The thickness of the hydration rind can be identified in petrographic thin sections cut normal to the surface and observed under a microscope.

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A distinct diffusion front can be recognized by an abrupt change in refractive index at the inner edge of the hydration rind. These fronts or rinds of hydration are more dense than the unhydrated inside, and the unhydrated zone has different optical properties. Friedman and Smith reasoned that the degree of hydration observed on an obsidian artifact could tell archaeologists how long it had been since that surface was created by a flintknapper. Hydration begins after any event which exposes a fresh surface e. Providing one can identify which process created the exposed surface or crack in the rock, it is possible to date when that process took place.

Hydration rind thickness is a non-linear function of time. The hydration rate is primarily a function of temperature, though chemical composition of the sample is also an important factor. For this reason, it is necessary to calibrate the samples within a limited geographical area against a sample of known age and similar chemical composition.

Hydration forms at different rates on different obsidians. Under the same conditions of temperature and humidity some glasses will hydrate rapidly while others are very slow. What controls the process? There is a very strong relationship between the rate of hydration and the quantity of intrinsic water found within the glass.

This is the water trapped in the obsidian at the time the lava hardens into a natural glass. The presence of intrinsic water opens up the glass structure and allows the atmospheric water to diffuse inward from the surface to form the hydration rim. The more intrinsic water present within an obsidian artifact, the faster it will hydrate and the faster the hydration rim will form. Three steps are required to determine a calendar date from an obsidian artifact.

These are the determination of: A hydration rate is determined for every artifact through a measurement of the amount of intrinsic water that is present. This is done by either a direct infrared spectroscopic measurement of the volcanic glass, or by a determination of the volcanic glass density made by submersion in a heavy liquid.

Once the quantity of water is known a hydration rate is estimated. A small sample is cut out perpendicular to the edge of the obsidian artifact using a diamond-impregnated saw. A lapidary machine is used to grind down the obsidian sample until it is very thin. It is glued to a clear microscope slide with Canada balsam. The obsidian sample is ground a second time until it less than 50 microns in thickness. A microscope is then used to optically measure the hydration rind on the petrographic thin section.

The hydration layer is measured at x using a Watson image-splitting instrument. This is the most precise optical instrument that can be used. It has an error factor of about 0. In order to adjust the experimental hydration rate to the conditions at the archaeological site, the soil temperature and soil relative humidity need to be well estimated. On short term projects, ambient conditions can be estimated from weather records. Conlee , moico81 hotmail. We com- kschreiber anth. We briefly examine the effects of burial depth, but ul- 2.

Background timately leave that for future research. The effects of water vapor pressure have not been studied Obsidian hydration operates on the principle that like all as intensively. While important in controlled laboratory volcanic glasses, obsidian absorbs water. In fronts from the exposed surface of an artifact. By measuring the Andes, both temperature and water vapor pressure are the thickness of these diffusion fronts or bands or rinds , likely to be mainly controlled by elevation.

Thus, in our appli- hence the amount of water absorbed, this principle can be cation of the hydration model, both these factors are subsumed used to determine if one artifact is older than another relative under a single term. However, if the rate at which water diffuses into glass The effect of the glass chemistry on hydration is less well can be determined, the technique allows for more useful calen- understood.

Empirical data indicate that certain obsidians drical age estimates absolute dating. We focus on the latter hydrate faster than others within the exact same depositional approach.

The way archaeologists generally deal with this is front thickness is described by Eq. These effects are then summa- rized into a constant, expressed as part of D in Eq. As we where age is generally measured in years, D is a constant show below, this does not prove to be a large problem in the though see below , and X is the hydration rind measured in SNR because obsidian comes predominantly from a single microns.

In short, the age increases as the square of the hydra- geochemical source.

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• Obsidian Hydration Dating.

An issue related to glass chemistry is the intrinsic water Other than time, at least three other factors affect the rate of content of obsidian e. Studies hydration, including temperature, water vapor pressure, and indicate that the amount of water within obsidian can vary glass chemistry Friedman and Smith, ; Friedman and even within obsidian nodules from a single source, and that Obradovich, ; Jones et al. These factors are generally expressed as a constant our knowledge, this factor has not yet been addressed in the and subsumed into the term D in Eq. However, recent re- Andes and its effects are, as yet, unknown.

