History of the Earth
The Great Oxygenation Event was one of the most important events of the Proterozoic.  Though oxygen was undoubtedly released by photosynthesis well back in Archean times, it could not build up to any significant degree until chemical sinks--unoxidized sulfur and iron—had been filled; until roughly 2.3 billion years ago, oxygen was probably only 1 to 2% of its current level.(Stanley, 323)  Banded iron formations, which provide most of the world's iron ore, were also a prominent chemical sink; most accumulation ceased after 1.9 billion years ago, either due to an increase in oxygen or a more thorough mixing of the oceanic water column.p324