How Carbon Dating Works and Why It Stops at About 50,000 Years
Every plant and animal takes in a trace of radioactive carbon while it lives and stops when it dies. Here is how scientists turn that slow decay into a date, what can and cannot be dated, and why tree rings keep the clock honest.
Carbon dating, or radiocarbon dating, works out the age of once-living material by measuring how much radioactive carbon-14 it still contains. A plant or animal takes in carbon-14 while alive and stops when it dies; after that, the carbon-14 decays with a half-life of about 5,730 years, according to NIST. The method dates wood, charcoal, bone, shell and cloth back to about 50,000 years.
It is the most widely used dating tool in archaeology, and it earned its inventor, Willard Libby, the 1960 Nobel Prize in Chemistry. It also has firm limits that explain why it cannot date dinosaurs or rocks.
Where does carbon-14 come from?
Carbon-14 is made high in the atmosphere. Cosmic rays strike atoms in the upper air and knock loose neutrons, and some of those neutrons hit nitrogen-14 atoms and turn them into carbon-14, the National Institute of Standards and Technology explains. The new carbon-14 combines with oxygen to form carbon dioxide and mixes through the atmosphere.
Plants take in that carbon dioxide during photosynthesis, so their tissues hold a small, steady share of carbon-14 alongside ordinary carbon-12. Animals get the same mix by eating plants or eating other animals. As long as an organism is alive, its carbon-14 level roughly matches its surroundings.
How does carbon dating measure age?
When an organism dies, it stops taking in new carbon, and the carbon-14 it holds begins to decay back into nitrogen-14 at a fixed rate. Half of it is gone after about 5,730 years, half of the remainder after another 5,730, and so on. Comparing how much carbon-14 is left with how much a living thing would contain gives the time since death.
| Years since death | Carbon-14 remaining |
|---|---|
| 0 | 100% |
| 5,730 | 50% |
| 11,460 | 25% |
| 17,190 | 12.5% |
| 22,920 | 6.25% |
| 28,650 | About 3.1% |
| 40,110 | About 0.8% |
| 51,570 | About 0.2% |
Labs measure carbon-14 in one of two ways. Libby’s original method counted the faint radioactive decays from a sample over time, which needed a lot of material. Most labs today use accelerator mass spectrometry, which counts carbon-14 atoms directly. NobelPrize.org notes that modern machines can date tiny samples, in some cases about a milligram.
What can carbon dating be used on?
Carbon dating only works on material that was once alive and took in carbon from the atmosphere or ocean. Common samples include:
- Charcoal from ancient hearths and fires
- Wood, seeds, grain and other plant remains
- Bone, antler and ivory
- Shell, which needs a marine correction
- Textiles, leather, paper and parchment
- Peat and organic layers in lake mud
It cannot date stone tools, pottery clay or metal directly. Archaeologists date the organic material found with them, such as charcoal in the same layer or food residue on a pot, and make the link carefully.
Why can’t carbon dating go back further than about 50,000 years?
After about nine half-lives, so little carbon-14 is left that it is very hard to measure against background contamination. The Smithsonian’s Human Origins Program gives the useful range as about 50 to 50,000 years. NIST notes that labs have dated some samples to about 60,000 years and occasionally older, but results that old are near the edge of what instruments can detect.
A tiny amount of modern carbon can throw off an old date badly, because the sample has so little carbon-14 of its own. Even a trace of modern carbon can make a very old sample look thousands of years younger, which is why labs clean samples with strong chemical treatments before measuring them.
This limit is also why carbon dating is never used on dinosaurs, which died out about 66 million years ago, more than a thousand times beyond its range. Fossils that old are dated with other radioactive clocks in volcanic rock, explained in how fossils are dated.
Why do radiocarbon dates need calibration?
Raw radiocarbon dates need calibration because the amount of carbon-14 in the atmosphere has not been perfectly constant. It shifts with changes in the Sun’s activity, Earth’s magnetic field and the carbon cycle. A raw “radiocarbon age” therefore does not equal calendar years.
Scientists correct for this with calibration curves built from material whose age is known independently. Tree rings are the backbone: each ring grows in a known year, so measuring carbon-14 ring by ring shows what the atmosphere held at that time. NIST notes that tree rings and objects of known age, such as samples from Egyptian tombs of pharaohs with known reign dates, were used to check the method.
The current standard for the Northern Hemisphere is IntCal20, published in 2020 in the journal Radiocarbon. It extends calibration back to 55,000 years, combining tree rings with lake and marine sediments, cave formations (speleothems) and corals for the oldest part. Separate curves exist for the Southern Hemisphere and for marine samples, because the ocean holds older carbon.
How do scientists write a carbon date?
A calibrated date is written as a range with a probability, not a single year. A result might read “3,950 to 3,820 cal BP (95%),” meaning calibrated years before present, with a 95% chance the true age falls in that range. “Before present” is counted from 1950 by convention.
- The lab measures the sample’s carbon-14 and reports a radiocarbon age with an uncertainty, such as 3,600 plus or minus 30 years.
- Software compares that age against the calibration curve.
- The output is a calendar range, which can be lumpy where the curve wiggles.
- Archaeologists combine several dates with the order of layers at the site to narrow the result.
The same layering logic is used for fossils, and our guide on who the Neanderthals were shows why sites near the 50,000-year edge are so hard to date precisely.
Bottom line
Carbon dating measures how much carbon-14 remains in once-living material. With a half-life of about 5,730 years, it reaches back about 50,000 years, after which too little is left to measure reliably. Raw dates are calibrated with tree rings and other records, currently the IntCal20 curve, and results are given as calendar ranges with probabilities.
FAQ
Can carbon dating date rocks?
No. Rocks do not take in carbon from the atmosphere the way living things do. Geologists date rocks, especially volcanic layers, with other radioactive elements such as potassium and uranium that decay much more slowly.
How accurate is carbon dating?
For well-preserved samples a few thousand years old, calibrated dates can often be narrowed to a few decades or a century or two. Accuracy drops with older samples, contamination, or periods where the calibration curve is flat.
Who invented carbon dating?
American chemist Willard Libby and his team developed the method at the University of Chicago in the late 1940s. Libby received the 1960 Nobel Prize in Chemistry for the work.
What does “BP” mean in a date?
BP means “before present,” and in radiocarbon dating “present” is fixed at the year 1950. A date of 2,000 BP therefore refers to about 50 BC.
