Comparison of Different Dating Methods in a Lake with Annually Laminated Sediments

The studies of natural historical archives such as lake sediments require reliable age-scale. In order to determine the age for sediments accumulated during last 100–150 years the 210Pb method as well as the distribution of several artificial radionuclides (as 137Cs, 241Am etc.) have been used. But often due to diffusion processes these chemical elements can be mobile in sediments (Smith and Comans, 1996) and therefore the reliability of such datings decrease. Chemically stable markers such as spheroidal fly-ash particles which are emitted during the high-temperature combustion of fossil fuels have proven to be useful means for indirect dating of recent sediments (Renberg and Wik, 1984; Rose et al., 1995). In this investigation laminated sediments of Lake Rõuge Tõugjärv (SE Estonia) were studied to establish high-resolution sediment chronology for palaeoenvironment reconstructions of the last centuries and to compare the accuracy of different dating methods. Both CRS and CIC models were used to calculate 210Pb dates. The 137Cs and 241Am had subsurface peaks recording the 1986 Chernobyl accident and the weapons test fallout of 1963. Obtained dates were validated with independent age-scale of varve counting. Also the down-core distribution of fly-ash particle abundance was used and characteristic features in the particle concentration curve were dated according to the known fuel combustion history of the region. Varve analysis, the results of radiometric dating and the distribution of fly-ash particles show chronologies, which are in good agreement. The marker horizons of vertical distribution of fly-ash particles can be accurately dated which allow further application of this method to determine the age of other sediment profiles. When studies of homogeneous lake sediments demonstrated younger ages by 210Pb datings and were therefore corrected by 137Cs and 241Am age-scales, then in annually laminated sediments the chronologies obtained by 210Pb method and by the distribution of artificial radionuclides coincide well. The latter confirms good preservation of paleoinformation in the sediments where bioturbation is missing.