Carbon-12 and carbon-13 account for approximately 98.9% and 1.1% (respectively) of the naturally occurring carbon on Earth. : \delta \ce{^{13}C} = \left( \frac{\left( \frac\ce{^{13}C}\ce{^{12}C} \right)_\text{sample}}{\left( \frac\ce{^{13}C}\ce{^{12}C} \right)_\text{standard}} - 1 \right) \times 1000 ‰
Peedee Belemnite ("PDB"), a fossil
belemnite, was the original reference standard used for standardizing isotope ratio values. Due to the depletion of the original PDB, artificial "Vienna PDB", or "VPDB", is generally used today.
Paleoclimate and 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). Plants find it easier to use the lighter isotope () when they convert sunlight and carbon dioxide into food. For example, large blooms of
plankton (free-floating organisms) absorb large amounts of from the oceans. Originally, the was mostly incorporated into the seawater from the atmosphere. If the oceans that the plankton live in are stratified (meaning that there are layers of warm water near the top, and colder water deeper down), then the surface water does not mix very much with the deeper waters, so that when the plankton dies, it sinks and takes away from the surface, leaving the surface layers relatively rich in . Where cold waters well up from the depths (such as in the
North Atlantic), the water carries back up with it; when the ocean was less stratified than today, there was much more in the skeletons of surface-dwelling species. Other indicators of past climate include the presence of tropical species and coral growth rings.
Tracing food sources and diets Different photosynthetic pathways preferentially select for the lighter , but their selectivity differs. 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‰. 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‰. It follows that eating these different plants will affect the δ13C values in the consumer's body tissues. If an animal (or human) eats only C3 plants, their δ13C values will be from −18.5 to −22.0‰ in their bone
collagen and −14.5‰ in the
hydroxylapatite of their teeth and bones. In contrast, C4 feeders will have bone collagen with a value of −7.5‰ and hydroxylapatite value of −0.5‰. In case studies, millet and maize eaters can easily be distinguished from rice and wheat eaters. Studying how these dietary preferences are distributed geographically through time can illuminate migration paths of people and dispersal paths of different agricultural crops. However, human groups have often mixed C3 and C4 plants (northern Chinese historically subsisted on wheat and millet), or mixed plant and animal groups together (for example, southeastern Chinese subsisting on rice and fish). == Carbon-14 ==