Sample ; Rogers, ; Stevens, While easy to control under laboratory conditions e.

Dating methods in archaeology and paleoanthropology Part I

Our sample of hydration measurements comes from Mazer et al. Of these, arti- is quite dynamic in real world situations, fluctuating diurnally, facts are from 15 radiocarbon-dated site components. Site seasonally, and over longer time scales as well. Instead of components are defined as stratigraphically or spatially modeling these factors independently, archaeologists have em- restricted areas e.

For example, Rogers Grados, Two additional components come from Mar- has expressed EHT as a function of annual mean temperature, caya Vaughn, ; Vaughn and Glascock, , two from annual temperature range, and diurnal temperature range. All of these samples come for the temperature of buried artifacts Ridings, ; Rogers, from excavated contexts. All radiocarbon dates were cali- , under the assumption that underground conditions are brated using the on-line version of the Calib 5.

However, as we dis- Stuiver and Reimer, Moreover, the correction one of us KS during the Proyecto Nasca Sur for a summary tends to impose order on an assemblage in precisely the man- of the surveys see Schreiber and Lancho Rojas, All ner expected naturally in a stratified deposit. Thus it is difficult these loci represent single-component locations, where diag- to evaluate whether depth corrections improve the analysis be- nostic ceramics representing only one time period were cause depth significantly affects hydration, or whether it im- located.

For dating purposes we used the median age for tradi- proves because it reinforces stratigraphic differences. In any tionally recognized culture historical periods see Table 1. Culture historical periods, ages, and median age used for developing a hydra- There is no local source of obsidian in the SNR, and all mate- tion rate in the SNR rial was imported to the region.



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Based on visual inspection, a clean part of the artifact was chosen for analysis. For pieces of debit- most cases, these periods have been independently established age the platform was cut to determine if there was difference with radiocarbon dates from excavated deposits, though occa- between the dorsal and ventral surfaces, or in some cases the sionally the dates are based on analogies to neighboring actual platform. Weathered specimens are addressed in the regions outside the SNR. For projectile points the Note that some of the sites in the map contain multiple but base or notched area was generally cut.

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This cutting strategy spatially and temporally discrete components from where was employed specifically to identify and measure artifacts obsidian samples were drawn e. Elevation among the sites ranges from to Two parallel cuts ca. This was accom- garding estimation of EHT. Obsidian slices were analyzed by laser ablation-inductively coupled plasma-mass then mounted with Lakeside thermoplastic cement onto Fig. Study area, places mentioned in text, and sites from which samples were drawn. Generally, five similar types of weather-recording centers to help estimate specimens were mounted on each slide.

Samples were then EHT in these locations. As expected, EHT Prepared slides were measured using a Meiji petrographic decreases significantly with altitude, suggesting artifacts at microscope fitted with a Lasico digital filar eyepiece microm- higher elevations will hydrate slower than those at lower eter. Once a defined hydration rim or band was observed on elevations. Also seen in Fig. We take the middle of the monitor, to reduce parallax, and measured advantage of this correlation, and use it to derive EHT esti- using a micrometer.

Typically, 10 measurements were taken mates for sites in mid-elevation zones between the coast and on each specimen. However, imperfections in the stone, highlands. Hydration values were recorded to the nearest 0.

Double, or in rare cases triple, Such and elevation, and that produced in the only analogous study fracturing episodes must be separated in time by significant we know of in the Andes, shows some difference. Lynch margins, such that they form visually distinct hydration bands, and Stevenson report a 6. However, their study was lies on a site surface. If an older artifact is scavenged and based on observations exclusively over m in elevation reworked, it is the smaller band that more accurately dates while the majority of ours are under this mark.

If we examine the deposit from which it comes, and by extension the other only the weather stations over m in Fig. If an artifact was later damaged seem to be a slightly greater decrease in temperature with by trampling, it may be the larger band that more accurately elevation 4. When multiple rims were observed, This may indicate that the relationship between EHT and each was independently measured using the micrometer and elevation in the Andes is non-linear, with temperature change reported.

If so, the slope of the line defined by Eq.



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