N is for Nitrogen
Nitrogen is the most abundant gas in the Earth’s atmosphere. It is also an important component of the amino acids that make up proteins in the bodies of living organisms.
Isotopes of nitrogen fractionate according to trophic level, that is, where the organism lies on the food chain (more properly, the food web). This is the origin of the common saying in isotope geochemistry, “You are what you eat, plus a few permil.” For nitrogen, on average a consumer’s δ15N is 3‰ more positive than the organism that is being eaten (whether it be a plant or another animal).
This relationship of nitrogen isotopes in proteins can be used to determine predator-prey relationships between extinct animals, presuming that proteins are preserved.