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Russian Geology and Geophysics

2004 year, number 5

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

E.P. Chebykin, D.N. Edgington*, E.L. Goldberg, M.A. Phedorin, N.S. Kulikova, T.O. Zheleznyakova, S.S. Vorob'yova, O.M. Khlystov, O.V. Levina, G.A. Ziborova, and M.A. Grachev
Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia
* Great Lakes WATER Institute, University of Wisconsin, Milwaukee, USA
Keywords: Climate proxy, uranium-series isotopes, activity ratios, ICP-MS, diatoms, Lake Baikal
Pages: 497-513

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 230Th ( = 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.