<SUP>210</SUP>Pb-<SUP>226</SUP>Ra: radioactive disequilibrium in the deep sea

210Pb and 226Ra profiles have been measured in the North Atlantic and North Pacific Deep Water. The 210Pb activity is 25% to 80% of that of 226Ra, averages about 50% in each profile, and is lowest in the bottom water. This deficiency of 210Pb relative to 226Ra shows that 210Pb is rapidly and continually scavenged from deep water, probably by adsorption on particulate material sinking from the surface. A method is developed for simultaneous application of the vertical diffusion-advection model to 226Ra and 210Pb deep-water profiles, and the in-situ source terms for both isotopes are obtained as a function of the parametric upwelling velocity. For a positive source term for 226Ra in deep water, the parametric velocity is limited to a small range (3-12 m/y) and the deep-Pacific residence time of lead is 54 yr, a value two orders of magnitude smaller than residence times estimated from stable lead concentrations. The model calculations show that an in-situ scavenging process, acting throughout the water column, is required to remove the Pb. Box model calculations yield a similar short residence time for lead in North Atlantic Deep Water, indicating that radioactive disequilibrium for 210Pb, due to fast scavenging, is a general phenomenon through the deep sea