OPEN SYSTEM U-SERIES DATING OF MARINE MOLLUSC SHELLS

Marine terraces and shoreline deposits preserve a rich record of sea-level change in response to past climates and tectonic processes. Since Kaufman et al. (1971) demonstrated U-series dating of mollusc shells to be notoriously unreliable because fossil material was subject to open-system behaviour (uptake or loss) of uranium, accurate and independent dating of events beyond radiocarbon limits has been limited largely to U-series dating of marine deposits that contain corals. Subsequently reported U-concentrations and ²³⁴U/²³⁸U ratio distributions in fossil shells indicate differing rates of post mortem uranium-uptake in different shell parts and species, and account for different uranium concentrations and closed-system U-series ages obtained from different mollusc species at single localities. However, rather than taking into account this open system behaviour, most mollusc dating studies have pursued closed-system U-series dating of those shell parts that appear to accumulate their uranium shortly after burial (early U-uptake). Furthermore, where inconsistent or a range of dates are encountered, these are typically attributed to unresolved open-system uranium behaviour or the reworking of shells from older units. This approach is clearly of limited value because it can neither be used to obtain independent age estimates nor be applied to many sites. We have taken a new open-system approach to U-series dating of mollusc shells, that employs laser ablation ICP-MS to profile uranium and U-series isotope distributions within fossil shells. These high spatial resolution profiles provide the basis for constraining the uranium-uptake history and accurate U-series dating of fossil shells using physically meaningful open-system models. To date, we have focused on the distribution of uranium and U-series isotopes in fossil (MIS 5e and Holocene) and modern shells of the bivalve Anadara trapezia. Fossil shells are observed to accumulate significant uranium (up to 10 ppm) in the outer shell layer and to develop U-concentration and U-series isotope ratio profiles that are consistent with model predictions. Accurate ²³⁴U/²³⁸U and ²³⁰Th/²³⁴U activity ratios can be profiled in situ on samples that contain as little as 0.5 ppm uranium.