Obsidian Hydration Dating (OHD) is a technique that can be used over a wide age range; dates have been reported in the age range from 200 to 100,000 years ago. On the island Rapa Nui (Easter Island) obsidian-hydration has been used to date habitation sites (Vargas et al. 2006), archaeologists can use this information to estimate when depopulation occurred on the island. As fig.1 (Hunt & Lipo. 2009) shows (indicated by the arrow), the decline of the population from its maximum, c. 3,000–5,000 (Pollard et al. 2010), occurred between 1751-1800AD.
Fig.1. Habitations dated by obsidian hydration. The arrow indicates the decline in dated habitations, supporting a hypothesis of postcontact demographic decline. Values are plotted from data provided by Vargas et al. 2006. When a chipped surface of obsidian is exposed it slowly acquires a hydration rim by diffusion of water in to the artefact. The thickness of the rim depends on age, the type of obsidian and burial temperature. Limitations of this technique is the imprecision in measuring the width of the rims; in tropical regions the rim forms quickly whereas in colder regions such as the arctic it may take a few millennia for a rim of only 1 µm to form. (Aitken. 1990) Obsidian hydration dating is split into two steps, first the extent of the diffusion of water is measured, this is often done using optical microscopy on prepared thin sections of the artefact. The second step is to calculate the diffusion rate of the water into the glass. Under laboratory conditions, rind formation can be modelled as a diffusion process, this allows for the estimation of EHT-dependent hydration rates. EHT (effective hydration temperature) can be measured using surrounding temperature cells (Lee.1969) or can be extrapolated from temperature histories (Rogers.2007). Obsidian is abundant on Easter Island making obsidian hydration a popular dating technique for sites and artefacts on the island. However OHD is limited to regions where...
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