U-Th-Pa dating systematics of ';dirty carbonates' from the Dead Sea basin

The application of U-series dating for lacustrine carbonates depends on good knowledge of the composition and fraction of non-authigenic U and Th derived from admixed detritus and from water column-absorbed Th. In cases where these components are significant (i.e., ';dirty carbonates'), such as lacustrine carbonates and spring deposits, the possibility for open system behavior is generally overlooked due to its more negligible effect on the resulting ages. We present U-Th-Pa ages and supporting major and trace element concentrations of authigenic aragonite from a 450 meter-thick sediment core covering the last 200 kyrs from the center of the Dead Sea, which was recovered during the Dead Sea Deep Drilling Project (DSDDP). Previous efforts to determine U-Th ages of lacustrine deposits in the Dead Sea basin focused on estimation or direct measurement of the composition of detritus and absorbed Th, which were used to correct the measured values. Our results show that the composition of the detritus and hydrogenous end members may vary significantly between sites within the same basin, challenging the use of a common value representing the contaminating phases. We thus advocate the independent determination of the end member values for each set of coeval samples and apply these values to the calculation of isochron and single sample ages. Moreover, while the measured values (i.e. before correction) produce ages that are inherently erroneous, the combined 234U-230Th and 235U-231Pa schemes can be used to construct concordia plots that serve as an immediate filter for the validity of data sets and in some cases can be used to directly extract the true age of a given sample set, thus overcoming the necessity to develop complex correction schemes. The results of the combined 234U-230Th and 235U-231Pa dating provide a new powerful filter for the evaluation of the true ages of carbonates in the Dead Sea and will be discussed in the context of improving the quality and methodology of U-series dating of dirty carbonates.