Proterozoic Atmosphere
Before the Proterozoic Eon, there was little oxygen in earth's atmosphere, but during this eon, oxygen increased in the atmosphere due to the development of photosynthetic organisms. Photosynthetic organisms create oxygen from sunlight. An example of a photosynthetic organism is the cyanobacteria which dates back to about 2.7 BYA, but significant amounts of oxygen didn't exist until about 2.3 BYA. It took a while for the photosynthetic organisms of this eon to create enough oxygen to change the environment.
As the oxygen increased in the atmosphere, iron that dissolved into the ocean during the Archean Eon solidified into magnetite (Fe3O4) which was left on the ocean floor. Once the dissolved iron from the ocean was used up, the oxygen was released into the atmosphere, which is why it took so long for there to be enough oxygen to change the environment.
When iron is exposed to oxygen it rusts, which changes its color to reddish brown. This red color is characteristic of red beds that are made of the chemical reaction between oxygen and iron, which created hematite (Fe2O3). Until all of the iron was rusted by the oxygen in the ocean, the oxygen couldn't build up in the atmosphere.
As the oxygen increased in the atmosphere, iron that dissolved into the ocean during the Archean Eon solidified into magnetite (Fe3O4) which was left on the ocean floor. Once the dissolved iron from the ocean was used up, the oxygen was released into the atmosphere, which is why it took so long for there to be enough oxygen to change the environment.
When iron is exposed to oxygen it rusts, which changes its color to reddish brown. This red color is characteristic of red beds that are made of the chemical reaction between oxygen and iron, which created hematite (Fe2O3). Until all of the iron was rusted by the oxygen in the ocean, the oxygen couldn't build up in the atmosphere.