URANIUM-SERIES DISEQUILIBRIUM, SEDIMENTATION, DIATOM FRUSTULES, AND PALEOCLIMATE CHANGE IN LAKE-BAIKAL

The large volume of water, approximately one-fifth of the total surfac e fresh water on the planet, contained in Lake Baikal in southeastern Siberia is distinguished by having a relatively high concentration of uranium (ca. 2 nM), and, together with the surface sediments, an unusu ally high U-234/U-238 alpha activity ratio of 1.95. About 80% of the i nput of 234 uranium to the lake, with a U-234/U-238 ratio of 2.0, come s from the Selenga River, Profiles of uranium, as well as the extent o f isotopic disequilibrium in a 9 m sediment core collected on Academic Ridge, generally show high values during interglacial periods corresp onding to high diatom frustule numbers (DiFr) and biogenic silica (BSi ) data that have been reported elsewhere. During glacial periods (low DiFr and BSi), uranium progeny (U-234 and Th-230) were in secular equi librium with low concentrations of their parent U-238. Radionuclide di stributions were interpreted in terms of a quantitative model allowing for adsorption of riverine inputs of uranium onto two classes of sedi menting particles with differing U-238/Th-232 ratios and uranium proge ny in secular equilibrium. If the U-234/U-238 activity ratio of adsorb ed uranium has remained constant, mean sedimentation rates can be inde pendently estimated as 3.6 +/- 0.6 and 3.7 +/- 0.9 cm . kyr(-1) for th e decay of U-234 and in-growth of Th-230, respectively. These rates ar e consistent with a mean rate of 3.76 cm . kyr(-1), calculated by opti mization of the correspondence between adsorbed U-238 and delta(18)O i n dated oceanic sediments, The adsorbed uranium apparently tracks vari able river flow during interglacials and is drastically reduced during periods of glaciation. Evidently, uranium has not been significantly redistributed within Baikal sediments over at least the past 250 kyr a nd is a unique, biologically non-essential, tracer for climate-sensiti ve processes, which provide their own internal geochronometers, potent ially useful for ages up to 1 Myr BP.