MORE ACCURATE U-TH AGES OF OSTRICH EGGSHELLS (OES) WITH IMPROVED SAMPLE PROCESSING

Dating the Middle Stone Age (~250-30 ka) is of great importance to studies of human evolution because this time period witnessed the origin of modern Homo sapiens in Africa, its migration out of the continent and across the rest of the world, advances in stone tool manufacture, and changes in foraging strategies. Techniques such as AMS ¹⁴C can only be applied to the youngest end of this interval. Because many human groups collected ostrich eggs, U-Th dating of ostrich eggshell (OES) has often been used to establish the age of archaeological sites.

Measurement of the ²³⁴U and ²³⁸U and their decay product, ²³⁰Th, will yield an accurate age, if the OES remains a closed system to U and Th after burial. However, multiple lines of evidence indicate an open system, making U-Th dating of ancient OES more complicated. U concentration gradients across ancient OES suggest diffusion of U from soil into OES crystal structures (Sharp et al, 2014). Estimation of U diffusion rates (Sharp et al., 2015) into OES enable calculation of burial age. High ²³²Th concentrations in outer layers, interpreted as detrital infiltration (Sharp et al. 2014), can be avoided by removal of these surfaces.

We present new analyses of modern OES that indicate no significant U or Th present when OES form, supporting the need to characterize post-burial U diffusion rates and processes. OES cross-section images, 3D high resolution X-ray computed tomography and 2D element maps indicate that complex networks of pores in OES facilitate infiltration of U, detrital Th, and secondary calcite into the interior layer. U and Th isotope analyses of the internal layer of ancient OES, indicate that inclusion of pore fill yields younger ages likely due to the incorporation of U-bearing secondary calcite and high ²³²Th concentrations requiring large correction factors. Removal of pore fill from analyzed samples minimizes ²³²Th concentration, lowers U concentration, and results in statistically consistent ages from multiple samples from the same site. Post-burial secondary calcite growth in pores may also complicate OES dating by AMS ¹⁴C. Avoidance of pore fill will result in more accurate OES ages.

Sharp et al., 2014, U-series dating of ostrich eggshells, Goldschmidt Mtg., Sacramento, CA.

Sharp et al., 2015, U-Th Burial Dates on Ostrich Egg Shell, AGU Mtg, San Francisco, CA.