|Calendar years versus Radiocarbon Years
The years on the Ice Age Forest Timeline and other dates used in this module are to be read as calendar years unless stated as radiocarbon years.
Radiocarbon dating can provide ages for materials less than 40,000 years old. Radiocarbon or 14C (pronounced "carbon fourteen") is a radioactive form of carbon. Normal 12C has 6 protons and 6 neutrons, whereas 14C has 6 protons and 8 neutrons. 14C is unstable, and it decays at a constant rate. The half-life of 14C is 5,700 years. This means that an organism that died 5,700 years ago will have only one half of its original 14C. An organism that died 11,400 years ago will have only one quarter of its original 14C, and so on. After about 40,000 years, too little 14C remains to be measured.
Plants take up 14C during photosynthesis, and animals acquire 14C from eating plants. An assumption of 14C dating is that the amount of 14C in the atmosphere has remained constant. However, this assumption is not exactly true, so that radiocarbon ages become progressively too young with age; 5000 14C years is about 5700 calendar years, 11,000 14C years is about 13,000 calendar years, 20,000 14C years is almost 24,000 calendar years, and so on.
A correction is possible based upon dating of tree-rings of known age and from paired radiocarbon and uranium-series dates of corals. Tree rings of known age are from ancient bristlecone pine trees from Nevada and California, from oak logs buried in peat bogs in Europe, from archeological sites, and from other sources. Many dates reported in the scientific and popular literature are uncorrected radiocarbon dates. Computer programs now exist to easily calibrate radiocarbon years to calendar years, and the trend in recent years is to use calibrated years.