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

Coastal surface waters in the South Atlantic Bight contain elevated activities of 223Ra (t1/2 = 11.4 days) and 224Ra (t1/2 = 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 223Ra and 224Ra 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 228Ra. 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.