A New Radiotracer for Particulate Carbon Dynamics: Examination of ²¹⁰Bi‐²¹⁰Pb in Seawater

Abstract 210 Bi ( t 1/2 = 5.01 d) is theoretically a radionuclide for tracing the particle cycle over a timescale of hours to days. However, it has been rarely investigated in marine environments due to its very short half‐life and low activity. Here, 210 Bi and 210 Pb were examined in the water column on the shelf/slope of the northern South China Sea (SCS), as well as their atmospheric deposition. In rainwater, the 210 Bi/ 210 Pb ratio averaged 0.54 ± 0.28, indicating the influence of atmospheric deposition on the disequilibrium between 210 Bi and 210 Pb in surface seawater. On the shelf, 210 Bi/ 210 Pb averaged 0.73 ± 0.10 in the euphotic zone and 1.25 ± 0.10 below, supporting a quick removal of 210 Bi from the euphotic zone and regeneration in the twilight zone. On the slope, deficits in 210 Bi ( 210 Bi/ 210 Pb of 0.81 ± 0.07) were also observed in the productive low euphotic zone. The concurrence of 210 Bi deficits and higher particulate organic carbon (POC) concentrations implied that POC largely dominates the deficit and excess of 210 Bi. Based on a simple model, the removal fluxes of 210 Bi at the euphotic base were 728 ± 73 dpm m −2 d −1 and 216 ± 89 dpm m −2 d −1 on the shelf and slope. The residence time of particulate 210 Bi was 14 ± 2 d. The 210 Bi‐derived export flux of POC was 1.7 ± 0.7 mmol‐C m −2 d −1 out of the euphotic zone over the slope. These results lay the foundation for 210 Bi/ 210 Pb to quantify the sinking and remineralization of particulate organic matter in coastal seas.