RADIUM ISOTOPES IN COASTAL WATERS ON THE AMAZON SHELF

Radium isotope distributions on the continental shelf adjacent to the Amazon River are a complex function of salinity. The river and seawater endmembers for each radium isotope are low. Data from intermediate salinities plot as a highly scattered region above the conservative mixing line connecting the river and ocean endmembers. Spatial and temporal variability in sediment mixing and water residence times cause the scatter in the radium isotope data. We have used the distributions of Ra-226, Ra-228, and Ra-224 collected during four cruises representing different conditions on the shelf to construct inventories of radium due to nonconservative additions from sediments. Using estimates of water residence times, we calculate fluxes of these radium isotopes from the Amazon system. The amount of fresh river sediment entering the system each year is adequate to support the nonconservative flux of Ra-226 from the shelf sediments; however, the ultimate release of Ra-226 from the sediments requires weeks to months. To support the nonconservative flux of Ra-228 from this system, sediments of the 1-2 m thick surface mixed layer must be resuspended every two years. The resulting flux supports 15% of the Ra-228 entering the surface Atlantic Ocean. Release of Ra-224 from suspended sediments, primarily fluid mud, is more than adequate to support the inventory of Ra-224 on the Amazon shelf. Comparing the AmasSeds low discharge cruise (November 1991) to a low discharge sampling in December 1982 revealed no significant changes in the Ra-226 and Ra-228 distributions.