Examples of carbon reservoirs are found in the atmosphere, the lithosphere (the Earth’s crust), the oceans, and the biosphere (living organisms). Land plants and the food chains they support acquire most of their carbon from the atmosphere, whereas marine food chains acquire carbon mainly from the oceans. About 7.5 kg of C-14 is produced each year in the upper atmosphere, and its mixing with carbon in the oceans is less complete than its mixing with atmospheric carbon. Upward flow of deep ocean water also brings ancient, non-radioactive carbon to the surface waters. Therefore marine organisms are relatively depleted in C-14, and modern marine plants and animals can yield apparent ages of hundreds of years. This discrepancy is called the reservoir effect.

It was once thought that the reservoir effect was about 400 years in all the oceans, but it is now known that the size of the effect varies geographically and through time. Every regional study that employs radiocarbon dates on marine organisms must establish the appropriate correction factor for that region.