Testing Australian megafaunal avian eggshells for 230Th/U burial dating

Abstract:

While there is clear overlap between the timing of human arrival in Australia, the onset of climatic aridification, and the extinction of many megafauna ~65 – 45 ka, current geochronological techniques (e.g., ¹⁴C, OSL, AAR) are not precise and/or accurate enough to distentangle them. Eggshells of large flightless birds are suitable for ²³⁰Th/U dating (capable of producing closed-system ages with s ~ 0.1%), as they are made of ~mm-thick, 97-99% low-Mg calcite with 1-3 wt.% organics, and they are found both in natural fossiliferous deposits and the paleoanthropological record. ²³⁰Th/U burial dating of ostrich eggshell yields reproducible ages in blind tests with ¹⁴C ages and accounts for secondary U uptake via diffusion [1], as early, rapid U uptake is likely mediated by eggshell structures [2]. Similar structures are apparent in other avian eggshells, like the Australian extant emu (Dromaius novellaehollandiae) and the extinct Genyornis newtonii, such that they may also be suitable for ²³⁰Th/U burial dating.

To test this, we characterized [U] and Th/U in modern emu and ancient Australian avian eggshells via femtosecond laser ablation ICP-MS. Profiles of modern emu eggshell show near-zero [U] in continuous calcite structures (e.g., prismatic zone internal to eggshell surfaces) and non-negligible primary U and Th at transitional crystallographic boundaries. In ancient eggshells, secondary U is highest in the external zone (~10² – 10³ ppb) and consistently elevated throughout the prismatic zone (~10 – 10² ppb), similar to the U patterns observed in the prismatic zone of ostrich eggshells. Th/U ratios are high at crystal boundaries and in macroscopic pores hosting secondary contaminants (≳ 0.1), and they are lowest in the prismatic zone (≲ 0.01). This suggests that with fast-diffusive U and low Th/U, the prismatic zone is analogous to the palisade crystals of ostrich eggshell: i.e., the optimal sampling locus for ²³⁰Th/U burial dating. Ongoing efforts for strategic sampling may provide a way to apply ²³⁰Th/U burial dating to Australian avian eggshells.

References: [1] Sharp et al. (2019) Quaternary Science Reviews v. 219, 263 – 276. [2] Niespolo et al. (2019) AGU Fall Meeting, Abstract #PP34B-06