It is stored in biomass, organic matter in sediments, and in carbonate rocks like limestone. The Carbon Cycle. The primary source of carbon/CO 2 is outgassing from the Earth's interior at midocean ridges, hotspot volcanoes, and subduction-related volcanic arcs. Much of the CO 2 released at subduction zones is derived from the metamorphism of carbonate rocks subducting with the ocean crust.