Distortions in the radiocarbon age of samples derived from environments where samples from a particular carbon reservoir may not be in complete equilibrium with the atmosphere, so that their normalized isotope ratio is not the same as that of contemporaneous terrestrial wood. The most common example of the effect occurs with marine shells because the surface ocean waters where they grow have a significant contribution from older deep‐ocean waters. The magnitude of the correction necessary for marine shell ages will depend on the geographical location: for mid‐latitude samples the gap is probably about 400 years while for higher latitudes the correction may rise to more than 1000 years. In all cases the correction is subtracted from the conventional radiocarbon age. Also known as the marine effect or the environmental effect.