How Does Carbon Dating Work?
Carbon dating, also known as radiocarbon dating or carbon-14 dating, is a technique used to learn the age of organic (here meaning “organism-derived”) remains. It works in part by comparing the amounts of the naturally occurring carbon isotopes—namely, carbon-12, carbon-13, and carbon-14—in a sample of the remains. Because carbon-14, as a radioactive isotope, has a known half-life of 5,730 ± 40 years, researchers can calculate the time elapsed since the sampled organism died by determining the proportion of carbon-14 in the sample. For instance, a piece of wood charcoal with only 80 percent of the carbon-14 expected in a living tree indicates that the tree was chopped down about 1,845 years before the sample was collected.
The number following the ± sign in half-lives and radiocarbon dates indicates the margin of error—the extent to which an estimate may deviate from the exact value. It is discussed further in the final paragraph, but, in general, it means that there is a 95 percent probability that the true date of a sample or event will be found within a range of possible dates above or below the estimate, as indicated by adding and subtracting the margin of error from the number before the ± sign.
Step 1: Cosmic rays create radioactive carbon
Three isotopes of carbon exist in nature: carbon-12, carbon-13, and carbon-14. The vast majority of carbon is in the form of the stable isotopes carbon-12 (about 98.8 percent) and carbon-13 (about 1.1 percent). About one part per trillion (ppt) of all carbon consists of carbon-14, a radioactive form of the element, which is created when energetic neutrons, ejected from atoms in the upper atmosphere by cosmic rays, collide with atoms of nitrogen-14. The collisions eject a proton from each nitrogen-14 atom, transforming the atom into carbon-14. The carbon-14 then bonds with oxygen to form radioactive carbon dioxide (CO2).
Step 2: Organisms take in carbon
Carbon enters living things as part of the compound CO2. Plants absorb CO2 as part of photosynthesis. Animals then eat the plants—or, in the case of carnivores, other animals that consumed plants—and thus take in the carbon.
Step 3: Organisms die
While an organism is alive it is constantly taking in new carbon, which means that the ratio of different carbon isotopes it contains is more or less stable. Once it dies, however, it stops taking in carbon. The amount of carbon-12 and carbon-13 in the remains then stays constant, whereas the carbon-14 will decrease at a constant rate because of radioactive decay.
Step 4: Carbon-14 undergoes beta decay
Over a given amount of time, a calculable amount of carbon-14 will deteriorate from a sample. It does this through the process of beta decay, by which carbon-14 becomes nitrogen-14 again. The rate at which carbon-14 decays is known: its half-life is 5,730 ± 40 years. This means that in a sample of, say, two grams of carbon-14, about one gram will be left after about 5,730 years.
Step 5: Samples are collected and analyzed
Researchers collect samples for carbon dating if they are working with remains that they believe are between about 50,000 and 500 years old. (After decaying for 50,000 years or more, there is so little carbon-14 left that it is difficult to measure, and the technique’s margin of error is too large to be useful for dates within the last few centuries.) The researchers then send the samples to a laboratory, where each is processed. Ultimately, the amounts of different isotopes of carbon in the sample are measured by using an accelerator mass spectrometer. The ratio of stable carbon (usually carbon-12) to carbon-14 is determined, and the approximate age of the sample is calculated on the basis of how much carbon-14 has decayed.
Step 6: Radiocarbon dates are calibrated and reported
After the sample has been analyzed, the age estimate needs to be calibrated. When carbon dating was first developed, scientists believed that the ratio of different carbon isotopes in the atmosphere was constant, meaning that researchers could use the modern ratio when calculating the percentage of carbon-14 remaining. Later investigation, however, demonstrated that the ratio of these isotopes changes over time. Consequently, a radiocarbon date is calibrated by means of a calibration curve, which is a standardized tool used to correct raw radiocarbon dates. The calibration curve can be made by comparing the sample’s amount of carbon-14 with that of carbon dating samples of known age. It can also be made by means of dendrochronology, a method of determining the age of a piece of wood by analyzing tree rings. Each ring’s carbon-14 content indicates the level of the isotope in the atmosphere during that ring’s year of growth. The sample’s carbon-14 content can then be compared with that of tree rings of known age.
- Related Topics:
- carbon-14
- carbon-14 dating
The radiocarbon date may be reported as a calendar year ce or bce or in years bp. The latter designation, bp, stands for “before present”—“present” being defined by an international convention as 1950 ce. If the date has been calibrated, it is generally reported as cal bce, cal ce, or cal bp. The date often has a ± sign after it. The number that follows the sign represents the margin of error—the number of years by which the estimated date may be earlier or later than the true date. Adding and subtracting the margin of error from the estimated date gives the confidence interval, which consists of an upper bound and a lower bound denoting the range within which the true date would be expected to fall. Published radiocarbon dates normally express the confidence interval at two standard deviations. This means that there is a 95 percent probability that the true age of the sample is within the range given. For example, if the estimated date is 2000 ± 50 cal bce, the margin of error is 50 years, making the confidence interval 2050–1950. Therefore, the probability that the true date of the sample is between 2050 and 1950 bce is 95 percent.