Rapid U-series dating of young fossil corals by laser ablation MC-ICPMS

We demonstrate the utility of uranium-series age dating using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS) to ‘range find’ a large suite of fossil corals in order to select those for subsequent detailed and high resolution paleoclimatological analyses. The high abundance of unaltered and long-lived (100–200 years) microatolls on Kiritimati Island, Kiribati, offers the exciting prospect of constructing a millennial, if not continuous, coral paleoclimate record for the central Pacific spanning the period from the mid-Holocene to present. However, with a multitude of collected corals and Kiritimati sample sites, we required an efficient, cost-effective and reliable analytical method to optimize selection of quality coral belonging to a prescribed age. Two modern and 41 fossil microatoll corals from Kiritimati were analysed by the range-finding LA MC-ICPMS technique with a subset also analysed by high-precision solution MC-ICPMS and AMS radiocarbon to assess the reliability and accuracy of the laser ablation technique. Coral ages ranged from 6400 to 900 yr BP and laser ablation age precisions, constrained by the low 230Th count rate, ranged from 9 to 45% at the 2SE level. These ages generally agreed with ages measured by solution U-series and AMS 14C. However, conflicting results for (234U/238U)initial in samples analysed by LA- and solution-MC-ICPMS indicates that the laser ablation method may not be as suitable as previously thought for detecting subtle geochemical alteration. With 40–50 sample analyses per day, the laser ablation method offers a rapid, efficient and cost-effective means to histogram coral ages with multi-centennial accuracy, even at such low 230Th count rates typical for young Holocene samples. In addition to in situ corals, the LA MC-ICPMS technique could be further applied to corals recovered from drill core.