Denis Scholz1,2 and Augusto Mangini2
1Bristol Isotope Group, School of Geographical Sciences, University of Bristol, University Road, BS8 1SS Bristol, United Kingdom (firstname.lastname@example.org)
2Heidelberger Akademie der Wissenschaften, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany
Chronologies of past sea-level change rely, in large part, on U-series dating of fossil reef corals. Many fossil corals, however, show evidence for post-depositional diagenetic alteration, and it is generally accepted that the accuracy of U-series coral ages is more limited due to coral diagenesis than analytical precision. Unaltered corals are usually identified by application of so-called strict reliability criteria. In addition, in recent years three models were developed, which aim to correct the effects of diagenesis and allow the calculation of model ages (Thompson et al., 2003; Villemant and Feuillet, 2003; Scholz et al., 2004).
However, application of 230Th/U-dating to different sub-samples of individual coral specimens revealed that the errors of conventional coral ages as well as those of the method of Thompson et al. (2003) do not account for the true age variability (Scholz and Mangini, 2007). Furthermore, the widely used reliability criteria are not sufficient to identify all diagenetically altered corals. Analysis of different sub-samples from one coral specimen allows (i) estimating the real age variability, (ii) testing if the assumptions of the models are fulfilled, and (iii) investigating the diagenetic processes in more detail.
Here we present U-series data of a large number of fossil corals of the species Acropora palmata collected on Barbados. These corals grew during Marine Isotope Stage 6.5 and show substantial variations in their (234U/238 U) and (230Th/238 U) activity ratios and also their U concentration. Investigation of several sub-samples of individual coral specimens suggests that some sub-samples gained U that was lost by surrounding corals. We term this diagenetic scenario U-redistribution (Scholz et al., 2007). Even small amounts of U-redistribution may have a substantial effect on the 230Th/U-age.
Scholz D., Mangini A., and Felis T. (2004) U-series dating of diagenetically altered fossil reef corals. Earth and Planetary Science Letters 218, 163-178.
Scholz, D., Mangini, A., 2007. How precise are U-series coral ages? Geochimica et Cosmochimica Acta 71, 1935-1948.
Scholz, D., Mangini, A., Meischner, D., 2007. U-redistribution in fossil reef corals from Barbados, West Indies, and sea level reconstruction for MIS 6.5. In: Sirocko, F., Claussen, M., Litt, T., Sanchez-Goni, M. F. (Eds.), The climate of past Interglacials. Elsevier, Amsterdam, pp. 119-140.
Thompson W. G., Spiegelmann M. W., Goldstein S. L., and Speed R. C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210, 365-381.
Villemant B. and Feuillet N. (2003) Dating open systems by the 238U-234U-230Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210, 105-118.