Radiocarbon samples which obtain their carbon from a different source (or reservoir) than atmospheric carbon may yield an apparent age called a reservoir age. The Ocean has a reservoir age of about 402 radiocarbon years (see Stuiver et al. 1986). This apparent age 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. The reservoir age is incorporated in the modelled marine curve of Stuiver et al. (1998).

Regionally, the ¹⁴C of the ocean surface deviates from this modelled marine curve due to variations in upwelling, ocean currents, and inter-hemispheric atmospheric ¹⁴C. Therefore, when dating marine shells it is essential to know the difference (ΔR) between the modelled radiocarbon age of surface water and the known ¹⁴C age of surface water at that time represented by the marine shell sample. The ΔR for a specific location (s) can be calculated from known age shells, collected prior to atmospheric bomb testing using the formulae:

Where Rs(t) is the ¹⁴C age of known-age sample and Rg(t) is the ¹⁴C age of the model mixed ocean layer for the calendar year (AD) of the known-age sample. The ΔR standard error is calculated by the formulae: