Step by Step
1
How the method works, step by step
Living organisms continuously absorb carbon-14 (a radioactive isotope) from the atmosphere while they're alive, in roughly the same proportion as the atmosphere itself. After death, the organism stops absorbing new carbon, and the C-14 it already contains starts decaying at a known, steady rate. By measuring how much C-14 remains in a sample, scientists can calculate how long ago the organism died.
2
The half-life
Carbon-14 has a half-life of approximately 5,730 years, meaning half of the original C-14 decays away every 5,730 years. This fixed, predictable decay rate is what makes the calculation possible.
3
The upper limit
Radiocarbon dating is reliable only up to roughly 50,000 years — beyond that, so little C-14 remains that it becomes too difficult to measure accurately, and other dating methods (like K-Ar dating, covered in the next lesson) are needed instead.
4
AMS: measuring tiny samples
Accelerator mass spectrometry (AMS) counts individual C-14 atoms directly, rather than relying on older methods that measured radioactive decay events over time. This allows scientists to date very small samples — a huge practical advantage, since it means precious or limited archaeological material doesn't need to be destroyed in large quantities for dating.
5
What materials it works on
Radiocarbon dating only works on organic material — bone, wood, charcoal, shell, and seeds — because only living things absorb carbon from the atmosphere. It cannot be used to directly date inorganic materials like stone tools or pottery.
6
Calibration curves
Atmospheric C-14 levels haven't been perfectly constant throughout history, so raw radiocarbon dates need to be corrected using calibration curves (built from independently dated material like tree rings) to produce an accurate calendar-year estimate.
Applied Walkthrough
1
An archaeologist finds a small fragment of charred wood in a hearth feature.
2
Using AMS, the lab needs only a tiny sample to measure the remaining C-14 atoms directly.
3
The raw measurement suggests the wood stopped absorbing carbon (i.e., burned) a certain number of half-lives ago; after applying a calibration curve to account for historical atmospheric fluctuations, the lab reports a calibrated calendar date.
4
Because charcoal is organic, this works — but if the archaeologist instead wanted to date a stone tool found in the same hearth, radiocarbon dating wouldn't apply directly; they'd need to rely on the organic material found alongside it or use a different method entirely.
Exam Application
Exams test whether you can explain how radiocarbon dating works in your own words, state both the half-life and the upper dating limit, and which materials radiocarbon dating can and can't be used on.
⚠ Common Trap
Don't use radiocarbon dating on inorganic material like stone tools or metal — a very common exam trap is applying C-14 dating to something that never contained organic carbon. Also remember the ~50,000-year ceiling: anything older needs a different method.
✓ Quick Self-Check
1. What is the half-life of carbon-14?
Approximately 5,730 years.
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2. What is the upper reliable limit of radiocarbon dating?
Roughly 50,000 years.
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3. What does AMS stand for, and what advantage does it provide?
Accelerator mass spectrometry; it counts individual C-14 atoms directly, allowing very small samples to be dated.
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4. What types of material can radiocarbon dating be used on?
Organic material only — bone, wood, charcoal, shell, and seeds.
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5. Why are calibration curves needed?
Because atmospheric C-14 levels have fluctuated historically, so raw radiocarbon dates need correction to produce an accurate calendar-year estimate.
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