Ages of continental shelf waters determined from 223Ra and 224Ra

Coastal surface waters in the South Atlantic Eight contain elevated activities of Ra-223 (t(1/2) = 11.4 days) and Ra-224 (t(1/2) = 3.66 days). These elevated dissolved activities are due to continual input of the short-lived tracers near the coast. The presence of these tracers in an offshore water sample implies that the water is less than six half-lives removed from the source of the tracer. The absence of the tracer implies the water is more than six half-lives old or has been diluted substantially with old water containing little tracer. In this paper I use models based on Ra-223 and Ra-224 to derive ages of continental shelf waters. These models compensate for tracer dilution by normalizing the short-lived tracers to each other or to long-lived Ra-228. If the water on the inner shelf (0-20 km from shore) is assigned a zero age, ages for waters 80-100 km from the inner shelf are 20-30 days.