Putting Numbers on the Sequence
Stratigraphy orders events; absolute dating hangs calendar numbers on the sequence. Choose the wrong method, and it is like timing a sprint with a sundial.
| Method | Clock | Dates | Range |
|---|---|---|---|
| Radiocarbon | Decay of carbon-14 | Once alive | to ~50,000 yr |
| Dendrochronology | Annual tree-ring widths | Preserved timber | to ~12,000 yr |
| Potassium-argon | Argon in cooled lava | Volcanic rock | 100k to billions |
| Thermoluminescence | Radiation since heating | Pottery, flint | to ~500,000 yr |
Living things constantly exchange carbon with the atmosphere, holding a steady trace of radioactive carbon-14. Death stops intake; C-14 then decays with a half-life of 5,730 years ( or ). Half the original C-14 means one half-life; a quarter means two.
The Fine Print & Calibration
Past ten half-lives almost nothing survives, hence the ~50,000-year ceiling. This is why dinosaurs are radiocarbon's most popular impossible request.
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 provide a correction curve back through the millennia.
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 (the old wood problem). A reused oak beam dates its felling, not the house's later renovation. The lab delivers a number; the archaeologist supplies what it means.