EXPANDING THE APPLICATIONS OF 230TH/U BURIAL DATING OF OSTRICH EGGSHELLS TO BURNT SAMPLES AND BY LASER ABLATION METHODS

Uranium-thorium (or ²³⁰Th/U) burial dating of ostrich eggshell (OES) fragments can provide robust ages for archaeological sites beyond ¹⁴C range, perhaps up to c. 500 ka, but examples thus far have required large (100s mg) samples due to low (10s to 100s ppb) U. We explore the potential of ²³⁰Th/U burial dating in previously under-studied sample types – burnt OES, and OES with high (~ppm) U – via conventional solution and laser ablation ICP-MS methods (LA-ICP-MS). This would expand the utility of ²³⁰Th/U burial dating of OES, as burnt OES may not be suitable for other typically reliable dating approaches (such as ¹⁴C), and while less precise than conventional methods, LA-ICP-MS requires an order of magnitude less material (~10s of mg). We examined burnt and high-U OES from Kisese II (Tanzania), a painted rockshelter hosting Middle and Later Stone Age (MSA/LSA) deposits. U concentrations in the dated portion of OES range from ~300 – 1800 ppb and detrital Th is low ((²³⁰Th/²³²Th)_act ~34 – 2300) to yield precise results (median 1.7%, 2σ); internal reliability criteria inherent to the ²³⁰Th/U burial dating method indicate conventional ²³⁰Th/U burial ages of burnt OES also accurately date the time of deposition. Ages range from ~30 – 45 ka and span the MSA/LSA transition at Kisese II. ²³⁰Th/U burial ages are within error or distinguishably older (5 – 10 ka) than AMS ¹⁴C ages of splits of the same burnt eggshells. Because internal reliability criteria for ²³⁰Th/U burial ages were met, we interpret that the ¹⁴C system may be producing erroneously young ages in burnt eggshells due to factors such as secondary carbon contamination or recrystallization of primary eggshell carbonate occurring after burning. For a high-U OES, ²³⁰Th/U analyses via LA-ICP-MS produce low-precision (15% 2 S.E.) age estimates and overlap with conventional results of the same eggshell. Low-precision ages by LA-ICP-MS may still provide accurate and less costly age estimates requiring less destructive analyses. This may benefit archaeological/paleontological deposits lacking other means of dating, particularly where eggshell material is sparse, recovered only in small fragments (<1 cm²), or too precious (e.g., beads, decorated OES, vessels) for more destructive solution methods.