This apparent age of oceanic water is caused both by the delay in exchange rates between atmospheric CO2 and ocean bicarbonate, and the dilution effect caused by the mixing of surface waters with upwelled deep waters which are very old (Mangerud 1972).
A reservoir correction must therefore be made to any conventional shell dates to account for this difference.
In order to ascertain the ages of samples which were formed in equilibrium with different reservoirs to these materials, it is necessary to provide an age correction.
New leaves are produced in weeks while larger fruit and vegetables form over the period of a month or two.
The atmospheric concentration of C becomes the fingerprint of this radioisotope in a given year's food supply.
Herbivores lag the atmosphere slightly because their primary carbon source is on the order of months old.
Reservoir corrections for the world oceans can be found at the Marine Reservoir Correction Database, a searchable database online at Queen's University, Belfast and the University of Washington.
Human bone may be a problematic medium for dating in some instances due to human consumption of fish, whose C14 label will reflect the ocean reservoir.