How Radiocarbon Dating Works

Why Living Things Have Carbon 14

The upper atmosphere is continuously struck by cosmic rays, which convert nitrogen 14 into carbon 14. Plants take it up through photosynthesis and animals eat the plants, so living things hold a steady known ratio of carbon 14 to carbon 12 as long as they are exchanging carbon with the environment.

What Happens at Death

Exchange stops. Carbon 14 begins decaying with its 5,730 year half life while carbon 12 stays put, so the ratio falls in exact half life steps. Measuring the remaining ratio and comparing it with the expected starting value gives the time since death.

How Far Back It Works

Reliable dating extends to roughly 50,000 years, about nine half lives, where only 0.2 percent of the original carbon 14 remains. Beyond that the signal falls below what can be reliably distinguished from background radiation, and results become unreliable.

Accelerator Mass Spectrometry

Older methods counted decays per minute. Accelerator mass spectrometry counts the carbon 14 atoms themselves directly, which is far more precise and extends the usable range to much smaller samples and shorter ages.

Why Dates Come With a Range

A reputable laboratory never returns an exact year. Calibration against known dated samples such as tree rings, sample contamination, and the material chosen all shift the result. Expect a range, and treat any single confident year as overconfident.

Known Limits

Contamination from old carbon in the soil can make a sample appear older, and contamination from young carbon can make it appear younger. Materials like bone that exchange carbon with soil are the most vulnerable, which is why pretreatment is standard practice.