Isotopes of carbon
has 14 known isotopes, from to as well as, of which only and are stable. The longest-lived radioisotope is, with a half-life of 5700 years. This is also the only carbon radioisotope found in nature, as trace quantities are formed cosmogenically by the reaction + → +. The most stable artificial radioisotope is, which has a half-life of. All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. Lighter isotopes exhibit beta-plus decay into isotopes of boron and heavier ones beta-minus decay into isotopes of nitrogen, though at the limits particle emission occurs as well.
List of isotopes
Carbon-11
Carbon-11 or is a radioactive isotope of carbon that decays to boron-11 with a half-life to 20.34 minutes. This decay mainly occurs due to positron emission, with around 0.19–0.23% of decays instead occurring by electron capture.It is produced by hitting nitrogen with protons of around 16.5 MeV in a cyclotron. The causes the endothermic reaction
It can also be produced by fragmentation of by shooting high-energy at a target.
Carbon-11 is commonly used as a radioisotope for the radioactive labeling of molecules in positron emission tomography. Among the many molecules used in this context are the radioligands DASB|DASB and 25I-NBOMe|Cimbi-5.
Stable isotopes
Carbon-12 and carbon-13 account for approximately 98.9% and 1.1% of the naturally occurring carbon on Earth. However, the ratio of stable 13C and 12C in a material can vary due to differences in precursor source and isotopic fractionation induced by a variety of biogeochemical processes. The quantities of the different isotopes are commonly measured via isotope ratio mass spectrometry and expressed as parts per thousand divergence from the ratio of a standard:Peedee Belemnite, 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. Plants find it easier to use the lighter isotope when they convert sunlight and carbon dioxide into food. For example, large blooms of plankton 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, 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, 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 follow a C3 photosynthetic pathway that will yield δ13C values averaging about −26.5‰. Grasses in hot arid climates 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 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, or mixed plant and animal groups together.