Th Isotopes in the Santa Monica Basin: Temporal Variation, Long-Term Mass Balance and Model Rate Constants CHIH-AN HUH

Distribution and flux of 234Th, 232Th and 230Th in the water column of central Santa Monica basin observed over a period of seven years show seasonal and interannual variabilities. A steady-state model is applied to the integrated data to calculate long term average flux and model rate constants of Th isotopes. Mass balance calcu-lations show that the basin acts like a closed system for short-lived 234Th, but not for the long-lived isotopes 230Th and 232Th. Most 230Th in the basin is transported from elsewhere. Of the incoming Th, 40–55 % of the 230Th and 14–26 % of the 232Th enter the surface water in dissolved form. In the upper 100 m, the residence time of dissolved Th with respect to adsorption onto suspended particulates, 70–80 days, is about one order of magnitude higher than the residence time of suspended particles with respect to aggregation into sinking particles, 7–10 days. 1