Th isotopes in the Santa Monica basin: Temporal variation, long-term mass balance and model rate constants

Abstract

Distribution and flux of234Th,232Th and230Th 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 calculations show that the basin acts like a closed system for short-lived234Th, but not for the long-lived isotopes230Th and232Th. Most230Th in the basin is transported from elsewhere. Of the incoming Th, 40–55% of the230Th and 14–26% of the232Th 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.