Amazon and Mississippi river concentrations of uranium, thorium, and radium isotopes

Concentrations of uranium and thorium series isotopes were measured in two large samples of river water and associated sediment. From these data an oceanic residence time for uranium of 4 m.y. is calculated. The concentration of ²³²Th in these rivers is an order of magnitude greater than in surface Atlantic waters. The amounts of ²³⁰Th and ²²⁶Ra being supplied to the ocean directly from rivers are less than 4% of that coming from other sources.