Distribution of natural radioactive elements in the Holocene-Pleistocene deep-water sediments of lake Baikal and chronological constructions

In the Pleistocene-Holocene section of the Baikal deep-water sediments shal es are periodically enriched with the siliceous shells of diatomic algae up to nearly pure diatomite. As the terrigenous shale-silty material is dilut ed with amorphous silica, the concentrations of all chemical elements, exce pt for organic carbon and uranium, decrease in the sediments. As in seas an d oceans, the blossom of diatomic algae in the Baikal basin is associated w ith the intensification of a supply of dissolved silica and nutrients under the climatic conditions close to the present-day ones, and the uranium con centration in the diatomic silts resulted from the formation of humic acids , which served as a precipitant. In the glacial periods, when erosion in th e catch basins stopped, the contents of silicon, nitrogen, and phosphorus d ecreased below the level of their concentrations necessary for the vital ac tivity of diatoms. That is why the diatom appearance in the section is a si gn of paleoclimate. The diatomic muds, as compared with shales, are charact erized by increased concentrations of U and reduced contents of Th; therefo re, the Th/U ratio is a contrasting indicator of the chemical differentiati on of the section. By configuration, the distribution curves for the Th/U r atio, biogenic silica, and organic carbon are consistent with the isotope-o xygen curves constructed on the chronological scale developed in detail in the last 20 years. This permitted establishment of the age boundaries of pa leoclimatic epochs in the Baikal section, which are in good agreement with the chronostratigraphic constructions carried out by S. A. Arkhipov for the West-Siberian plain and are comparable with the West-European glaciation a ges. Direct dating of the sediments by non-equilibrium uranium is hampered by the high content of terrigenous admixture in the diatomic silts and by t he nonstationary initial U-234/U-238 ratio. It is shown that direct radiome tric dating of the Baikal sediments will be favored by obtaining agreed age estimates by U-234 and Th-230 (or Ra-226).