Prehistoric settlement chronology on Rapa Nui, Chile: obsidian hydration dating using infrared photoacoustic spectroscopy

• Evaluation of a predictive model for obsidian hydration rates.
• Obsidian dates and AMS dates are in good agreement for many contexts.
• The use of infrared spectroscopy in the obsidian hydration dating process.

The prehistoric Polynesian inhabitants of Rapa Nui (Easter Island) utilized obsidian for nearly 700 years in many activities connected with daily life. The near ubiquitous occurrence of the natural glass in both domestic residences and religious structures makes the application of obsidian hydration dating highly suitable for the investigation of cultural change. We have applied previously developed calibrations that estimate hydration rates for obsidian based upon the structural water content of the glass as determined by infrared spectroscopy. The archaeological ages estimated by this method were compared with accelerator mass spectrometry (AMS) dates using short-lived woody species endemic to the island. The convergence between the two dating methods is strong and we suggest that obsidian hydration dating may be used on Rapa Nui to reliably date contexts where suitable material for AMS dating may not be available.