Isotopes of carbon

Carbon is a fundamental building block for life on Earth. While we often think of carbon as a single element, it actually exists in many different forms called isotopes. There are 14 known isotopes of carbon in total. Most of these isotopes are unstable, meaning they undergo radioactive decay to become different elements. Only two isotopes are stable: carbon-12 and carbon-13. These stable forms do not change over time. Carbon-12 is the most abundant, making up about 98.9% of all natural carbon. Carbon-13 is much rarer, accounting for about 1.1%. Understanding these isotopes helps scientists study everything from ancient climates to human diets.

Unstable isotopes change through specific processes called radioactive decay. Lighter isotopes often undergo beta-plus decay, which turns them into isotopes of boron. Heavier isotopes typically undergo beta-minus decay, turning them into isotopes of nitrogen. At the extreme limits of stability, some isotopes may even emit particles. For example, the two lightest isotopes decay into helium through several short-lived stages involving lithium, beryllium, and boron. These transitions happen because the nucleus of the atom is trying to reach a more stable state. The speed of this change is measured by a half-life, which is the time required for half of the radioactive atoms to decay.

Carbon isotopes can be categorized by their stability and how they are formed. The stable isotopes, carbon-12 and carbon-13, are found naturally in large amounts. Carbon-14 is a unique radioisotope found in trace amounts in nature. It is produced cosmogenically in the upper atmosphere. This happens when neutrons from cosmic radiation strike nitrogen atoms. Most other carbon isotopes are artificial and must be created in a laboratory. For instance, carbon-11 is an artificial radioisotope with a very short half-life of 20.34 minutes. It is produced in a machine called a cyclotron by hitting nitrogen with protons.

Scientific discovery and standardization have been shaped by these isotopes. In 1919, the dalton was defined using the mass of an unbound carbon-12 atom. The dalton is a unit used to measure atomic mass. Researchers have also identified various isotopes throughout the 20th century. Carbon-10 was studied in 1934, and carbon-14 was identified as a trace isotope in 1936. Carbon-11 is used today in medical technology. It is used for radioactive labeling in positron emission tomography, or PET scans. This allows doctors to track specific molecules in the body.

Measuring the ratio of these isotopes provides vital data for scientists. They use a technique called isotope ratio mass spectrometry to find these quantities. Results are often expressed in parts per thousand, known as per mille. Scientists often compare these results to a standard called Peedee Belemnite, or PDB. Because the original PDB fossils are being depleted, researchers now use an artificial version called Vienna PDB. By looking at the ratio of carbon-13 to carbon-12, known as δ13C, scientists can reconstruct the history of our planet.

These ratios act as a signature for biological and environmental processes. Plants prefer to use the lighter carbon-12 during photosynthesis to create food. This preference creates different isotopic signatures in different types of plants. For example, C3 plants like wheat and trees show different values than C4 plants like maize or sugar cane. Because animals eat these plants, the signature moves up the food chain. Scientists can analyze bone collagen or tooth enamel to see what an animal or human ate. This helps researchers map how ancient people migrated or how crops spread across the world.

Carbon-14 is especially important for understanding the passage of time. Because it is constantly being created in the atmosphere and absorbed by living things, it acts like a biological clock. When an organism dies, it stops taking in new carbon-14. The existing carbon-14 then begins to decay at a steady rate. With a half-life of 5,700 years, this decay is slow enough to be measured. Archaeologists use this process, called radiocarbon dating, to estimate the age of organic remains. This method is a primary tool for dating artifacts and understanding human history.

Beyond archaeology, isotopes help us understand the health of our oceans. In the ocean, the ratio of carbon isotopes can indicate how much carbon dioxide is being exchanged between the air and the sea. Large blooms of plankton absorb massive amounts of carbon from the water. If the ocean layers do not mix well, the sinking of dead plankton can change the isotopic makeup of the surface water. By studying the skeletons of tiny sea creatures called foraminifera, scientists can track these ancient changes. This helps us understand how the climate and nutrient cycles have shifted over millions of years.