DISTRIBUTION OF NATURAL RADIOACTIVE ELEMENTS IN THE HOLOCENE-PLEISTOCENE DEEP-WATER SEDIMENTS OF LAKE BAIKAL AND CHRONOLOGICAL CONSTRUCTIONS

The Pleistocene-Holocene section of deep-water Baikal sediment clays demonstrates a periodic enrichment with siliceous diatom frustules up to nearly pure diatomites. As the terrigenous clay-silty matter is diluted with amorphous silica, the concentrations of all chemical elements in the sediments, except for organic carbon and uranium, decrease. As in seas and oceans, the acme of diatoms in the Baikal basin is associated with intense supply of dissolved silica and nutrients under the climatic conditions close to those at present, and uranium concentration in the diatom oozes is related to production of humic acids which served as a precipitant. In glacial periods, when erosion in drainage systems stopped, the supply of Si, N, and P became insufficient for diatom growth; therefore, their appearance in the section is a paleoclimatic mark. The diatomic muds contain much higher concentrations of uranium and lower concentrations of thorium than the clays. Therefore, the Th/U ratio is a distinct marker of chemical differentiation in the section. The patterns of distribution of Th/O, biogenic silica, and organic carbon are similar to the oxygen-isotope curves constructed for the last two decades on the basis of a detailed chronological scale. This permitted us to draw boundaries of paleoclimatic epochs, which are in good agreement with chronostratigraphic constructions performed by Arkhipov for the West-Siberian Plain and with West-European glaciations. Direct dating of the sediments by nonequilibrium uranium is hampered by the high content of terrigenous matter in the diatom oozes and by the nonstationary initial ²³⁴U/²³⁸U ratio. It is shown that concordant estimates by ²³⁴U and ²³⁰Th (or ²²⁶Ra) would allow direct radiometric dating of the Baikal sediments.