Dn. Edgington et al., URANIUM-SERIES DISEQUILIBRIUM, SEDIMENTATION, DIATOM FRUSTULES, AND PALEOCLIMATE CHANGE IN LAKE-BAIKAL, Earth and planetary science letters, 142(1-2), 1996, pp. 29-42
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.