The rise of oxygen and ozone Biogeochemical evolution

During stage 1 (3.85-2.45 Gyr ago) the atmosphere was largely or entirely anoxic, as were the oceans, with the possible exception of oxygen oases in the shallow oceans. 

During stage 2 (2.45-1.85 Gyr) atmospheric oxygen levels rose to values estimated to have been between 0.02 and 0.04 atm. The shallow oceans became mildly oxygenated, while the deep oceans continued to be anoxic. 

During stage 3 (1.85-0.85 Gyr) atmospheric oxygen levels did not change significantly. Most of the surface oceans were mildly oxygenated, as were the deep oceans. 

Stage 4 (0.85-0.54 Gyr) saw a rise in atmospheric oxygen to values not much less than 0.2 atm. The shallow oceans followed suit, but the deep oceans were anoxic, at least during the intense Neoproterozoic ice ages. 

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