Various independent lines of evidence suggest that photoautotrophic carbon fixation is a very ancient process that had attained control of the terrestrial carbon cycle from at least 3.5 Ga (if not 3.8 Ga). Since it is generally accepted that only biological processes could substantially increase the negligible oxygen pressures sustained on the early Earth by inorganic photochemistry (probably between 10–8 and 10–14 present atmospheric level (PAL)), the advent of photoautotrophy would provide a limiting condition for the oxygenation of terrestrial near-surface environments. However, as the size of the free oxygen reservoir is determined by the kinetics of the geochemically relevant oxygen-consuming reactions, we may assume that the start of photosynthetic oxygen production did not result in an instantaneous buildup of modern PO<inf>2</inf> levels. Low atmospheric oxygen pressures generally inferred for the Archaean and Lower Proterozoic Earth would, therefore, indicate extremely rapid processes of oxygen consumption in certain compartments of the ancient exogenic system (notably the hydrosphere) rather than the non-existence of photosynthetic oxygen production at this time.

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