Carbon is of course key to all life on Earth; if you pinch your skin right now you’re touching a huge number of carbon atoms, they literally hold you together. Carbon has 2 stable isotopes; both have 6 protons, but carbon can be stable with either 6 or 7 neutrons, leading to atoms with mass 12 and mass 13. Carbon can also pick up one more neutron and become carbon-14, but this atom is unstable and undergoes radioactive decay with a half life just over 5000 years.
Carbon isotopes are hugely useful in the geologic record because life doesn’t just use carbon, it cares about what type of carbon it uses. Organisms that create solid molecules out of carbon really, really like to take up carbon-12 and really dislike carbon-13.
Most of Earth’s carbon comes out of the planet’s mantle as a basically homogeneous pool. That carbon is then broken up into 2 reservoirs; the kind taken up by life and the kind that floats around. The part taken up by life becomes “light carbon”, made mostly of carbon-12 and missing its carbon-13. The remaining carbon becomes “heavy carbon”, with extra carbon-13.
This relationship can be found even in ancient rocks. Rocks made mostly of the soft tissues of organisms, such as coal or hydrocarbons, are extremely low in carbon-13. The heavy carbon portion winds up forming rocks of a different type – carbonate rocks such as limestones and marbles. Added together, the composition of organic matter and the composition of carbonate formed at a given time generally equals the composition coming out of the mantle. However, any time there’s a change to the biosphere, both pools of carbon “see it”.
If suddenly a large portion of the biosphere dies, there is less carbon-12 locked up in organic rocks and carbonate rocks will suddenly have more carbon-12. On the other hand, when there is a large and active biosphere, carbonate rocks become even richer in carbon-13. Carbon isotopes, therefore, show major swings when mass extinctions happen and they also track other big changes such as variations in the climate.
This graph shows carbon isotopes in the world’s oceans as recorded in the shells of corals over the last few hundred years. Over the last century, the carbon isotopes have become much more negative, meaning they’re much more enriched in carbon-12. This change in ocean compositions can’t be coming out of the mantle – the mantle sits at about a value of -5, so the carbon in the oceans is actually getting less like the mantle and more like an “organic source” over time.
This swing to more carbon-12 rich samples is caused by fossil fuel burning. Fossil fuels are ancient organic matter and so they’re much more carbon-12 rich than the mantle or the oceans. Burning fossil fuels releases that carbon-12 and makes carbon around the world richer in carbon-12. The oceans, plants, animals, even the food you eat today will all bear the isotopic signature of fossil fuel burning. Source: The Earth Story Image source: Dept. Earth & Planetary Science