Uranium and thorium isotope distribution in an offshore bottom sediment core of the Selenga Delta, Lake Baikal, Siberia

Distribution of uranium and thorium isotopes in a short sediment core obtained offshore of the Selenga Delta in Lake Baikal, Siberia, was investigated to establish their sedimentary behaviors and to look for a linkage to paleoenvironmental changes. The sediments were composed of dominantly fine detrital materials (70-85%) and a relatively high sedimentation rate (ca. 0.03 cm y(-1)). The depth profile of U-238 content in bulk sediment samples showed a large variation of 70-123 Bq kg(-1), while Th-232 profile showed a relatively narrow range from 36 to 56 Bq kg(-1). The observed U-234/U-238 activity ratios revealed a marked disequilibrium ranging from 1.53 to 1.84 with a mean value of 1.71 +/- 0.07, demonstrating the presence of 50-80% authigenic U-238 in the bulk sediments. The distribution of this authigenic U-238 did not display any clear correlation with variations in sediment composition (organic, carbonate, Bio-SiO2 and mineral contents) including grain size median. The profile of terrigenous U-238 showed a relatively similar pattern to that of Th-232. Results of sequential leaching indicate that U-238 in Fe-Mn oxyhydroxides fractions were responsible for the distribution of authigenic U-238 rather than in Bio-SiO2 fraction. The distribution of authigenic U-238 in the bottom sediments may be explained by the fluctuation of U adsorption capacity on particles including organic matter and Fe-Mn oxyhydroxides before they entered the lake. This study highlights the potential use of authigenic and terrigenous U (Th) signatures in sediments to trace the behavior of U (Th) and to reconstruct environmental (e.g., hydrological) changes in the lake catchment area.