Influence of Boundary Scavenging and Sediment Focusing on234Th,228Th and210Pb Fluxes in the Santa Barbara Basin

Abstract

Sediment trap samples were collected from the Santa Barbara Basin to examine the flux of234Th,228Th and210Pb. Mean ratios of measured to predicted fluxes are 2, 10 and 21 for234Th,228Th and210Pb, respectively. Boundary scavenging may supply a portion of all three radionuclides. However, sediment focusing supplies additional228Th and210Pb but little234Th. Sediment temporarily deposited on the shelf around the basin contains little excess234Th due to its short half-life.228Th and210Pb fluxes to the sediments in the Santa Barbara Basin are several times greater than predicted from their supply in the overlying water column. These enhanced fluxes require lateral transport of228Th and210Pb to the basin through boundary scavenging and sediment focusing.234Th flux is poorly correlated with228Th and210Pb fluxes because individual radionuclide sources to the basin differ (i.e. water column production, boundary scavenging and/or sediment focusing). Correlations between228Th and210Pb fluxes are high due to the strong influence of sediment focusing on both radionuclides. There is a strong correlation between both the228Th and210Pb 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 correlation between228Th and210Pb fluxes and both total mass and lithogenic fluxes.234Th flux is poorly correlated with total mass and major sediment component fluxes.