Temporal variations of 234Th activity inthe water column of Dabob Bay: Particle scavenging

Monthly measurements of total, dissolved, and particulate 234Th, along with fluxes of total mass and particulate 234Th, were made throughout 1987 in Dabob Bay, Washington. Vertical distri-butions of total 234Th showed a large deficiency relative to238U throughout the water column during all sampling periods. Total 234Th activities n urface waters were generally bigher in winter and lower in summer, in response to seasonal biological activity. In spite of this variation, thesteady state assumption seems valid for modeling 234Th of the bay because the magnitude of variation isinsignificant in the mass balance calculation. The range of residence times of total, dissolved, and particulate 234Th estimated horn an irreversible scavenging model are 4-70, <: l-10, and 3-60 d, respectively. The residence time of 234Th is controlled by the suspended particles whose distribution is determined bybiological production i the surface layer and resuspension ofbottom sediments. Vertical f uxes of 234Th measured by sediment raps also showed wide seasonal variation a d generally increased monotonically from 200-l,000 dpm me2 d-l at 20 m to 2,000-4,000 at100 m. The flux of 234Th predicted with the steady state scavenging model tends to be about twice th