Influence of boundary scavenging and sediment focusing on Th-234, Th-228 and Pb-210 fluxes in the Santa Barbara Basin

Sediment trap samples were collected from the Santa Barbara Basin to examin e the flux of Th-234, Th-228 and Pb-210. Mean ratios of measured to predict ed fluxes are 2, 10 and 21 for Th-234, Th-228 and Pb-210, respectively. Bou ndary scavenging may supply a portion of all three radionuclides. However, sediment focusing supplies additional Th-228 and a Pb-210 but little Th-234 . Sediment temporarily deposited on the shelf around the basin contains lit tle excess Th-234 due to its short half-life. Th-228 and Pb-210 fluxes to t he sediments in the Santa Barbara Basin are several rimes greater than pred icted from their supply in the overlying water column. These enhanced fluxe s require lateral transport of Th-228 and Pb-210 to the basin through bound ary scavenging and sediment focusing. Th-234 flux is poorly correlated with Th-228 and Pb-210 fluxes because indi vidual radionuclide sources to the basin differ (i.e. water column producti on, boundary scavenging and/or sediment focusing). Correlations between Th- 228 and Pb-210 fluxes are high due to the strong influence of sediment focu sing on both radionuclides. There is a strong correlation between both the Th-228 and Pb-210 flux and total mass flux and lithogenic flux, but a weak correlation with the other major sediment components (e.g. organic carbon, carbonate and biogenic opal). In the Santa Barbara Basin, a large fraction of the total mass flux is lithogenic material (50-80%), hence the strong co rrelation between Th-228 and Pb-210 fluxes and both total mass and lithogen ic fluxes. Th-234 flux is poorly correlated with total mass and major sedim ent component fluxes. (C) 2000 Academic Press.