²¹⁰Pb-²²⁶Ra: radioactive disequilibrium in the deep sea

Abstract

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.