Consider five specific dating methods in common use by archaeologists and paleontologists. Two of them, radiocarbon and potassium/argon (or argon/argon) dating, use the decay of radio- isotopes and are therefore ‘absolute’ methods, independent of the sample environment. They are relevant in different time periods
because the radioactive atoms decay at different rates e 14C (car-
bon-14) has the (comparatively) short half-life of around 5700 years, while 40K (potassium-40) decays much more slowly with a half-life of 1.25 billion years. The other three methods are
commonly called ‘trapped charge methods’ and are important in filling the gap between the numerical methods. Electron spin resonance (ESR), thermoluminescence (TL) and optically stimu- lated luminescence (OSL) measure the accumulation of radiation damage in a sample from radioactive substances in the soil and in the sample itself. These methods, conventionally called ‘chrono- metric’, therefore depend on environment and uncertainties about environmental change over time affect the calculated ages. TL