Our planet has got many systems or cycles in place that allow for the re-use of materials which are vital for life. These include nitrogen, water and carbon. The recycling of these vital materials helps support the continued existence of life that would otherwise end. We, as all living things, do not simply use this cycle but are a part of it.
Plants absorb carbon from the atmosphere in the form of CO2 during photosynthesis turning that carbon and sunlight into energy (carbohydrates) to grow. These plants are then eaten by animals and the carbon chains are consumed during digestion. The majority of the carbon is released in the form of CO2 in respiration through a process called Aerobic Respiration. When the animals and plants die some of the CO2 is released into the atmosphere through decomposition. Sometimes however the organic material doesn’t decompose and the carbon is stored and it is this stored carbon which makes dating organic matter possible. See Carbon Dating
In relatively recent years we now extract carbon from their deposits underground and burn it to produce energy and plastics, and we call that stored carbon ‘fossil fuels’. Burning those fossil fuels releases the CO2 back into the atmosphere.
Some organisms in the sea absorb carbon through their diet and convert it into calcium carbonate which goes into producing their shells. When they die these shells are deposited onto the sea bed forming limestone. This limestone eventually comes into contact with oxygen in the air as the sea bed is moved around by tectonic movement. The limestone reacts with the oxygen to produce carbon dioxide which is then released back into the atmosphere.