232Th–228Ra–228Th disequilibrium in East China Sea sediments

Five sediment cores collected from the East China Sea have been analyzed for 228Ra and 228Th (via 228Ac and 208Tl, respectively) by γ-ray spectrometry with a 150% efficiency HPGe detector. In four cores, the 228Th/228Ra activity ratio decreases from >1 at the core top to a subsurface minimum around a depth of 5 cm and then increases towards unity at deeper layers. Such disequilibrium relationships between 228Th and 228Ra downcore manifest the decay with depth of 228Th scavenged from the water column and the ingrowth of 228Th towards equilibrium with 228Ra after burial. Profiles of 228Ra are characterized by an upward decrease to minimum values at core tops, indicating diffusion of 228Ra to seawater. From deficiencies of 228Ra relative to 232Th in the studied sediment cores, fluxes of 228Ra are estimated to be 1.2–3.1 dpm cm-2 (year)-1 at the steady state. Assuming the study area is representative of the East China Sea Shelf, it is estimated from the measured sedimentary 228Ra fluxes and water column 228Ra data that the residence time of the East China Sea shelf water with respect to the exchange with the open ocean water via Kuroshio is about 1–3 years.