Abstract:
238U, 234U, and 230Th concentrations in samples of Baikal bottom sediments from the submerged Akademichesky Ridge were measured by ICP-MS, which is 100 times as fast as the traditional alpha spectrometry. These concentrations, and the respective activities A8, A4, and A0, show abrupt oscillations. The A4/A8 ratio varies more smoothly between 1.0 and 1.8 and correlates with the diatom record. A0 increases with depth in high-A4 intervals because 234U decays to give 230Th (τ1/2 = 245.3 kyr). According to our geochemical model, excess 234U (A4/A8 > 1) is supplied into Lake Baikal in the soluble form. This model, along with new data, was used to determine U-Th ages of six sediment intervals. In the suggested linear age-depth model for the core, the A4/A8 profile is similar to the SPECMAP record for the past 140 kyr. Fluxes of sediment components were estimated from sedimentation rates, water content, BiSi, and clay density. The clay flux during cold MIS 2, 3, and 4 (controlled mostly by mountain glaciers) was four times as high as in MIS 5.5, 5.3, 5.1, and 1. The flux of authigenic uranium adsorbed by sediments from the Baikal water was almost zero during MIS 2 and 4, presumably because of strongly reduced Selenga discharge, the main source of dissolved uranium. The input of nutrients ceased for the same reason, and the development of diatoms was inhibited. Repeated cycles of aridization and the related discontinuity in river nutrient input may have been a major mechanism responsible for the formation of the unique assemblage of endemic organisms in Lake Baikal.
