So it by this time enough free oxygen had been produced to oxidise all the reduced iron and oxygen levels in the atmosphere started to rise. As we saw in the lecture on the carbon cycle, the carbon dioxide content of the atmosphere is only a small part of the overall carbon cycle that involves the oceans, lithosphere and biosphere. The only way new carbon dioxide can enter the system is through volcanic activity or submarine hydrothermal springs. This is one reason why the Cretaceous had such a warm climate. Carbon is lost from the system when it is precipitated in carbonate rocks, although a portion is recycled through subduction of limestones.
Green plants convert carbon dioxide to oxygen and organic matter.
The oxygen enters the atmosphere and allows animals to breathe and oxidises dead organic matter back to carbon dioxide. Some organic matter escapes oxidation and is preserved to varying extents, e.g., as fossil fuels. This reduces the amount of carbon dioxide in circulation and increases the amount of oxygen. Hence, carbon dioxide is stored in a large number of reservoirs in addition to the atmosphere. Oxygen is also stored in reservoirs, of which the most important are oxidised soils and minerals.