²³⁰Th/U burial dating of ostrich eggshell

Authors:

Warren D Sharp, Christian A Tryon, Elizabeth M Niespolo, Nick D Fylstra, Alka Tripathy-Lang, J Tyler Faith

Abstract:

Obtaining precise and accurate dates for late Quaternary terrestrial sequences containing Middle Stone Age or Middle Paleolithic archaeological materials (ca. 30–300 ka) can be challenging. Many such sequences lie beyond the limit of radiocarbon dating (∼50 ka) and lack volcanic ashes suitable for ⁴⁰Ar/³⁹Ar dating. We report a dating approach with about ten times the range of radiocarbon dating (∼500 ka) based on a novel protocol for ²³⁰Th/U dating of ostrich eggshell, a common component of African, Near Eastern and Asian archaeological sequences. Uranium in ostrich eggshell, typically at levels of tens to hundreds of ppb, is acquired after burial. However, accurate burial ages can be determined from measured ²³⁰Th/U ages when corrected for uranium uptake via diffusion. Using ostrich eggshells from a Pleistocene-Holocene east 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 simple model for diffusive uptake of uranium. We assess our method by comparing the ²³⁰Th/U burial ages (median uncertainty 2.3%, 1 SD) with radiocarbon dates on carbonate from the same eggshells. We find good agreement in seven out of nine eggshells, confirming the accuracy of their ²³⁰Th/U model ages. Eggshells that fail to yield reliable ²³⁰Th/U ages may be recognized by anomalous patterns of apparent ²³⁰Th/U ages, providing a reliability criterion innate to the ²³⁰Th/U data. Ostrich eggshells are resistant to diagenesis simplifying ²³⁰Th/U dating compared to bones and teeth. Eggshells of ostrich, emu, rhea and other living and extinct ratites found on six continents share similar structure and composition, suggesting our new approach to ²³⁰Th/U dating of eggshell may be applicable in diverse late Quaternary terrestrial settings.

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