14C is produced by thermal neutrons from cosmic radiation in the upper atmosphere, and is transported down to earth to be absorbed by living biological material.
Since dead tissue does not absorb 14C, the amount of 14C is one of the methods used within the field of archeology for radiometric dating of biological material.
12C and 13C are measured as the isotope ratio δ13C in benthic foraminifera and used as a proxy for nutrient cycling and the temperature dependent air–sea exchange of CO2 (ventilation).
[11] Plants find it easier to use the lighter isotopes (12C) when they convert sunlight and carbon dioxide into food.
Other indicators of past climate include the presence of tropical species and coral growth rings.
[citation needed] Grasses in temperate climates (barley, rice, wheat, rye, and oats, plus sunflower, potato, tomatoes, peanuts, cotton, sugar beet, and most trees and their nuts or fruits, roses, and Kentucky bluegrass) follow a C3 photosynthetic pathway that will yield δ13C values averaging about −26.5‰.
[citation needed] Grasses in hot arid climates (maize in particular, but also millet, sorghum, sugar cane, and crabgrass) follow a C4 photosynthetic pathway that produces δ13C values averaging about −12.5‰.