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

Abstract

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 137Cs 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, 137Cs 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 137Cs particle transport and diagenetic release of cesium in the deep water. The major 137Cs inventory (>70%) resides in the upper 400m of water, with a mean residence time of 480 ± 90 years. Less than 11% of the total 137Cs inventory was found in the sediments at water depths of 900m in 1987. Unlike 137Cs, more than 80% of the fallout 239+240Pu has been deposited in the Santa Monica Basin sediments. A mean residence time of 13 ± 2 years was measured for 239+240Pu in the basin water. The ubiquitous 239+240Pu 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. The 239+240Pu flux for settling particles correlates remarkably well with mass flux, and it increases with depth in the water column. Plutonium deposition in anoxic sediments also appears to be useful as a time marker for age-dating of sediments and estimating the sediment-accumulation rates. The sedimentation rates determined by plutonium are shown to be comparable to those estimated by excess 210Pb in the cores. Radionuclide fluxes measured from sediment trap particles in 1986–1988 were lower by a factor of 0.6 ± 0.2, compared to the average fluxes determined from the box core samples. Radionuclides, such as 210Pb, 137Cs, and 239+240Pu have been shown to be useful tools in the study of biogeochemical processes in the Santa Monica Basin.