Predicting the oceanic flux of radionuclides on sinking biogenic debris

The vertical flux of long-lived radionuclides, and of metals in general, in marine systems has been linked to the flux of particulate matter^1-2. Various forms of particulate matter have been shown to be capable of transporting radionuclides in the sea, with greatest interest in recent years focusing on debris of biological origin³. Here we calculate the degree to which radionuclide flux out of the euphotic zone can be attributed to sinking biodetritus deriving ultimately from phytoplankton. We use experimentally determined concentration factors in phytoplankton, dissolved radionuclide concentrations in surface waters and new production estimates for specific ocean regions. Our predictions of radionuclide fluxes are generally comparable with sediment-trap measurements of radionuclide fluxes in these waters, suggesting that the downward flux from open ocean surface waters of particle-reactive radionuclides is governed principally by sinking biogenic debris.