230Th dating of coral abraders from stratified deposits at Tangatatau Rockshelter, Mangaia, Cook Islands: Implications for building precise chronologies in Polynesia

Polynesian archaeology has relied primarily on C-14 dating to establish the timing and tempo of human colonization of the eastern Pacific, ensuing cultural development, and human-environmental interactions. Despite advancements in C-14 dating including accelerator mass spectrometry, selection of short-lived plant remains to avoid in-built age, and the use of Bayesian models to refine C-14-based chronologies, large uncertainties introduced during calibration to calendar ages can severely limit the resolution of C-14 dates in this context. Th-230 dating of coral abraders, a common artifact in many Polynesian archaeological sequences, can potentially provide much more precise dates. We report 25 Th-230 dates for Acropora and Porites coral abraders from a well studied, stratified archaeological sequence at Tangatatau Rockshelter, Mangaia (site MAN-44, Cook Islands), develop screening criteria to identify reliable Th-230 dates from buried contexts, and compare the results with a recent Bayesian C-14 chronology for the site. Visual characteristics (gray color, microcrystalline sugary texture) can aid identifying altered corals before analysis but are non-diagnostic of the suitability of samples for dating. Some corals that were fully remineralized to calcite preserved suitable U and Th isotopic compositions and produced ages in agreement with C-14 and pristine aragonitic coral Th-230 dates, suggesting that unlike subaerially exposed corals, buried corals may be susceptible to mineral alteration without perturbing the U-series isotopic systematics, possibly due to burial under reducing conditions. In general, corals with reliable Th-230 dates preserve initial (U-234/U-238) ratios in equilibrium with seawater and have (Th-230/Th-232) ratios > 10. Reliable dates at MAN-44 cluster within layers, preserve stratigraphic order, and agree with the previous C-14 chronology. Precise Th-230 dates (median +/- 7.4 yr, all errors 2 sigma) from coral abraders support early Polynesian arrival on Mangaia (by 1011.6 +/- 5.8 CE) and the arrival of the sweet potato (Ipomoea batatas) no later than 1361-1466 CE, indicating that trans-Pacific voyaging had introduced this South American native plant to the Cook Islands by the mid-15th century. With suitable samples, Th-230 dating of corals from stratified sequences has great potential for developing refined chronologies for Polynesian and other Pacific archaeological sites.