Uranium-series chronology for sediments of Lake Hovsgol, Mongolia, and the 1-Ma records of uranium and thorium isotopes from the HDP-04 drill core

摘要

Uranium and thorium isotopes in an 81-m long sediment core (HDP-04) of Lake Hovsgol, Mongolia, were measured to investigate their downcore distributions and to explore potential linkage to paleoenvir-onmental changes. Three-dimensional isochron techniques using isotope-ratio diagrams in ~(238)U-~(234)U-~(230)Th-~(232)Th system presented by Ludwig and Titterington were applied to age date the lake sediments at the depths of 11.42,14.71 and 14.83 m in the HDP-04 section, the estimated ages of these horizons are 66 ± 8,122 ± 11 and 128 ± 22 ka, respectively. The (238)~U concentration throughout the entire section fluctuated by a factor of 12, ranging from 19.9 to 232.1 mBq/g with anomalously high ~(238)U peak at 23.8 m in depth, while the ~(232)Th concentration varied only by a factor of about two between 24.3 and 54.0 mBq/g. The discrimination of the bulk (238)~U into authigenic and terrigenous ~(238)U fractions was attempted, based on the measured ~(232)Th as a correction index for terrigenous materials. In the upper 24 m corresponding to the last 250 ka, the authigenic ~(238)U was higher in interglacials and lower in glacials. This depth profile of authigenic ~(238)U contents was almost identical pattern with that found in a sediment core (VER98-1-6) from the Academician Ridge, Lake Baikal. Further, this profile can be correlated well with that of photosynthetic pigment contents, one of proxies of paleoproductivity, suggesting that the variation of authigenic ~(238)U contents were associated with the environmental change around Lake Hovsgol.