Putting Numbers on the Sequence
Stratigraphy orders events; absolute dating hangs calendar numbers on the order. Each method is a natural clock with a specific tick, range, and material — choosing the wrong one is like timing a sprint with a sundial.
| Method | Clock | Dates | Range |
|---|---|---|---|
| Radiocarbon (C-14) | Decay of carbon-14 after death | Anything once alive | to ~50,000 yr |
| Dendrochronology | Annual tree-ring widths | Preserved timber | to ~12,000 yr, precise to the year |
| Potassium-argon (K-Ar) | Argon accumulating in cooled lava | Volcanic rock around fossils | 100,000 yr to billions |
| Thermoluminescence (TL) | Radiation dose since last heating | Pottery, burnt flint, hearths | to ~500,000 yr |
How the Radiocarbon Clock Runs
Living things constantly exchange carbon with the atmosphere, holding a steady trace of radioactive carbon-14. Death stops the intake; the C-14 then decays with a half-life of 5,730 years. Measure the surviving fraction and the elapsed time follows:
Half the original C-14 means one half-life (~5,730 years); a quarter means two (~11,460); an eighth, three (~17,190). Past ten half-lives almost nothing survives — hence the ~50,000-year ceiling, and hence why dinosaurs (66+ million years) are radiocarbon's most popular impossible request.
The Fine Print
Willard Libby's method (1949, Nobel 1960) has a subtlety: atmospheric C-14 was never perfectly constant, so raw radiocarbon years drift from calendar years. The fix is calibration — tree-ring sequences of known calendar age carry their own measured C-14, providing a correction curve back through the millennia. Dendrochronology thus does double duty: the year-exact dating of timbers and the ruler that keeps radiocarbon honest.
Date the death, not the use: C-14 dates when the organism died. A hearth burning 300-year-old driftwood dates the tree, not the fire ("old wood problem"); a reused oak beam dates its felling, not the house's third renovation. The laboratory delivers a number — the archaeologist must supply what the number means.
A Clock Made of Disappearance
The animation follows one organism's carbon. In life, C-14 holds steady — intake balances decay. At death the intake line is severed and the decay curve takes over, halving the stock every 5,730 years: one half, one quarter, one eighth, the bars shrinking down the exponential staircase.
Watch the far end of the curve: by ten half-lives the remaining C-14 sinks into the measurement noise floor — the ~50,000-year ceiling appears on screen not as a rule but as a consequence. Then the calibration beat: raw radiocarbon years bend against the tree-ring curve, and the date steps from "radiocarbon age" to calendar age. The clock is elegant; reading it honestly is the craft.