Paper No. 256-13
Presentation Time: 4:45 PM
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.