URANIUM-SERIES ISOTOPES AS PROXIES OF LATE PLEISTOCENE CLIMATE AND GEOCHRONOMETERS IN BOTTOM SEDIMENTS OF LAKE BAIKAL

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 ²³⁰Th (τ_1/2 = 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 picnometric clay density. The clay flux during MIS 2, 3, 4 (controlled mostly by mountain glaciers) was four times as high as in the interglacials (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 diatoms could not develop. 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.