Vm. Gavshin et al., Distribution of natural radioactive elements in the Holocene-Pleistocene deep-water sediments of lake Baikal and chronological constructions, GEOL GEOFIZ, 39(8), 1998, pp. 1045-1058
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).