Evidence is provided for seasonality in the bomb 14C content of surface ocean water in temperature and polar regions. The GEOSECS results show that the surface waters in the north temperate Atlantic had 14C/12C ratio 35¿/00 higher just before later fall deepening of the mixed layer than just before the reestablishment of the summer mixed layer six months later. This is the difference expected from the summer exchange of CO2 between the atmosphere and the surface mixed layer.