230Th/U Burial Dating of Ostrich Eggshell: Initial Tests of a Novel Pleistocene Geochronometer

Precise, accurate ages for late Quaternary terrestrial sequences containing hominin fossils and archaeological materials can be challenging to obtain but are essential for understanding human biological and behavioral evolution. We have developed a novel protocol for ²³⁰Th/U dating of ostrich eggshell, a common component of Pleistocene archaeological sequences in Africa, the Near East and Asia. ²³⁰Th/U dating has about ten times the range of radiocarbon dating (ca. 500 rather than 50 ka) and ancient ostrich eggshell is geochemically suitable for the ²³⁰Th/U technique. Uranium in ostrich eggshell, typically at levels of tens to hundreds of ppb, is acquired after burial. However, burial ages can be determined from measured (apparent) ages if uranium uptake occurs via single-stage diffusion. Using ostrich eggshells from a Pleistocene-Holocene eastern African site, we have: (1) characterized the spatial distribution of uranium and common thorium (²³²Th) concentrations in ostrich eggshell by laser ablation ICP-MS, (2) determined apparent ²³⁰Th/U ages on outer and inner layers of eggshells by solution ICP-MS analyses of selectively abraded eggshells, and (3) calculated ²³⁰Th/U burial ages using a model for diffusive uptake of uranium. In a blind test, we applied our new approach to eggshells with robust ¹⁴C ages (all ≤ 50 ka), allowing us to assess the ²³⁰Th/U burial ages (median uncertainty ~2%, 1 sigma) by direct comparison with precise independent ages. Seven of nine eggshells yield concordant ²³⁰Th/U and ¹⁴C ages, confirming the accuracy of their ²³⁰Th/U model ages and showing that onset of U uptake was rapid. Eggshells that failed to yield reliable ²³⁰Th/U ages are characterized by anomalous patterns of apparent ²³⁰Th/U ages, providing a reliability criterion innate to the ²³⁰Th/U data. Ostrich eggshells tend to be stable in the soils of semi-arid to arid regions in which they are commonly found, simplifying ²³⁰Th/U dating relative to more commonly targeted, highly reactive biominerals such as bones and teeth. Eggshells of living and extinct ratites found on four continents, including those of the emu, Genyornis newtoni, rhea and others are structurally and compositionally similar to ostrich eggshell, suggesting that ²³⁰Th/U dating of eggshell may be broadly applicable in late Quaternary terrestrial settings.