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 14 °C 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 animal's 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 animal's 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