 One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth's atmosphere.Though oxygen is believed to have been released by photosynthesis as far back as Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized sulfur and iron had been exhausted.Until roughly 2.3 billion years ago, oxygen was probably only 1% to 2% of its current level.