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230Th/U burial dating of eggshells

Elizabeth Niespolo

Many Pleistocene archaeological and paleontological sites beyond the c. 50-thousand-year 14C limit remain poorly constrained in age or undated entirely. Yet, they host key evidence about terrestrial ecosystems, the biological and cultural evolution of H. sapiens, and human geographic range expansion out of Africa. Many such deposits host giant avian eggshells, as large flightless birds producing giant eggs have resided on five continents in the Pleistocene and eggs served as a food source for foraging humans. Eggshells are furthermore made of calcite and are resistant to diagenesis in deep time, making them potential candidates for uranium-series (230Th/U) geochronology; however, eggshells do not have primary U in them, rendering conventional 230Th/U dating ineffective. Laser ablation measurements that compare modern and ancient avian eggshells indicate that while modern eggshells have negligible U, ancient eggshells host significant concentrations of U and Th that vary with the eggshells’ petrographic structures. I’ll share a novel approach to dating eggshells, first tested with ostrich eggshells, called 230Th/U “burial dating”, which explicitly accounts for U in ostrich eggshell acquired from soil pore water. U and Th concentration profiles from laser ablation data optimize subsampling approaches to correct for post-depositional U uptake, and 232Th/U profiles allow screening to avoid “dirty” samples that may produce imprecise, inaccurate 230Th/U ages. Careful subsampling and the use of a simple model of U uptake provide reliability criteria inherent to the U-Th data to determine accurate 230Th/U burial ages. Resultant 230Th/U burial ages of ostrich eggshells from African archaeological sites preserve stratigraphic order and agree with independent dates. In other avian eggshells, laser ablation data suggest primary petrographic structures control secondary uptake of U, indicating that 230Th/U burial dating may apply to well-preserved eggshells of other avian taxa.

Speaker: Elizabeth Niespolo, Caltech

Tuesday, 09/28/21


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Natural Science Annex

UC Santa Cruz
Room 101
Santa Cruz, CA 95064