The amount of C14 in the atomsphere varies over timeAtmospheric14C,New Zealand[1] andAustria.[2] The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere. Atmospheric nuclear weapon tests almost doubled the concentration of14C in the Northern Hemisphere.[3]
Radiocarbon dating, also known as theC14 dating method, is a way of telling how old an object is. It is a type ofradiometric dating.
The method uses theradioactiveisotopecarbon-14. Most organic matter containscarbon. Carbon has differentisotopes, which are usually not radioactive.14C is the radioactive one, itshalf-life (time it takes to reduce its radioactivity by half) is about 5,730 years. This makes it possible to tell the age of substances that contain carbon. The method works to about 60,000 years old. Dates obtained are usually written asbefore present ('present' is 1950).
Plants take up atmosphericcarbon dioxide byphotosynthesis, and are eaten by animals, so every living thing is constantly exchanging carbon-14 with its environment as long as it lives. Once it dies, however, this exchange stops.
In 1958 Hessel de Vries showed that the concentration of carbon-14 in the atmosphere varies with time andlocality. The relatively short-lived14C is constantly renewed bycosmic ray bombardment onatmosphericnitrogen. Since the bombardment is slightly variable, and for other reasons, the14C taken into organic matter is also slightly variable. That leads to errors in the chronology. However, under about 20,000 years the results can be compared withdendrochronology, based on tree rings. For the most accurate work, variations are compensated by means of calibration curves.
The method was developed byWillard Libby and his colleagues at theUniversity of Chicago in 1949. In 1960, he was awarded theNobel Prize in Chemistry for this work. He first demonstrated the accuracy of radiocarbon dating by accurately estimating the age of wood from anancient Egyptian royal barge of which the age was known from historical documents.[4][5]
