4.5 Generating carbon — the legacy of volcanoes
What is the origin of the carbon within the carbon cycle? Figure 1.9 showed that the greatest proportion of the global carbon store is locked into carbonate rocks. Over the 4.5 billion years of the Earth's history, carbon must have moved from the atmosphere into the oceans and thence into carbonates. How did the atmospheric carbon originate?
The Earth's atmosphere as a whole was derived mainly from gases brought to the surface from the Earth's interior. For example, the 1991 eruption of Mount
Etna in Sicily released an estimated 0.01 × 1012 kg of carbon in the form of CO2 (Figure 1.12). Most volcanic carbon comes from the steady degassing of lava flows rather than from volcanic vents.
In the short term, volcanic sources release insignificant volumes of CO2 compared with other fluxes of carbon, but over geological time, degassing of the Earth's interior can reasonably account for all the carbon in the natural surface system.