Carbon age dating
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.After about 50,000 years, the radiocarbon concentration remaining is too small to be measured for the purpose of radiocarbon dating. This is necessary to remove errors in raw radiocarbon dates caused by fluctuations in the amount of radiocarbon in the atmosphere in the past.The archaeologist Colin Renfrew (1973) called it the development of this dating method 'the radiocarbon revolution' in describing its great impact upon the human sciences.
Plants obtain all their carbon atoms from the atmosphere."Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure.We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 19)].The ratio of radiocarbon to stable carbon atoms in the atmosphere has varied in the past.This is because the amount and strength of cosmic radiation entering the earth's atmosphere has varied over time.