Uranium-series dating of marly sediments: application to Jaroszów fossil lake (SW Poland)

A U-series (230Th/U) study was carried out on the carbonate sediments from the fossil lake of Jaroszów (SW Poland). Six marly samples of unit 5 have been dated with isochron method. Two procedures for obtaining sub-samples have been investigated, the Successive Leaching (SL) and the Full Leaching (FL). It appears that with SL method thorium isotopes are subjected to isotope fractionation. On the other hand, FL method presents no significant isotopic fractionation and gives a wide range of values for isotopic ratios, which is very convenient for plotting isochrons. A simple graphical representation of sample sub-sets was created to detect isotopic fractionation. Even though FL procedure is efficient, results are not in agreement with stratigraphy. Three possible explanations can be given for these stratigraphic inconsistencies: (1) detrital contamination, (2) open system, and (3) mixing of carbonates of different ages.