The naturally occurring isotopes of carbon, of which there are three: 12C making up about 98.9 per cent; 13C about 1.1 per cent; and 14C whose amount is negligible, but which is detectable because it is radioactive. The relative abundance of these isotopes varies and the study of this variation is an important tool in geologic research, especially radiometric dating. Carbon-isotope dating is a method of radiometric age-dating using the amount of the heavy, radioactive isotope carbon-14 remaining in organic matter. Carbon-14 has a half-life of 5730 ± 40 years and the amount of the isotope present can be used to date materials up to about 50 000 years old (see radiocarbon dating). Measurement of the ratio of carbon-13 to carbon-12 allows the recognition of carbonate precipitated from a variety of different sources. This ratio in plant-derived organic matter varies depending on which photosynthetic pathway was involved in its fixation (C3 or C4/CAM, see crassulacean acid metabolism). The distinctive ratio persists in the tissues of consumer organisms, allowing their dietary history to be reconstructed.
Subjects: Ecology and Conservation.