Photosynthesis is the primary process by which carbon moves from the atmosphere into living things. In photosynthetic pathways  is absorbed slightly more easily than , which in turn is more easily absorbed than . The differential uptake of the three carbon isotopes leads to / and / ratios in plants that differ from the ratios in the atmosphere. This effect is known as isotopic fractionation.
To determine the degree of fractionation that takes place in a given plant, the amounts of both  and  isotopes are measured, and the resulting / ratio is then compared to a standard ratio known as PDB. The / ratio is used instead of / because the former is much easier to measure, and the latter can be easily derived: the depletion of  relative to  is proportional to the difference in the atomic masses of the two isotopes, so the depletion for  is twice the depletion of . The fractionation of , known as , is calculated as follows:
where the ‰ sign indicates parts per thousand. Because the PDB standard contains an unusually high proportion of , most measured  values are negative.
For marine organisms, the details of the photosynthesis reactions are less well understood, and the  values for marine photosynthetic organisms are dependent on temperature. At higher temperatures,  has poor solubility in water, which means there is less  available for the photosynthetic reactions. Under these conditions, fractionation is reduced, and at temperatures above  the  values are correspondingly higher, while at lower temperatures,  becomes more soluble and hence more available to marine organisms.
The  value for animals depends on their diet. An animal that eats food with high  values will have a higher  than one that eats food with lower  values. The animals own biochemical processes can also impact the results: for example, both bone minerals and bone collagen typically have a higher concentration of  than is found in the animals diet, though for different biochemical reasons. The enrichment of bone  also implies that excreted material is depleted in  relative to the diet.
Since  makes up about 1% of the carbon in a sample, the / ratio can be accurately measured by mass spectrometry. Typical values of  have been found by experiment for many plants, as well as for different parts of animals such as bone collagen, but when dating a given sample it is better to determine the  value for that sample directly than to rely on the published values.
The carbon exchange between atmospheric  and carbonate at the ocean surface is also subject to fractionation, with  in the atmosphere more likely than  to dissolve in the ocean. The result is an overall increase in the / ratio in the ocean of 1.5%, relative to the / ratio in the atmosphere. This increase in  concentration almost exactly cancels out the decrease caused by the upwelling of water (containing old, and hence -depleted, carbon) from the deep ocean, so that direct measurements of  radiation are similar to measurements for the rest of the biosphere. Correcting for isotopic fractionation, as is done for all radiocarbon dates to allow comparison between results from different parts of the biosphere, gives an apparent age of about 400 years for ocean surface water.