Excess ²²⁷Ac in deep ocean water

Abstract

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, t_1/2, 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.