Improved ¹⁴C and novel ²³⁰Th/U burial dating of Australian megafaunal avian eggshells: implications for the extinction of Genyornis and early human arrival to Sahul

Late Pleistocene human arrival to Australia may have coincided with and influenced the extinction of Australian megafauna, including the large flightless bird Genyornis newtoni . However, current geochronological tools cannot resolve the timing, tempo, and order of events, to discern whether human and/or natural environmental perturbations contributed to extinctions in Australia. Eggshells from Genyornis are widely preserved in sediments, including variably burnt eggshells indicative of human predation. Diverse preparation methods for eggshell carbonate ¹⁴C measurements indicate that, beyond ∼45 thousand years before present (ka BP), physical and chemical preparation methods can impact the accuracy of ¹⁴C ages. Uranium-thorium (²³⁰Th/U) “burial dating” of both extinct Genyornis and extant Dromaius novaehollandiae (emu) eggshell can reliably date Genyornis and Dromaius eggshells. Precise (±1-4%, 2σ) ²³⁰Th/U burial ages of ∼25-50 ka BP of well-preserved Dromaius eggshells agree with ¹⁴C ages of the same sample. These results are consistent with early, rapid uptake of U upon burial followed by closed evolution of the ²³⁰Th/U system in eggshells. Thin section petrography also informs the preservation and suitability of samples for ²³⁰Th/U burial dating. The ¹⁴C and ²³⁰Th/U burial ages of the youngest, well-preserved Genyornis eggshells near the ¹⁴C limit also agree and corroborate the youngest Genyornis eggshells are ∼46-44 ka BP. Proximal archaeological sites are contemporaneous or pre-date the youngest Genyornis within regions. Because eggshells are found in both paleontological and archaeological deposits, these dating tools provide a promising new way to date both the extinction of Genyornis and the timing of human arrival to Australia.