Scavenging of ex234Th, ex230Th, and ex210Pb by particulate matter in the water column of the California Continental Margin

The accumulation of ex230Th, organic carbon and total mass at Station M (a 4100-m site 220 Km off Central California) is not supported by sediment trap-collected fluxes. The flux of ex230Th to sediment traps only accounts for 10% of the ex230Th accumulation in sediments. The accumulation of ex230Th in sediments exceeds water column production by a factor of 20. This discrepancy is reconciled by the vertical flux of very large aggregate flocs, which are not efficiently sampled by sediment traps. This process was examined through measurement of ex234Th, ex210Pb, and ex230Th on sediment trap material, large aggregate flocs and surface sediments. Aggregate flocs and sediments were collected during two cruises with the submersible Alvin during major floc deposition events in August and September of 1994. Sediment trap material was analyzed for the summer/fall 1994 and the winter/spring of 1995. sults indicate that a single floc deposition event may exceed the integrated annual sediment trap flux for organic carbon, mass and ex230Th at Station M. Sediment trap flux data for ex210Pb and ex230Th, as well as lithogenic material (as Ti), suggest seasonality in their water column inventories. Seasonality implies residence times on the order of one-year or less for the water column inventories of ex210Pb and ex230Th at Station M. Bloom-induced floc deposition events represent an important mechanism for removal of suspended material, organic carbon and dissolved radionuclides in the waters of the California continental margin.