Uranium geochemistry on the Amazon shelf: Evidence for uranium release from bottom sediments

Abstract

In Amazon-shelf waters, as salinity increases to 36.5 × 10 −3, dissolved uranium activities increase to a maximum of 4.60 dpm 1 −1. This value is much higher than the open-ocean value (2.50 dpm 1 −1), indicating a source of dissolved uranium to shelf waters in addition to that supplied from open-ocean and riverine waters. Uranium activities are much lower for surface sediments in the Amazon-shelf seabed (mean: 0.69 ±.09 dpm g −1) than for suspended sediments in the Amazon River (1.82 dpm g −1). Data suggest that the loss of particulate uranium from riverine sediments (and the consequent input of dissolved uranium to shelf waters) is probably the result of uranium desorption from the ferric-oxyhydroxide coatings on sediment particles, and/or uranium release by mobilization of the ferric oxyhydroxides. The total flux ( i.e., riverine flux plus desorbed-remobilized particulate flux) of dissolved 238U from the Amazon shelf (about 1.2 × 10 15 dpm yr −1) constitutes about 15% of uranium input to the world ocean, commensurate to the Amazon River's contribution to world river-water discharge (approximately 18%). Measurement of only the riverine flux of dissolved 238U underestimates, by a factor of about 5, the flux of dissolved 238U from the Amazon shelf to the open ocean.