Precise two chronometer dating of Pleistocene travertine: The Th-230/U-234and Ra-226(ex) /Ra-226(0) approach

In order to determine the geochemical evolution of a freshwater limestone c ave system located in central Switzerland (Hell Grottoes at Baar/Zug,) youn g postglacial tufaceous limestone and travertine precipitates were investig ated using the Th-230/U-234 ingrowth system. Additional analyses of further radionuclides within the U-238 decay chain, i.e. Ra-226 and Pb-210, showed that the Th/U chronometer started with insignificant inherited Th-230 over the entire formation period of the travertine setting (i.e. Th-230(0) = 0) . A contribution from detrital impurities with Th-230/U-234 in secular equi librium could be precisely subtracted by applying isochron dating of cogene tic phases and recently formed travertine. The resulting precise 230Th/U-23 4 formation ages were found to be consistent with the geological stratigrap hy and were furthermore used to demonstrate the applicability of the next g eologically important chronometer in the U-238-decay series, based on decay of excess Ra-226 normalized to the initial, i.e.Ra-226(ex)/Ra-226(0). This system is suitable for dating phases younger than 7000 yr when the correct ion of a detritus component increasingly limits the precision of the Th-230 /U-234 chronometer. Analytical solutions of the coupled U-234/Th-230/Ra-22 6 radionuclide system predicted that the Ra-226(ex) /Ra-226(0) chronometer is independent of the actual 230Th activity build up from decay of U-234, i f the systems starts with zero inherited Th-230(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated Ra-2 26 excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4-5 half-lives of Ra-226. This is concluded becau se (i) the Ra-226(ex)/Ra-226(0) ages agreed well with those derived from 23 0Th/234U, GO all data plot within uncertainty on the Ra-226(ex)/Ra-226(0) d ecay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such bou ndary conditions hold, Ra-226(ex)/Ra-226(0) should be applicable to materia ls which are suitable for Th-230/U-234 dating in sedimentology and oceanogr aphy, i.e. travertine, corals, phosphorites, etc., and should strongly supp ort Th-230/U-234 for samples that have been formed a few thousand years ago . (C) 2001 Elsevier Science Ltd. All rights reserved.