REFINING CHRONOMETERS AND THE ROLE OF HUMANS IN THE MEGAFAUNAL EXTINCTIONS OF AUSTRALIA (Invited Presentation)

The timing of human arrival to Australia and the role of humans in the extinction of many megafauna are still debated despite decades of study, in part because these events lie near the effective limit of ¹⁴C dating. The large flightless bird Genyornis newtoni overlapped with early human presence in Australia ~65-45 thousand years ago (ka) before going extinct, evidenced by pattern-burnt eggshells and unmodified fragments recovered from stratified anthropogenic deposits. An abundance of eggshell across the continent is preserved in sediments predating their extinction, and due to the calcite composition of avian eggshells, Genyornis eggshell material is potentially ideal for ¹⁴C and uranium-series (²³⁰Th/U) geochronology, providing a means to assess the timing of their extinction. We evaluate standard preparation methods for eggshell carbonate ¹⁴C measurements and find that, beyond ~45 ka, preparation methods impact the precision and accuracy of ¹⁴C ages. We further validate that ²³⁰Th/U burial dating of megafaunal avian eggshells is a promising approach to date paleontological and paleoanthropological deposits within and beyond the limit of ¹⁴C dating. To extend and test the ²³⁰Th/U burial dating technique on both Genyornis eggshells and those of the extant emu, we have measured: 1) U and common Th (²³²Th) profiles across modern and ancient eggshells via laser ablation ICP-MS (LA-ICP-MS), and 2) solution ICP-MS ²³⁰Th/U ages of two sub-samples within each eggshell to produce ²³⁰Th/U burial ages (cf. [1]). Precise (±1-4%, 2s) ²³⁰Th/U burial ages of ~25-50 ka emu eggshells are in close agreement with ¹⁴C ages of the same sample. ¹⁴C and ²³⁰Th/U burial ages of the youngest, well-preserved Genyornis eggshells near the ¹⁴C limit also agree. Results are consistent with early, rapid uptake of U upon burial followed by closed evolution of the U-Th system in these eggshells. Thin section petrography, LA-ICP-MS results, and ²³⁰Th/U isotope data inform on the preservation and suitability of samples for ²³⁰Th/U burial dating, providing a promising new way to date Pleistocene fossiliferous deposits hosting eggshells, the last appearance of Genyornis, and the timing of human arrival in Australia.

References:

[1] Sharp et al. 2019, QSR 219, 263-276, DOI:10.1016/j.quascirev.2019.06.037