A Reassessment of Obsidian Hydration Ages of Projectile Point Types from the Coso Volcanic Field Alexander K. Rogers Abstract employed the best information and methods Data relating projectile point types, obsidian hydration, and available at the time for including the effects of radiocarbon dating of artifacts from the Coso Volcanic Field have played a significant role in reconstructing cultural temperature; however, subsequent developments sequences in the western Great Basin and Mojave Desert. This in the theory of hydration have indicated that their paper re-evaluates the obsidian hydration data published by temperature assumptions were incorrect. This paper Gilreath and Hildebrandt (1997) in light of better estimates of effective hydration temperature, and concludes that the reports a correction to the temperature calculation, previously assessed ages are too young, sometimes by and reanalyzes the hydration rim data in this light. significant amounts. This affects assessments of the time of introduction of the bow and arrow, which appears to be 2000+ rcybp instead of the conventionally-assumed 1500 rcybp. Background to the Issue Introduction Studies of Coso obsidian hydration typically take the Lubkin Creek site (CA-INY-30) as the reference Gilreath and Hildebrandt (1997), in their site, which lies at an altitude of approximately comprehensive study of obsidian use at the Coso 3000 feet above mean sea level (amsl). Gilreath Volcanic Field, have provided a widely-used data and Hildebrandt (1997) assumed that the effective base for Coso obsidian studies. Their research hydration temperature (EHT) difference between related, inter alia, projectile point types with sites would be roughly proportional to the obsidian hydration measurements and radiocarbon difference in mean annual temperature of the sites. ages. In particular, their data have been used to infer They used a mean annual temperature of 13.5° C the periods of use of various projectile point types for Lubkin Creek, and 15.7° C for Haiwee, and in the Coso region. further assumed that the temperature at Haiwee represents that at the Coso Volcanic Field (Gilreath It has long been known that obsidian hydration is a and Hildebrandt 1997:16, also 164, Table 83). temperature-sensitive process, (e.g., Friedman and Based on these assumptions, they computed a rim Long, 1976), so it is necessary to take account of correction factor of 0.8723, using the methodology temperature in comparing hydration rims between of Basgall (1990) (Gilreath and Hildebrandt sites. The analysis by Gilreath and Hildebrandt 1997:16). Since they assumed the Coso Volcanic Pacific Coast Archaeological Society Quarterly, Volume 39, Number 4 24 Rogers Field temperature to be 2.2° C warmer than the Doremus 2000, 2002; Stevenson, Carpenter, and reference site, hydration would proceed more Scheetz 1989; Stevenson, Mazer, and Scheetz rapidly and any rim at Coso would correspond 1998). No other form of functional dependence is to a younger age than the same rim at Lubkin currently suggested by theory; Haller observed in Creek. However, it is shown in this study that the 1963 that, if any dependence other than quadratic is temperature assumption is incorrect, and the Coso found, “it is more likely the fault of the experiment Volcanic Field is actually 3.1° C cooler than Lubkin rather than any inherent feature of the diffusion Creek. This implies that hydration would have process” (Haller 1963:217). When obsidian data proceeded more slowly at Coso than at Lubkin are expressed in radiocarbon years before the Creek, which significantly alters the ages assigned present (rcybp, by convention referenced to 1950), to the rims. the quadratic form is still the best fit, giving the smallest overall error in age estimation, but the rate Gilreath and Hildebrandt also used the Basgall constant is different (Rogers 2006). equation for converting rim thickness to age (Gilreath and Hildebrandt 1997:16). Although this The hydration coefficient varies with effective equation is not supported by physical theory, and hydration temperature, or EHT (e.g., Hull 2001; gives excessively old ages for large rims, it does Ridings 1996; Rogers 2007a; Stevenson et al. not differ significantly from the quadratic form for 1989, 1998, 2004; Onken 2006), with relative more recent ages. This equation is not the source of humidity (Friedman et al. 1994; Mazer et al. 1991; the problem here. Onken 2006; Rogers 2008a), and with structural water concentration in the obsidian (Ambrose and Theory Stevenson 2004; Rogers 2008b; Stevenson et al. 1998, 2000). Obsidian Hydration Rogers (2007a) developed a rigorous solution to The use of obsidian hydration to estimate age of an the time-varying temperature case, and further obsidian artifact was first suggested by Friedman developed an algebraic model to compute EHT and Smith (1960). Hydration of obsidian has both from annual temperature and annual and diurnal a physical and a chemical aspect, and is known variation. These models are the basis of the present as a diffusion-reaction process (Doremus 1994, study. 