On the role of Bi-210 in the apparent disequilibrium of (210)pb-Po-210 at sea

Abstract

The disequilibrium of the grandparent-daughter pair Pb-210 ( t(1/2)=(22.3 years))-Pb-210(t(1/2=138 days)) has been used to estimate the export fluxes of particulate organic carbon in the ocean using particulate-matter-associated Pb-210. Pb-210 is produced from Bi-210, not from Pb-210. The half-life of Bi-210 (t(1/2=5.01 days)) is sufficiently long compared to the rates of biological particle formation and decomposition or dissolution occurring at sea. The role of Bi-210 has not yet been assessed quantitatively in the apparent disequilibrium between Pb-210 and Pb-210, partly due to the non-existence of Bi-210 depth profile measurements at sea up to now. However, greater affinity of Bi-210 over( 210)Po and Pb-210 was found recently in coastal waters and phytoplankton Bi-207 uptake experiments. Build upon these findings, we developed a primitive and simple analytical approach to elucidate the role of Bi-210 in the Po-2(10)-Pb-210 pair in the ocean using a simplified two-box irreversible steady-state ocean model. We assumed that the activity concentrations in the dissolved and particulate phases of Pb-210, Bi-210, and Po-210 in a given water column are solely determined by the concentration of the particles, their input and output, the distribution coefficients between dissolved and particulate phases, and decay constants of these radionuclides in the steadystate ocean. The Bi-210 contribution to the Pb-210-Po-210 activity difference in seawater is found to be significant, therefore, it needs to be considered in estimating particle fluxes using Pb-210-Po-210 secular equilibrium at sea.