Size-fractionated thorium isotopes (228Th, 230Th, 232Th) in surface waters in the Jiulong River estuary, China.

Abstract Thorium isotopes ( 228 Th, 230 Th, 232 Th and 234 Th) are useful tracers for studying particle dynamics and trace element scavenging in marine environments. In this study, surface waters were collected along a salinity gradient from the Jiulong River estuary, China, for determination of activity concentrations of 228 Th, 230 Th and 232 Th in different size fractions, namely, the >53 μm, 10–53 μm, 2–10 μm, 0.4–2 μm, 10 kDa–0.4 μm and the 228 Th, 230 Th and 232 Th in the Jiulong River estuarine waters were significantly higher than most of the previously reported values in coastal and oceanic seawaters, suggesting a higher lithogenic U and Th contribution from the Jiulong River Basin. When normalized to the particulate mass concentration, the activity concentrations of the three thorium isotopes decreased with increasing particle size, demonstrating the important role of surface areas of particles in controlling the scavenging of thorium from the water column. The partitioning of three thorium isotopes showed a common characteristic, i.e., the >53 μm fraction had the least share (0–1%), while the 10–53 μm fraction had the largest share of Th isotopes. The average value of the 230 Th/ 232 Th activity ratio ( 230 Th/ 232 Th) A.R. increased from 0.8 in the >53 μm fraction to 3.7 in the 10 kDa–0.4 μm fraction, indicating that the radiogenic Th isotopes are preferentially scavenged by the small size particles. ( 230 Th/ 232 Th) A.R. in the 228 Th had the highest concentration in the mid-salinity region, showing a non-conservative behavior with additional input. In contrast, dissolved 232 Th showed a concave profile, indicating a net removal of 232 Th during the mixing of fresh water with seawater. The behavior of Th isotopes in the