Obsidian hydration relative dating
These include the composition of the glass, effective hydration temperature (EHT), the relative humidity (RH) of the environment, the glass surface area-to-solution volume (SA/V), solution p H, solution composition, solution-flow rate, and even the exposure time (e.g., Clark et al. Furthermore, since it is widely recognized that hydration involves the replacement of alkali ions by hydrogen or hydronium ions (the first and second reactions), it should be apparent that p H is one of the most critical variables determining hydration rates. 1989 Obsidian as a Time Keeper: An Investigation in Absolute and Relative Dating. Cultural Resource Management Program, Sonoma State University, Rohnert Park, California.
While there is general consensus regarding the geochemical reactions that lead to hydration, and a growing awareness of the variables that influence hydration rates (e.g., Clark et al. 1991 Soil Temperature and Relative Humidity Investigation as Relevant to Obsidian Hydration Studies.
Several researchers have observed a shift from a square-root relation (classic diffusion) between hydration-rind thickness and time to a linear relation, based on changes in p H (e.g., White and Claassen 1979; Wicks et al. In turn, p H, in a closed system, is influenced by how long hydration has occurred (Tremaine 1989).
As alkali ions leach into solution and are replaced by hydrogen or hydronium ions (the first two reaction types), the p H of the solution will increase.
An analogy with anthropological systems theory is useful; hydration is not a unilinear process, but one with many feedback loops between interacting variables.
As such, it does not yield easily to simple modeling strategies.
In vie^^ ofthe likelihood of large errors in the Coprin obsidian dates, the assertion that the Late Classic Coner phase should be e- tended beyond A. Las constantes utilizadaspara determinar la ferha alargada de la fase Coner en el Clrisico Tardio en Coprin, Honduras, son de validez discutible y lesfalta verificaczbn ~ndependiente. L'n error de muy pocos grados Kelvin puede resultar en errores de varzos siglos. Materials Research Society Symposium Proceedings 123.
The most significant implication for Maya archaeology is that population decline in the Copan pocket and its hinterlands at the end of the Late Classic was slower than first thought, and that the Late Classic Coner phase continued well beyond the collapse of the elite political structure, to A. In order to assess the validity of this claim, it is necessary to review the current state of obsidian-hydration theory and the methods employed to generate the Copin dates.
THEORETICAL BACKGROUND In recent years, the natural and artificial weathering of glass has been scrutinized by scientists interested in using glass to contain highly radioactive nuclear waste (e.g., Fillet et al. 1980; Shade and Strachan 1986; Spinoza and Means 1986; Strachan 1984; Umeki et al. In her masterful review of this body of literature, Tremaine (1989: 13-14) has characterized the weathering of glass as consisting of four geochemical reactions: (1) The leaching of alkali ions (such as Na, Al, Mg, Ca, and K) from the glass into the liquid water or water vapor contacting it; (2) the replacement of these ions with H or H, O ions from the liquid or vaporized solution by adsorption or diffusion; (3) the surface dissolution of the silica network of the glass: and (4) the precipitation of reaction products on the glass surface.
Radiocarbon or tree-ring dates have typically been used to support either a linear relation or a square-root relation (hydration-rind thickness in direct proportion to the square root of time [see below]), but other exponential relations between about one-third and one have also been proposed (e.g., De Atley and Findlow 1980; Ericson 1975,1988; Findlow et al.
1982; Friedman and Trembour 1983; Hurtado de Mendoza 198 1; Jackson 1984; Kimberlin 1976; Meighan 1983; Minor 1977; Tremaine 1989). Scheetz 1989 Obsidian Dating: Recent Advances in the Experimental Determination and Application of Hydration Rates.