This calibration step eliminates any concern about fluctuations in historic radiocarbon to stable carbon ratios or decay rates.Radiocarbon dating is a valuable tool to chronologists and archaeologists. Carbon has different isotopes, which are usually not radioactive.Plants take up atmospheric carbon dioxide by photosynthesis, and are eaten by animals, so every living thing is constantly exchanging carbon-14 with its environment as long as it lives. In 1958 Hessel de Vries showed that the concentration of carbon-14 in the atmosphere varies with time and locality.Radiocarbon is not stable; over time radiocarbon atoms decay into nitrogen atoms.This tendency to decay, called radioactivity, is what gives radiocarbon the name radiocarbon.Plants obtain all their carbon atoms from the atmosphere.Thus, the ratio of radiocarbon to stable carbon in a living plant is the same as the ratio of radiocarbon to stable carbon in the atmosphere at any given time.
Although the radiocarbon and OSL ages agree in some samples, the radiocarbon ages are older than the corresponding OSL ages at the 550-cm depth horizon (late Pleistocene) and in the 100–300-cm interval (early to late Holocene).
Animals (and humans) get their carbon atoms primarily from what they eat (i.e., plants).
Thus the ratio of radiocarbon to stable carbon in living animal tissue is also virtually the same as the ratio of radiocarbon to stable carbon in the atmosphere at any given time.
The starting ratio of radiocarbon to stable carbon is locked in at that point. The purpose in each of these methods is to determine the ratio of radiocarbon to stable carbon in the sample.
From then on, the ratio of radiocarbon to stable carbon will decrease, because the unstable radiocarbon atoms will slowly decay. From this measurement the age in radiocarbon years is calculated. Modern radiocarbon dates are calibrated using long tree-ring chronologies.