Radionuclide concentrations, fluxes, and residence times at Santa Monica and San Pedro Basins

Our participation in the California Basin Study (CaBS) in 1986-1988 has produced a radionuclide data base that allows us to trace the particle and water movement in the Santa Monica and the San Pedro Basins. These data enable us to calculate the radionuclide inventories in the three compartments of the basin, the water column, the settling particles, and the sediments. We have also determined the fluxes of several radionuclides associated with the sinking particles, the residence time in the water column, and the rate of deposition in the sediments. The biogeochemical environment of the Santa Monica Basin is uniquely suited to the application of tracer techniques to study the sedimentation processes. Subtle, but measurable changes in the distribution of fallout of super(137)Cs from 1977 to 1988 were noted in the water column. Compared to the decay corrected measurement in 1973-1977 at the Geosecs and Farallon Islands stations off the coast of California, super(137)Cs inventory in the water column in Santa Monica Basin has not changed significantly for the past decade. Increases in the deep water concentration profiles of super(137)Cs partially resulted from particle transport and diagenetic release of cesium in the deep water. The major super(137)Cs inventory (> 70%) resides in the upper 400m of water, with a mean residence time of 480 plus or minus 90 years. Less than 11% of the total super(137)Cs inventory was found in the sediments at water depths of 900m in 1987. Unlike super(137)Cs, more than 80% of the fallout super(239+240)Pu has been deposited in the Santa Monica Basin sediments. A mean residence time of 13 plus or minus 12 years was measured for super(239+240)Pu in the basin water. The ubiquitous super(239+240)Pu subsurface concentration maximum observed in the Pacific oceanic waters at about 350m depth is also evident in the basin water. We have no definitive explanation for the persistence of the subsurface maximum except that it appears to be correlated with the density gradient at 26.5 to 26.9 sigma-t.