SCAVENGING OF EX(234)TH, EX(230)TH, AND EX(210)PB BY PARTICULATE MATTER IN THE WATER COLUMN OF THE CALIFORNIA CONTINENTAL-MARGIN

The accumulation of ex(230)Th, organic carbon and total mass at Statio n M (a 4100-m site 220 Km off Central California) is not supported by sediment trap-collected fluxes. The flux of ex(230)Th to sediment trap s only accounts for 10% of the ex(230)Th accumulation in sediments. Th e accumulation of ex(230)Th in sediments exceeds water column producti on by a factor of 20. This discrepancy is reconciled by the vertical f lux of very large aggregate flocs, which are not efficiently sampled b y sediment traps. This process was examined through measurement of ex( 234)Th, ex(210)Pb, and ex(230)Th on sediment trap material, large aggr egate flocs and surface sediments. Aggregate flocs and sediments were collected during two cruises with the submersible Alvin during major f lee deposition events in August and September of 1994. Sediment trap m aterial was analyzed for the summer/fall 1994 and the winter/spring of 1995. Our results indicate that a single flee deposition event may ex ceed the integrated annual sediment trap flux for organic carbon, mass and ex(230)Th at Station M. Sediment trap flux data for ex(210P)b and ex(230)Th, as well as lithogenic material (as Ti), suggest seasonalit y in their water column inventories. Seasonality implies residence tim es on the order of one-year or less for the water column inventories o f ex(210)Pb and ex(230)Th at Station M. Bloom-induced hoc deposition e vents represent an important mechanism for removal of suspended materi al, organic carbon and dissolved radionuclides in the waters of the Ca lifornia continental margin. (C) 1998 Elsevier Science Ltd. All rights reserved.