Uranium-series isotopes from Eemian lake deposits, Hollerup, Denmark
We report the results of a U-series isotope study on Eemian lake sediments from Hollerup, western Denmark. The purpose is to examine the possibility of dating these, and similar, sediments with the 238U-23"h method. Two sedimentary facies were studied; carbonate-present, and virtually carbonate-free but organic-rich sediments. All carbonate-present sediments (5-95%) have (230ThP38U) values higher than unity and relatively radiogenic (23"W232Th) 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 (230Th/238U) 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 results 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 on sediments that have been more deeply buried andor are less compacted and sealed from percolation of oxidized groundwater.