SEDIMENTATION DYNAMICS IN THE SANTA-MONICA SAN-PEDRO BASIN OFF LOS-ANGELES - RADIOCHEMICAL, SEDIMENT TRAP AND TRANSMISSOMETER STUDIES

A large number of sediment cores and sediment trap samples collected from different parts of the Santa Monica-San Pedro (SM-SP) Basin during 1985-1988 were studied for radionuclides, trace metals and other sedimentary components. The radiochemical data are presented here to give a basin-wide view of the sedimentation dynamics. 210Pb stratigraphy indicated that sedimentation rates were higher and more variable (30-80 mg cm-2y-1) in the more dynamic slope region, but were uniformly low (13.4-18.8 mg cm-2y-1) in the flat, deep basin. From the sediment record, it was suggested that sedimentation rates were decreasing and the area of anoxia had been expanding, at least during the past one to two centuries. Turbidite layers found in the sediment cores suggested higher frequency and more recent occurrence toward the basin margins. 210Pb geochronologies indicated that the recent turbidites might be related to major storms which occurred during the past two decades. Sediment traps deployed in the basin recorded very large short-term spatial and temporal variabilities of mass flux, with unusually high fluxes corresponding to recorded large wave events. Trap-measured near-bottom mass fluxes averaged over all collection periods (622 days) were consistent with 210Pb-based sediment accumulation rates. With few exceptions, trap-measured fluxes decreased offshore but increased with depth at any location, strongly suggesting lateral input of materials. Transmissometer data demonstrated the existence of nepheloid plumes off the eastern slope of the SM-SP Basin. The offshore decrease of sedimentation rate in the eastern part of the SM Basin was consistent with the fact that nepheloid plumes were confined to the basin slopes and that the major transport pathway of suspended particles, as indicated via progressive vector analysis, was alongshore from the SP Basin toward the SM Basin. Concentrations and fluxes of radionuclides measured in the near-bottom trap in the deep basin compared favorably with those registered in bottom sediments. Based on water column, sediment trap and sediment core data, self-consistent flux balances can be constructed for 228Th and 210Pb. Flux balances for 234Th were less well defined. The cyclic pattern of uranium profiles in deep basin sediments appeared to be in phase with the sedimentary record of CaCO3 and the historical record of primary production and anchovy biomass. It is suggested that the removal of uranium from the water might be regulated by longterm regional changes in biological processes and sedimentation environments. © 1990.