230Th/U burial dating of ostrich eggshell, a new chronometer for Pleistocene terrestrial strata

Figure: Paleoanthropological sites hosting ostrich eggshells tested for 230Th/U burial dating. Figure from Niespolo, E.M., Sharp, W.D., Fylstra, N., Avery, G., Blegen, N., Faith, J.T., Henshilwood, C.S., Klein, R., Van Niekerk, K., Weisz, D.G., Tryon, C.

Many Middle-Late Pleistocene paleoanthropological sites beyond the ~50 ka 14C limit remain poorly constrained in age or undated. Yet, they host key evidence about human evolution, including the earliest occurrences of H. sapiens, the development of modern human behaviors (defined by the Middle Stone Age in Africa and the Middle Palaeolithic in the Near East >300 – 30 ka), and the expansion of H. sapiens out of Africa. Many such sites host ostrich eggshell (OES), made of ~2-mm thick, 97-99% low-Mg calcite (with 1-3% organics). 230Th/U burial dating of OES is a novel approach that explicitly recognizes that U in OES is acquired from soil pore water, and OES yield accurate 230Th/U ages when corrected for prolonged U uptake. LA-ICP-MS profiles of U and 232Th in ancient OES from > 10 Sub-Saharan archaeological sites ~10 to >500 ka old are similar, suggesting U uptake is self-limiting and controlled by primary eggshell structures, and that [U] is controlled by soil composition. 232Th/U profiles allow screening to avoid detritus-rich samples with high common Th. Thin sections, SEM imaging, and erfc-1(U/U0) v. distance plots indicate primary OES microstructures control U diffusivity. OES 230Th/U burial ages preserve stratigraphic order and agree with independent dates in blind tests. The potential of 230Th/U burial dating will be presented from Ysterfontein 1, a Middle Stone Age shell midden in South Africa, which indicates it is among the oldest, most precisely dated shell middens known. Preliminary data suggest primary structures control secondary U element uptake in other giant avian eggshells from Australia, as well as in a Jurassic theropod eggshell, corroborating that in well preserved eggshells, diagenetic processes may be restricted to microstructural modification, even in deep time.