Uranium uptake history, open-system behaviour and uranium-series ages of fossil Tridacna gigas from Huon Peninsula, Papua New Guinea

Molluscs incorporate negligible uranium into their skeleton while they are living, with any uranium uptake occurring post-mortem. As such, closed-system U-series dating of molluscs is unlikely to provide reliable age constraints for marine deposits. Even the application of open-system U-series modelling is challenging, because uranium uptake and loss histories can affect time-integrated uranium distributions and are difficult to constrain. We investigate the chemical and isotopic distribution of uranium in fossil Tridacna gigas (giant clams) from Marine Isotope Stage (MIS) 5e (128–116 ka) and MIS 11 (424–374 ka) reefs at Huon Peninsula in Papua New Guinea. The large size of the clams enables detailed chemical and isotopic mapping of uranium using LA-ICPMS and LA-MC-ICPMS techniques. Within each fossil Tridacna specimen, marked differences in uranium concentrations are observed across the three Tridacna growth zones (outer, inner, hinge), with the outer and hinge zones being relatively enriched. In MIS 5e and MIS 11 Tridacna, the outer and hinge zones contain approximately 1 ppm and 5 ppm uranium respectively. In addition to uptake of uranium, loss of uranium appears prevalent, especially in the MIS 11 specimens. The effect of uranium loss is to elevate measured [230Th/238U] values with little effect on [234U/238U] values. Closed-system age estimates are on average 50% too young for the MIS 5e Tridacna, and 25% too young for the MIS 11 Tridacna. A complex, multi-stage uptake and loss history is interpreted for the fossil Tridacna and we demonstrate that they cannot provide independent, reliable geochronological controls on the timing of past reef growth at Huon Peninsula.This research was undertaken with the support of an Australian Postgraduate Award to BA, and by Australian Research Council Funding (LE0560956 to MM, SE and RG; DP0559159 to SE)

High-precision U-series measurements of more than 500,000 year old fossil corals

Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ∼ 600 ka samples with an uncertainty of better than ± 15 ka (2σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [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(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman 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(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of 230Th and 234U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model

The timing of sea-level high-stands during Marine Isotope Stages 7.5 and 9: Constraints from the uranium-series dating of fossil corals from Henderson Island

Direct dating of fossil coral reefs using the U-series chronometer provides an important independent test of the Milankovitch orbital forcing theory of climate change. However, well-dated fossil corals pre-dating the last interglacial period (\u3e130 thousand years ago; ka) are scarce due to, (1) a lack of sampling localities, (2) insufficient analytical precision in U-series dating methods, and (3) diagenesis which acts to violate the assumption of closed-system U-series isotopic decay in fossil corals. Here we present 50 new high-precision U-series age determinations for fossil corals from Henderson Island, an emergent coral atoll in the central South Pacific. U-series age determinations associated with the Marine Isotope Stage (MIS) 9 interglacial and MIS 7.5 interstadial periods are reported. The fossil corals show relatively little open-system U-series behaviour in comparison to other localities with fossil coral reefs formed prior to the last glacial cycle, however, open-system U-series behaviour is still evident in most of the dated corals. In particular, percent-level shifts in the [230Th/238U]act composition are observed, leading to conventional U-series ages that are significantly younger or older than the true sample age. This open-system U-series behaviour is not accounted for by any of the open-system U-series models, indicating that new models should be derived. The new U-series ages reported here support and extend earlier findings reported in Stirling et al. (2001), providing evidence of prolific coral reef development on Henderson Island at 320 ka, most likely correlated with MIS 9.3, and subsequent reef development at 307 ka during MIS 9.1, while relative sea-level was potentially 20 m lower than during MIS 9.3. The U-series ages for additional well-preserved fossil corals are suggestive of minor reef development on Henderson Island during MIS 7.5 (245–230 ka) at 240.3 ± 0.8 and 234.7 ± 1.3 ka. All U-series observations are consistent with the Milankovitch theory of climate change, in terms of the timing of onset and termination of the dated interglacial and interstadial periods. The best preserved samples also suggest that the oceanic 234U/238U during MIS 9 and MIS 7.5 was within five permil of the modern open ocean composition. [Retrieved from publisher\u27s website: http://www.sciencedirect.com/science/article/pii/S001670371000147X