How can radiocarbon help date material from living things?

A charred seed found in an old hearth can carry a clue to time inside its carbon. That clue began while the plant was alive, exchanging carbon with its surroundings. Radiocarbon dating follows a physical change in that carbon, then connects the measurement to a historical question about what the seed represents.

Carbon-14 is a radioactive form of carbon produced in the atmosphere. Plants take in carbon dioxide during photosynthesis, and animals obtain carbon through food. Carbon that is actively exchanged reflects its sources, including a small radiocarbon component. Once that exchange ends for the material being studied, carbon-14 continues to decay without the same replenishment. Its abundance relative to stable carbon declines.

The physical half-life of carbon-14 is approximately 5,730 years. In an ideal closed sample, half of the original carbon-14 would remain after one half-life, a quarter after two and an eighth after three. These are expected proportions across many atoms; no one can predict the exact moment an individual atom will decay. The regular statistical pattern makes a clock possible, provided the initial composition and subsequent history are understood.

A laboratory measurement is not yet an exact calendar date. Atmospheric radiocarbon abundance has varied. Conventional radiocarbon ages also retain an agreed historical half-life convention and use BP, before present, with present fixed at 1950. Calibration compares the result with independently dated records, including tree rings, to account for the relationship between radiocarbon measurements and calendar time. Measurement uncertainty and variations in the calibration curve can leave one broad interval or several possible intervals.

The material determines which event is being dated. A seed usually contains carbon acquired during a short growth period. Wood contains carbon fixed when particular rings grew, potentially long before the tree was felled. A wooden beam could then be stored or reused. Dating its carbon therefore does not automatically date a building's construction. Identifying the sample and its context is part of the reasoning, not a detail to add after the numerical result.

Carbon from a different source can complicate the clock. Later contamination may add younger or older carbon. Food webs connected to reservoirs such as the ocean can also begin with radiocarbon proportions different from contemporary atmospheric carbon. Laboratories choose and prepare samples carefully, and interpretation must consider these histories. A very precise measurement cannot make an unsuitable sample represent the desired event.

After many half-lives, so little carbon-14 remains that background and contamination become increasingly important. The method is generally useful over tens of thousands of years, roughly up to 50,000 years for suitable material, rather than the millions of years needed to date non-avian dinosaur fossils. Within its range, a tiny biological remnant can connect a physical decay process with a human past, as long as the measurement, calibration and archaeological context support the same story.