The basic idea
Carbon exists in several forms, or isotopes. Most is carbon-12, a small amount is carbon-13, and a tiny trace is carbon-14, which is radioactive. Carbon-14 is produced continually in the upper atmosphere when cosmic rays strike nitrogen atoms. It combines with oxygen to form carbon dioxide, which plants absorb through photosynthesis and animals take in by eating plants or other animals.
As long as an organism is alive, it keeps exchanging carbon with its surroundings, so its proportion of carbon-14 stays roughly in step with the atmosphere. When it dies, that exchange stops. The carbon-14 already in the tissue slowly decays, while the stable carbon stays. Measuring how much carbon-14 remains, relative to stable carbon, gives a measure of how long ago the organism died.
The method was developed in the late 1940s by the American chemist Willard Libby, who received the Nobel Prize in Chemistry for it in 1960. It transformed archaeology because it works on a wide range of organic materials and does not depend on pottery styles or written records.
Half-life in plain terms
The decay of carbon-14 follows a regular pattern described by its half-life, the time it takes for half of the atoms in a sample to decay. The half-life is about 5,730 years. After one half-life, roughly half of the original carbon-14 remains; after two, about a quarter; after three, about an eighth.
Because the amounts become very small, the method has a practical limit. Samples older than roughly 50,000 years contain so little carbon-14 that it is hard to separate from background and contamination, and for such ages other techniques are used. For human history this covers all of the past of ancient civilizations, as well as much of the earlier prehistory.
What can be dated
Any material that was once living and contains carbon can in principle be dated: charcoal, wood, seeds, grain, bone, shell, textiles, leather and paper. Short-lived samples such as seeds, grain or twigs are generally the most useful, because they record the year or few years in which they grew, which is close to the time of human activity.
Stone and pottery cannot be dated directly, though soot, food residue or organic temper in a vessel sometimes can. Metal cannot be dated unless it is associated with organic material.
Measuring: counting atoms
Early laboratories counted the radioactive decays of carbon-14 in a sample, which required large amounts of material and long counting times. Since the 1970s and 1980s, accelerator mass spectrometry, usually called AMS, has become the standard in archaeology. It counts the carbon-14 atoms directly, using only a few milligrams of material. This makes it possible to date a single seed or a small fragment of bone from a precious object.
Why calibration matters
Libby assumed that the amount of carbon-14 in the atmosphere had always been constant. Experience showed this was not true. Production varies with changes in the Earth’s magnetic field and solar activity, and carbon moves between the atmosphere, the oceans and the land at different rates over time.
A raw radiocarbon age is therefore reported in radiocarbon years “before present”, where present is fixed by convention as 1950, and it must be converted into calendar years. This is done with a calibration curve built from samples whose calendar ages are known independently. The backbone of the curve comes from tree-ring dating, because each ring can be counted to an exact year and then measured for carbon-14. Other records, including corals, layered lake sediments and cave deposits, extend it further back. The international curves are periodically updated as new data accumulate.
Calibration is not a simple one-to-one conversion. The curve has wiggles and flat sections, called plateaus, where samples of quite different calendar ages have the same radiocarbon age. One such plateau affects dates in the centuries around 800 to 400 BCE and makes dating that period more difficult than neighbouring ones. In these cases a single measurement may correspond to a wide calendar range, or to more than one separate range.
Reading a result
A laboratory reports a radiocarbon age with a standard error, for example “plus or minus” a certain number of years. After calibration, the result is usually given as a range with a stated probability, often 68 percent or 95 percent. This means the true age is likely, but not certain, to fall within the range. Narrow ranges are possible in favourable parts of the curve, and wide ones elsewhere.
For this reason, a single date should not be treated as an exact year. Archaeologists often take several samples from a context and combine them using statistical methods, sometimes called Bayesian modelling, which incorporate known information such as stratigraphic order. This can tighten the ranges considerably, though the results depend on the assumptions put in.
Pitfalls and corrections
Several issues can bias results:
- Contamination: later carbon from roots, groundwater or handling can make a sample seem younger, and laboratories use cleaning procedures to remove it.
- The old wood problem: charcoal from the inner rings of a long-lived tree dates the growth of the tree, which may be decades or centuries earlier than the time the wood was burned.
- Reservoir effects: marine animals and people who eat a lot of seafood take up carbon that is older than atmospheric carbon, so their remains can seem older than they are. Corrections exist but are uncertain, and local conditions matter.
- Poor context: a sample is only as informative as its link to the event of interest, and a dated seed that has worked its way down into an older layer will mislead.
Where radiocarbon has made a difference
Radiocarbon dating has shaped the chronology of the earliest monuments, such as the Gobekli Tepe monuments, which were built long before pottery or metal in the region. It has been used to test the manuscripts known as the Dead Sea Scrolls, where results broadly supported the ages suggested by handwriting. It has also entered long arguments over the date of the volcanic eruption that buried the settlement of Akrotiri on Thera, where measurements from the island’s volcanic layers point to an earlier date than many archaeologists derived from Egyptian and Aegean links. That disagreement is still discussed, which shows both the power and the limits of the method.
Used carefully, radiocarbon dating complements historical records, tree rings and pottery sequences, and the strongest chronologies depend on all of them together.