14 DAY TRIAL // This is the most widely employed method of absolute dating: the radioactive isotope of the carbon, 14C. In the '40s, William Libby discovered that, with the passing of time, the 14C present in all dead organisms breaks down at a rhythm that can be measured. This finding found immediate applications in the newly born modern archeology.
The experts could date bones, wood, coal and other organic remains up to 50,000 years old, with an error edge of 80 years. But the radiocarbon method cannot date metal, stone or fossils in which organic matter has been replaced by minerals.
The element carbon has three isotopes: 12C, 13C and 14C. Opposite to the first two, 14C is radioactive and thus unstable. This means that it will break down, emanating very weak radiations. During photosynthesis, plants absorb from the air carbon dioxide, which enters in the composition of the organic chemicals resulted through the process. These biochemicals will form plant tissues that will feed plant-eating animals.
While plants and animals are alive, the proportion of the C14 in their tissue is the same with that from the atmosphere, extremely low. But when they die, concentration is constantly decreasing, due to the radioactive break down, which occurs at a known pace (50 % in 5,730 years). This allows estimating the time occurred from the death of the organism.
14C would have long ago disappeared from Earth in the absence of unremitting cosmic ray impacts on nitrogen in the Earth's atmosphere, which break down nitrogen to 14C.
The dating offered by the radioactive carbon is not extremely precise in years, because the level of atmospheric 14C has not been constant in time.
The concentrations of 14C can also be changed by variations in the cosmic ray intensity or Earth's magnetosphere induced by solar storms. There are also huge stores of carbon in organic matter, the ocean, ocean sediments, and sedimentary rocks. Climate changes can sometimes impair the carbon flow between these deposits and the atmosphere.
Human activities, and especially atomic bomb tests have multiplied the amount of 14C found in the atmosphere. Large quantities of CO2 from ancient fuels (which are depleted of 14C) decreased the percentage of 14C in the atmosphere.