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

Abstract

Polynesian archaeology has relied primarily on 14C dating to establish the timing and tempo of human colonization of the eastern Pacific, ensuing cultural development, and human-environmental interactions. Despite advancements in 14C dating including accelerator mass spectrometry, selection of short-lived plant remains to avoid in-built age, and the use of Bayesian models to refine 14C-based chronologies, large uncertainties introduced during calibration to calendar ages can severely limit the resolution of 14C dates in this context. 230Th dating of coral abraders, a common artifact in many Polynesian archaeological sequences, can potentially provide much more precise dates. We report 25 230Th 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 230Th dates from buried contexts, and compare the results with a recent Bayesian 14C 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 14C and pristine aragonitic coral 230Th 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 230Th dates preserve initial (234U/238U) ratios in equilibrium with seawater and have (230Th/232Th) ratios > 10. Reliable dates at MAN-44 cluster within layers, preserve stratigraphic order, and agree with the previous 14C chronology. Precise 230Th dates (median ± 7.4 yr, all errors 2σ) 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, 230Th dating of corals from stratified sequences has great potential for developing refined chronologies for Polynesian and other Pacific archaeological sites.