O2 predicted on the basis of model calculations appears to vary wildly depending on the assumed gas phase chemistry, the role of O2 reactions with Fe in the early oceans, and the transport mechanism. Oxygen levels reported as the result of recent theoretical calculations (Levine, 1982) range between lO " and 10" of the present atmospheric level (PAL). Enhanced levels of solar activity would have significantly increased the prebiological O2 levels. At the current level of uncertainty, relatively little can be deduced about the actual chemical mechanism responsible for fixing the O2 density prior to the establishment of photosynthetic sources of oxygen. 

Figure 2 Geological timeline expanding the mid-Precambrian period (Archaean and early Proterozoic). Note the burst of evolutionary activity in the period 2.3 to 2 billion years ago, as oxygen levels rose to about 5-18% of present atmospheric levels.

Figure 5 Changes in atmospheric composition over the Phanerozoic period, from 600 million years ago, based on the models of Robert Berner. Oxygen levels (top graph) reached a peak of 35% in the late Carboniferous and early Permian, before falling to 15% in the late Permian. Oxygen levels peaked a second time at 25 to 30% in the late Cretaceous (K) before falling to present atmospheric levels in the Tertiary (T). Carbon dioxide levels (bottom graph) fell from 0.5% in the Silurian (S) to around 0.03% by the end of the Carboniferous. Reproduced with permission from Graham et al.t and Nature.

Schidlowski, M. (1983b). Evolution of photo-autotrophy and early atmospheric oxygen levels. Precambrian Res. 20, 319-335. 

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