section_path: Physical science/Carbon cycle
section_name: Carbon cycle
paragraph_text: Carbon cycle
  
  Oceans contain the greatest quantity of actively cycled carbon in the world and are second only to the lithosphere in the amount of carbon they store. The oceans' surface layer holds large amounts of dissolved organic carbon that is exchanged rapidly with the atmosphere. The deep layer's concentration of dissolved inorganic carbon is about 15 percent higher than that of the surface layer and it remains there for much longer periods of time. Thermohaline circulation exchanges carbon between these two layers.
  
  Carbon enters the ocean as atmospheric carbon dioxide dissolves in the surface layers and is converted into carbonic acid, carbonate, and bicarbonate:
  
  It can also enter through rivers as dissolved organic carbon and is converted by photosynthetic organisms into organic carbon. This can either be exchanged throughout the food chain or precipitated into the deeper, more carbon-rich layers as dead soft tissue or in shells and bones as calcium carbonate. It circulates in this layer for long periods of time before either being deposited as sediment or being returned to surface waters through thermohaline circulation.