Isotopes of carbon can be used to interpret available vegetation and diet of ancient ecosystems and individual animals.
Carbon has two stable isotopes, carbon-12 and carbon-13. Carbon also has a rare radioactive isotope, carbon-14, which is used for assigning ages to objects less than 40,000 years old.
The carbon we measure usually comes into rocks or fossil teeth from plants. In some cases, the plants decay and are incorporated into the rock (like coal), or they are plants that the animals ate. Thus, what we’re measuring is plant carbon, which itself is a reflection of atmospheric carbon dioxide.
Different kinds of plants convert atmospheric carbon dioxide to glucose (and ultimately plant skeletal materials) in different ways, which is reflected in the carbon isotopes in the plants. We understand how the carbon isotopes in the plants are then converted into carbon isotopes in rock or fossils. This means we can measure carbon isotopes from teeth or rock and know something important about what the environment was like when the animal was alive or when the rock was merely sediments.