Hard water contains less Carbon-14 than the atmosphere, because dissolved carbonates are Carbon-14 free.
A fish caught in hard water has thus a higher Carbon-14 age than contemporaneous terrestrial samples.
Radiocarbon samples which obtain their carbon from a different source (or reservoir) than atmospheric carbon may yield an apparent age called a reservoir age.
Other corrections must be made to account for the proportion of throughout the biosphere (reservoir effects).
Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the 1950s and 1960s.
The radiocarbon dating method is based on the fact that radiocarbon is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen.
The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died.
La datation par le carbone 14 se fonde ainsi sur la présence dans tout organisme de radiocarbone en infime proportion (de l'ordre de 10).