2000, 2002). All that is known of the physics and chemistry of the process suggests the relationship The analysis reported in Rogers (2007a) employed between age and rim thickness should be quadratic, a simplified climatic model to represent climate i.e., of the form over archaeological intervals. The temperature model is based on three parameters: Ta, the annual 2 x = Dt (1) mean temperature; Va, the annual temperature variation (July mean - January mean); and Vd, the where t is age in calendar years, x is rim thickness mean diurnal temperature variation (average of in microns, and D is a constant, the diffusion or January and July). These parameters are derived hydration coefficient (e.g., Ebert, Hoburg, and from measured meteorological data from the web Bates 1991; Zhang, Stolper, and Wasserburg 1991; sites of the Regional Climate Centers. PCAS Quarterly, 39 (4) A Reassessment of Obsidian Hydration Ages of Projectile Points from the Coso Volcanic Field 25 An algebraic equation was developed to fit the temperatures, not the air temperatures reported computer model output data to facilitate practical by meteorological services. However, surface computations by the archaeologist (Rogers 2007a): temperatures at brush-covered sites approximate the air temperatures, so air temperatures are used here. -5 0.95 EHT = Ta(1 - 3.8×10 y)+0.0096 y (2a) Furthermore, Rogers (2008c) has shown that EHT differences between sites can be computed with where Ta is annual average temperature, and y is either air temperatures or surface temperatures, as the sum of the squares of the annual and diurnal long as they are used consistently. temperature variation constants, modified for depth, or It has been shown that depth correction for EHT is desirable, even in the presence of site turbation 2 2 y = (Va + Vd ) (2b). (Rogers 2007b). Burial depth of the obsidian samples in this analysis was not reported, so depth For buried artifacts, Va and Vd represent the was assumed to be zero, and all the samples were temperature variations at the artifact depth, which assumed to have been exposed to the same relative are related to surface conditions (Va0 and Vd0) by humidity. (Carslaw and Jaeger 1959:81) For this study the equation used to compute age is Va = Va0exp(-0.44z) (2c) t = 38.34(xRP)2 (4) and where t is age in rcybp, x is mean rim thickness in Vd = Vd0exp(-8.5z) (2d) microns, R is the rim correction factor for present temperature conditions (equation 3), and P is the Here z is burial depth in meters. The numerical paleoclimatic correction described below. Equation parameters in equations 2c and 2d are based 4 was derived from a data set of 21 radiocarbon- on recent measurements by the author in desert obsidian correlations, in which the obsidian conditions. readings were corrected for EHT, including burial depth, local temperature conditions, and Once EHT has been computed, the measured rim paleoclimatic shifts (Rogers 2008d). thickness is multiplied by a rim correction factor (a) to adjust the rims to be comparable to conditions at The rate constant in equation 4 reflects a mean for a reference site: obsidians of the Coso Volcanic Field at an EHT of 20.4° C. It has been shown that there are rate R = exp[-0.06(EHT-EHTr)] (3) variations between sub-sources in the Coso field (Fredrickson et al. 2006), but they were not used where EHTr is effective hydration temperature at here, for two reasons. First, obsidian samples in the reference site (usually Lubkin Creek, CA-INY- Gilreath and Hildebrandt (1997) were not identified 30, for Coso obsidian). to sub-source, and second, identifying to subsource is not, in itself, a good predictor of hydration rate. Onken (2006) has suggested that the temperatures Stevenson, Mazer, and Scheetz (1998) showed used in equations 2 and 3 ought to be surface that hydration rate is strongly influenced by the PCAS Quarterly, 39 (4) 26 Rogers hydroxyl ion concentration of the obsidian, which because the positive and negative perturbations in turn is determined by the intrinsic water content approximately compensate for one another. (Silver, Ihinger, and Stolper 1990). Furthermore, Further analysis of the simulation data showed that Stevenson et al. (1993) showed that the Coso sub- applying a rim correction to each artifact based on sources exhibit great internal variability in water its depth of recovery is, on the average, the best content, such that determining a sub-source is not, chronological analysis strategy, even in cases of in itself, sufficient to predict hydration rate for any extreme mixing. For cases where data are to be particular specimen (Rogers 2008b; Stevenson, aggregated, this strategy will lead to improvement Gottesman, and Macko 2000). Unfortunately in the mean of the rim data relative to use of there is at present no cost-effective and robust uncorrected rim data; for individual artifacts it will method of determining structural water content lead to better rim estimates on the average (Rogers in archaeological samples, and the Gilreath and 2007b), although the benefits for any specific Hildebrandt (1997) samples are no longer extant artifact are indeterminate.