Isotopic fractionation also occurs in the exchange reaction between carbon dioxide and ocean carbonate, there being an enrichment of 1.5% when carbon goes from atmosphere to ocean. Hence, relative to plant life there is an isotopic enrichment of ocean carbonate by about 5%. It so happens that this just compensates for the deficiency of carbon-14 in ocean carbonate that arises from the long residence time there and the directly measured carbon-14 activities are the same.