Radionuclide fluxes and particle scavenging in Cariaco Basin.

234Th, 228Th and 210Pb fluxes were measured in a series of sediment trap samples in Cariaco Basin from November 1995 to 8 May 1996. Sediment traps were located at 275, 455, 930 and 1255m depth, with the 275m trap just above the oxic/anoxic interface. Total mass, organic carbon, carbonate, biogenic opal, and lithogenic fluxes were also determined. Our results found that 234Th, 228Th and 210Pb fluxes are highly correlated with total mass (r2 = 0:92 - 0:94), major sediment component fluxes (r2 = 0:82 - 0:93), and one another (r2 = 0:94 - 0:99). Decreases in radionuclide, total mass and major sediment component fluxes with depth were attributed to physical disaggregation and remineralization. While the 210Pb specific activity loss from settling particles is the same as the bulk specific activity of the trapped particles between 455 and 930 m, the 210Pb specific activity loss is substantially higher than the bulk specific activity of the trapped particles between 930 and 1255 m. We attributed this fractionation to a loss of minor components from settling particles. This suggests that minor components are responsible for the initial water column scavenging of 210Pb in the Cariaco Basin. Steady state release of 210Pb from settling particles would produce an anoxic water column activity an order of magnitude greater than previously reported. Therefore, a non-steady state removal mechanism must exist and is potentially caused by high sediment flux events from earthquake triggered turbidity currents or major plankton blooms.