Bismuth-210, its parent, and daughter and their use as particle tracers in aquatic systems

Abstract

Disequilibrium between 210Pb (t½: 22.3 years) and its granddaughter 210Po (t½: 138.4 days) has been used to infer organic particle removal rates from the surface ocean on a timescale of weeks to months; however, the behavior of the short-lived intermediary daughter 210Bi (t½: 5.01 days) is not well known. Here, timeseries measurements of 210Pb, 210Bi, and 210Po in the Milwaukee Inner Harbor on Lake Michigan showed respective particle distribution (Kd) coefficients of 7.1 (± 0.4) × 105, 9.3 (± 1.7) × 105, and 10.2 (± 0.6) × 105 L kg −1, giving Kd ratios, or fractionation factors (F), of FBi/Pb = 1.3 ± 0.3, FPo/Bi = 1.1 ± 0.2, and FPo/Pb of 1.4 ± 0.1. After a large rain event, total 210Pb, 210Bi, and 210Po activities in the harbor water column decreased over a course of 12 days from 347 to 105 dpm 210Pb m−3, 397 to 88 dpm 210Bi m−3, and 144 to 58 dpm 210Po m−3. This is the first observation of a total 210Bi/210Po activity ratio of <1 in an aquatic system. A 1-D non-steady-state model of nuclide fluxes suggested that 92% of the reduction in 210Bi activity was due to removal from the water column, with the remainder accounted for by net in situ decay. The potential for using 210Bi as a tracer of high and variable particle scavenging rates in lakes and shallow marine systems is supported.