Uranium and Thorium Isotope Distribution in an Offshore Bottom Sediment Core of the Selenga Delta, Lake Baikal, Siberia |
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| Authors: | Aya Sakaguchi Masayoshi Yamamoto Keiichi Sasaki Kenji Kashiwaya |
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| Institution: | (1) Low Level Radioactivity Laboratory, K-INET, Kanazawa University, Wake, Nomi Ishikawa, 923-1224, Japan;(2) Kanazawa Gakuin University, Kanazawa Ishikawa, 920-1392, Japan;(3) Department of Earth and Environmental Science, K-INET, Kanazawa University, Kanazawa Ishikawa, 920-1192, Japan |
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| Abstract: | 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 238U content in bulk sediment samples showed a large variation of 70–123 Bq kg−1, while 232Th profile showed a relatively narrow range from 36 to 56 Bq kg−1. The observed 234U/238U 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 238U in the bulk sediments. The distribution of this authigenic 238U 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 238U showed a relatively similar pattern to that of 232Th. Results of sequential leaching indicate that 238U in Fe–Mn oxyhydroxides fractions were responsible for the distribution of authigenic 238U rather than in Bio-SiO2 fraction. The distribution of authigenic 238U 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. |
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| Keywords: | Adsorption Environmental changes Lake Baikal Sediment composition Sedimentary behavior Sediments Thorium Uranium |
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