Fallout Pu and Am in Baltic Sea ferromanganese concretions: Implications regarding their interactions in sediments

Three morphological types of ferromanganese concretions (discoidal, spheroidal, flat) collected from various basins in the Baltic Sea were analyzed for their contents of the transuranic radionuclides 238Pu, 239,240Pu, and 241Am. Our data show that the levels of these radionuclides in most of the concretions are comparable to those in the surrounding sediments, which are significantly higher than concentrations found in the overlying waters. The measurements also reveal that the levels of radionuclides in the concentrations are related to the oxidation-reduction (redox) conditions of the surrounding sediments; for example, more Pu and Am occur in nodule samples collected from the relatively less oxidizing environment of the Bothnian Sea compared to Bothnian Bay nodules. Thus, the diagenetic remobilization of transuranic radionuclides from the sediments into the pore waters appear to occur more readily under reducing conditions, accounting for increased amounts of these nuclides in concentrations formed under these conditions. (orig.)