U-TH BURIAL DATING OF OSTRICH EGGSHELLS: A NOVEL APPROACH TO DATING AFRICAN ARCHAEOLOGICAL SEQUENCES BEYOND THE 14C LIMIT

Established chronometers do not fully meet the needs of researchers investigating Middle Stone Age (MSA, ~300 – 30 ka) sites that co-occur with the first fossils of H. sapiens, provide evidence of accelerated human behavioral innovation, and coincide in time with geographic range expansion of H. sapiens outside Africa. MSA sequences commonly contain ostrich eggshells (OES) which consist of ~2-mm thick, low-Mg calcite with 1-3% organics and are amenable to ¹⁴C dating. OES are also geochemically suitable for U-Th dating, though previous attempts neglected to account for secondary U uptake from soil pore fluids upon burial. Our novel approach to U-Th dating of OES explicitly recognizes U as secondary and has produced U-Th dates concordant with ¹⁴C dates on splits of ~10 – 50 ka OES from Lukenya Hill, an East African rock shelter site. LA-ICP-MS profiling and mapping of OES from 10 additional archaeological sites reveal patterns of [²³⁸U] and [²³²Th] that guide selection of optimal OES and sampling positions for dating. U entering at the outer surface produces steep smooth gradients in [²³⁸U] through the “palisade” layer consisting of radially oriented crystals interior to OES surfaces. Minimum values of [²³⁸U] correspond to a change in crystal habit and decrease in micropore density, suggesting that U transport is mediated by shell structure. [²³²Th] is lowest away from OES surfaces and highest at visibly detritus-rich surfaces and pores. Erratic [²³⁸U] renders some OES unsuitable. Peak [U] in OES (~10¹ – 10³ ppb) varies with depositional context but not with age, suggesting U uptake is self-limiting and controlled by soil composition. For geochronology, we prepare two or more surface-parallel tablets from the palisade layer, determine their U-Th ages via solution MC-ICP-MS, and apply a diffusive uptake model to determine the time when the OES was first exposed to U in soil pore water. We term such dates “U-Th burial ages”. We analyze OES from deeply stratified archaeological deposits at and beyond the ¹⁴C (~45 ka) dating limit to determine if U-Th burial ages are reproducible within layers and preserve stratigraphic order. We examine OES >500 ka to evaluate the older limit of U-Th burial dating and if ²³⁰Th/²³⁸U and ²³⁴U/²³⁸U ratios converge towards secular equilibrium as expected if the U-Th system has remained closed.