Understanding the nature and rates of processes by which materials are transported and mixed in the deep ocean is essential for evaluating environmental effects and risks involved in human uses of the ocean, such as mining of manganese nodules and deep ocean dumping of wastes. I report here the first measurements of 227 Ac (half life, t1/2, 21.8 yr) in seawater. The results clearly show the presence of excess 227 Ac relative to its progenitor 231Pa (see Fig. 1) in deep waters below 3 km, suggesting that the 227 Ac is supplied by diffusion from sediments. Two other natural radionuclides, 222Rn (ref. 1) and 228Ra (ref. 2) which have bottom sources, have proved to be useful for determining rates of mixing from days to decades. The excess 227 Ac can be used as a novel tracer for basin-wide circulation and mixing on time scales up to 100 yr.