Carbon is the most important element in living organisms. It makes up 20% of their total body weight. The carbon cycle is the movement of carbon around the biosphere. When organisms die, carbon that was part of the live biomass pool becomes part of the dead biomass pool. Carbon is exchanged between the atmosphere and the ocean. Calcium carbonate can precipitate out of the water and form limestone and dolomite rock via sedimentation and burial. A small fraction of the organic carbon in the dead biomass pool is buried and incorporated into ocean sediments. Some of it may be transformed into fossil fuels. Petroleum Hydrocarbons are fossil fuels.