The atmosphere is the surrounding gas layer, the hydrosphere includes Earth’s water, the biosphere includes living things, and the geosphere includes rocks, soil minerals and Earth’s interior. Carbon is stored in all four: carbon dioxide in air, dissolved carbon in oceans, organic molecules in organisms, and carbon compounds in rocks. A reservoir is a store; a flux is a transfer between stores.
Sphere models simplify systems with overlapping boundaries. A living tree belongs to the biosphere even though it contains water. Dissolving atmospheric carbon dioxide transfers carbon into the hydrosphere. Burial can move organic carbon towards long-term geological storage. In a cycle diagram, check what an arrow represents and its direction. Carbon atoms circulate; they do not follow the same one-way path as energy through an ecosystem.
Example 1
Assign carbon in limestone and carbon dioxide in air to their main reservoirs.
Limestone is rock, so its carbon is in the geosphere. Carbon dioxide gas in air is in the atmosphere.
Example 2
Carbon dioxide dissolves in the ocean. Name the source and destination spheres.
The source is the atmosphere and the destination is the hydrosphere. The transfer is a flux, while the dissolved carbon becomes part of a reservoir.
Watch out: A reservoir stores carbon; an arrow normally represents a process transferring it between reservoirs.