Carbon dating measures how much radioactive carbon-14 remains in once-living material. Carbon-14 decays with a half-life of 5,730 years, so it works for samples up to about 50,000 years old.
Principle
Carbon-14 (C-14) is a radioactive isotope made in the atmosphere. Living things absorb it. When an organism dies, it stops, and C-14 decays with a half-life of 5,730 years.
Method
Measure the remaining C-14 compared with C-12 and calculate how long since death.
Range
Accurate to about 50,000 years. Beyond that, too little C-14 remains.
Calibration
Tree rings and other records correct for changes in atmospheric C-14.
History
Willard Libby developed it in the 1940s and won the 1960 Nobel Prize in Chemistry.
Uses
Dating bones, wood, charcoal, textiles and shells.
Not for rocks
Other methods like uranium-lead dating are used for rocks.
Accuracy
Samples can be dated to within decades to a few hundred years.
Principle
Carbon dating (radiocarbon dating) estimates the age of organic material by measuring carbon-14, a radioactive isotope. Cosmic rays turn nitrogen in the upper atmosphere into carbon-14, which combines with oxygen to form carbon dioxide. Plants take it in during photosynthesis, and animals get it by eating plants. While alive, an organism keeps the same ratio of carbon-14 to carbon-12 as the atmosphere, about 1 in a trillion. At death, intake stops, and carbon-14 decays with a half-life of 5,730 years.
Method
- A sample of bone, wood, charcoal, shell, cloth or seeds is cleaned to remove contamination.
- It is converted to carbon dioxide or graphite.
- Accelerator mass spectrometry (AMS) counts individual carbon-14 atoms and needs only milligrams of material. Older methods count radioactive decay and need grams.
- The ratio of carbon-14 to carbon-12 gives the radiocarbon age.
Range
The method works for samples up to about 50,000 years old. After about 9 half-lives, too little carbon-14 remains to measure reliably. For older material, other methods like potassium-argon dating or uranium-series dating are used.
Calibration
Atmospheric carbon-14 has varied over time, so raw radiocarbon ages must be calibrated using tree rings, corals, cave deposits and lake sediments. The IntCal curves, updated regularly (IntCal20 in 2020), convert radiocarbon years to calendar years. Calibrated dates are reported with a range, such as 3,100 plus or minus 30 years before present.
History
Willard Libby developed the method at the University of Chicago in the late 1940s and won the Nobel Prize in Chemistry in 1960. In 1950, it was used to date artifacts from known age like Egyptian tombs, which confirmed its accuracy.
Uses
Archaeologists date human remains, tools, fire pits and ancient textiles. It helped date the Dead Sea Scrolls (about 250 BCE to 70 CE), the Turin Shroud (1260 to 1390 CE in a 1988 test, which remains disputed) and Otzi the Iceman (about 3300 BCE).
Not for rocks
Carbon dating cannot date rocks or fossils that are millions of years old, nor materials that never lived, because they have no carbon-14. Geologists use uranium-lead and potassium-argon methods for rocks.
Accuracy
Accuracy is typically within 1% to 2% of the age for good samples. Contamination from younger carbon, like roots or preservatives, can give errors. The method assumes the sample took up carbon only from the atmosphere. Marine samples need a reservoir correction.
Half-life figures and limits
Radiocarbon dating measures carbon-14, a radioactive isotope formed in the upper atmosphere when cosmic rays hit nitrogen. Living things absorb it, and when they die, it decays with a half-life of 5,730 years. By comparing the remaining carbon-14 to carbon-12 using accelerator mass spectrometry, scientists estimate age. The method works up to about 50,000 years, since after about 9 half-lives too little remains to measure. Radiocarbon ages are calibrated with tree rings, cave deposits and corals, using curves such as IntCal20, because atmospheric carbon-14 has varied. Willard Libby developed the method in the late 1940s and won the 1960 Nobel Prize in Chemistry. It only works for organic material such as bone, wood, charcoal and shell, and rocks are dated with other methods such as uranium lead dating. Typical uncertainty is a few decades to a few centuries, depending on age and sample.
Based on data from the American Chemical Society and the National Science Foundation
