Uranium-series isotopes from Eemian lake deposits, Hollerup, Denmark

We report the results of a U-series isotope Study oil Eemian lake sediments from Hollerup, western Denmark. The purpose is to examine the possibility of dating these. and similar, sediments With the U-238-Th-230 method. Two sedimentary facies were studied carbon ate - present, and virtually carbonate-free but organic-rich sediments. All carbonate-present sediments (5-95%) have (Th-230/U-238) Values higher than unity and relatively radiogenic (Th-230/Th-232) values between 2 and 6.5. indicating postdeposition removal of U, probably by percolation of ground water. Carbonate-free sediments, but with high organic content (40-60%) had (Th-230/U-238) values lower than 1 and 'model ages' between 89 and 199 ky. The large spread in ages and variation in U content for these sediment samples that were deposited over a much shorter time interval, indicate a complex postdepositional migration pattern of U, probably involving several episodes of leaching and absorption of U from ground water. Our result suggest that lake sediments. such as those found in Hollerup. are Vulnerable to mobilization of U and its decay products and care should be taken when interpreting U-series disequilibrium data from such sediments. Future studies will concentrate oil sediments that have been more deeply buried and/or are less compacted and sealed from percolation of oxidized groundwater.