Present-day carbonate in deep ocean water may have an apparent age of several thousand years because of its long residence time there. Because there is some admixture of deep water into the surface ocean shells grown there have some apparent age too - around 400 years for mid-latitudes, rising to the order of 1000 years in regions of upwelling. The reservoir correction is determined by measurements on historically dated samples of the same material and origin as the sample being dated; samples prior to Industrial Revolution need to be used in order to avoid interference by fossil-fuel effect.