Isotopes of carbon

Carbon-11

Carbon-11 or ¹¹
C 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.

¹¹
C → ¹¹
B + e⁺
+ ν
_e + 0.96 MeV

¹¹
C + e⁻
→ ¹¹
B + ν
_e + 1.98 MeV

It is produced by hitting nitrogen with protons of around 16.5 MeV in a cyclotron. The causes the endothermic reaction

¹⁴
N + p → ¹¹
C + ⁴
He − 2.92 MeV

It can also be produced by fragmentation of ¹²
C by shooting high-energy ¹²
C 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 [¹¹
C]DASB and [¹¹
C]Cimbi-5. Due to the short half-life, the chemical reactions used to manipulate the radioisotope as generated and incorporate it into a biomolecule must be efficient. Popular key intermediates include [¹¹C]phosgene, for carboxylation-related reactions, though other synthons, such as [¹¹C]carbonyl fluoride and [¹¹C]carbon dioxide, are also being explored. For methylation, [¹¹C]iodomethane and related synthons are used.

Stable isotopes

Carbon-12 and carbon-13 account for approximately 98.9% and 1.1% (respectively) 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 (‰ or "per mille") divergence from the ratio of a standard:

${\displaystyle \delta {}^{13}\text{C}=\left(\frac{{\left(\frac{{}^{13}\text{C}}{{}^{12}\text{C}}\right)}_{\text{sample}}}{{\left(\frac{{}^{13}\text{C}}{{}^{12}\text{C}}\right)}_{\text{standard}}}-1\right)\times 1000}$ ‰

Peedee Belemnite ("PDB"), a fossil belemnite from the eponym Late Cretaceous geological formation in North and South Carolina (USA), was the original reference standard used for standardizing carbon isotope ratio values. Due to the depletion of the original PDB, an artificial "Vienna PDB" standard, or "VPDB", is generally used today.

Paleoclimate

¹²
C and ¹³
C 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 (ocean ventilation). Photosynthetic organisms, such as algae and plants find it easier to use the lighter carbon isotope (¹²
C) when they convert carbon dioxide and water into glucose and oxygen using sunlight and photosynthesis:

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

For example, large blooms of plankton (free-floating organisms) absorb large amounts of ¹²
C from the oceans. Originally, the ¹²
C 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 ¹²
C from the surface, leaving the surface layers relatively rich in ¹³
C. Where cold waters well up from the depths (such as in the North Atlantic), the water carries ¹²
C back up with it; when the ocean was less stratified than today, there was much more ¹²
C 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 ¹²
C, 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 (historically, northern Chinese subsisted on wheat (C₃) and millet (C₄)) or mixed plant and animal groups (for example, southeastern Chinese subsisting on rice and fish).

Carbon-14

Carbon-14 (also called radiocarbon) occurs in trace amounts and has a half-life of 5730 years. The primary source of ¹⁴
C on Earth is the reaction of ¹⁴
N with thermal neutrons from cosmic radiation spallation reactions in the upper atmosphere; this mixes throughout the atmosphere, and biological processes such as photosynthesis incorporate the ¹⁴
C into living organisms. Since organisms stop absorbing ¹⁴
C upon dying, measurement of the amount of ¹⁴
C in a sample may be used to estimate its age. This technique, called radiocarbon dating, is one of the principal methods of radiometric dating in archaeology.