Size-fractionated thorium isotopes (Th-228, Th-230, Th-232) in surface waters in the Jiulong River estuary, China
- 海洋环境－已发表论文 
Thorium isotopes (Th-228, Th-230, Th-232 and Th-234) 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 Th-228, Th-230 and Th-232 in different size fractions, namely, the >53 mum, 10-53 mum, 2-10 mum, 0.4-2 mum, 10 kDa-0.4 mum and the <10 kDa fractions. Our results indicated that the activity concentrations of Th-228, Th-230 and Th-232 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 mum fraction had the least share (0-1%), while the 10-53 mum fraction had the largest share of Th isotopes. The average value of the Th-230/Th-232 activity ratio (Th-230/Th-232)(A.R.) increased from 0.8 in the >53 mum fraction to 3.7 in the 10 kDa-0.4 mum fraction, indicating that the radiogenic Th isotopes are preferentially scavenged by the small size particles. (Th-230/Th-232)(A.R.) in the <10 kDa and 10 kDa-0.4 mum fractions were similar, however, suggesting a similar chemical composition and/or equilibrium partitioning between the low molecular weight and colloidal Th. It was very interesting to note that the geochemical behaviors of the three Th isotopes were different from each other. Dissolved Th-228 had the highest concentration in the mid-salinity region, showing a non-conservative behavior with additional input. In contrast, dissolved Th-232 showed a concave profile, indicating a net removal of Th-232 during the mixing of fresh water with seawater. The behavior of Th isotopes in the <10 kDa fraction followed those in the dissolved phases. The difference in geochemical behaviors among three Th isotopes was ascribed to their different sources in the estuary. (C) 2004 Elsevier Ltd. All rights reserved.