Systematic investigation of the suitability of two different skeletal materials of Diploria strigosa corals for 230Th/U-dating

Wednesday, 17 December 2014
Johanna Christina Obert1, Denis Scholz2, Thomas Felis3, William Brocas3 and Klaus P Jochum4, (1)Johannes Gutenberg University of Mainz, Mainz, Germany, (2)Institute for Geosciences, Johannes Gutenberg University Mainz, Mainz, Germany, (3)University of Bremen, Bremen, Germany, (4)Max Planck Institute for Chemistry, Mainz, Germany
Fossil reef corals are widely used in palaeoclimate research which requires precise absolute dating. However, post-depositional open-system behaviour often causes apparently inaccurate 230Th/U-ages. We systematically tested the suitability of two different skeletal materials of eight Diploria strigosa corals from Bonaire, Netherlands Antilles, for 230Th/U-dating: (i) The commonly used bulk sample material comprised of all skeletal elements and (ii) the denser theca walls.

The theca walls show less diagenetic alteration. The 230Th/U-ages range from 118.9 ± 2.0 to 130.6 ± 1.2 ka. In contrast, the ages determined on the bulk material range from 112.98 ± 0.80 to 174.0 ± 2.2 ka and, thus, show substantially older and younger ages than expected for the Last Interglacial. Furthermore, for all corals, at least one of the theca sub-samples has an initial (234U/238U) activity ratio similar to the modern seawater value. For the bulk material, only 50 % of the corals show this agreement. Thus, the bulk material is obviously more prone to open-system processes. Therefore, the extraction of the theca walls from the coral skeleton considerably improves the reliability of the 230Th/U-ages.

Comparison of the bulk material with the more pristine theca walls shows that several different open-system processes affected the fossil corals. For 50 % of the corals, no significant differences in ages, activity ratios and concentrations between the two materials are observed. For three of the eight corals, the bulk samples contain considerable amounts of 232Th indicating detrital contamination, probably in combination with uranium loss. These samples also yield the oldest ages predating the beginning of the Last Interglacial by up to 35 ka. For one coral, the bulk material yields younger ages than the theca wall material. This sample also contains elevated 232Th, which again suggests detrital contamination. In addition, the coral was subject to secondary aragonite precipitation introducing additional uranium.