Obsidian hydration dating: accuracy and resolution limitations imposed by intrinsic water variability

Obsidian hydration dating typically yields a range of ages for a single chronometric measurement, even after controlling for source chemistry and effective hydration temperature. Previously published data suggest that this range is due to hydration rate variations caused by variability in the concentration of intrinsic water, and specifically hydroxyl ions, in the obsidian. Such variability exists within any given obsidian source, and even within any particular specimen. This paper analyzes the effect on age estimates of intrinsic water variability and concludes that if a controlled sample of obsidian yields a range of ages, there is no way to tell whether the range is due to long site use or to variations in hydroxyl concentration. At present there is no robust and cost-effective protocol for measuring hydroxyl concentration. Suggested guidelines for obsidian hydration dating are developed using a case study. These findings do not invalidate obsidian hydration as a chronometric technique, but they do suggest limits to the temporal resolution achievable.     

Obsidian hydration dating on the South Coast of Peru

We compare over 230 obsidian hydration readings from 30 individual site components from the Southern Nasca Region (SNR) with independent age estimates based on radiocarbon dates and temporally diagnostic artifacts. Although there are problems with small sample sizes, and readings must be adjusted for elevation, a very strong relationship accounting for nearly 90% of the total variation in the data set is found. This suggests that obsidian hydration dating (OHD) works in the SNR and is a viable means of independently estimating age. Residual values from our regression suggest that hydration age estimates are usually within 15% of the radiocarbon estimates. Finally, we present an equation other scholars can use to estimate age for Quispisisa obsidian in the SNR.     

Humans,climate or introduced rats – which is to blame for the woodland destruction on prehistoric Rapa Nui (Easter Island)?

When the first Polynesian settlers arrived on Rapa Nui, about 70% of the island was covered with dense woodland in which Jubaea palms dominated. Our investigations of extended soil profiles provide evidence that more than 16 million palm trees grew on the island. Nearly all palms were removed by the 16th century. Teeth marks on nutshells of the Jubaea palms from the 13th or 14th centuries attest to the activity of Pacific rats (Rattus exulans) on Rapa Nui, which were probably imported there by the first Polynesians settlers. Did the rats perhaps prevent the germination of palm seeds and thus the regeneration of the dense palm woodland of Rapa Nui?     

Stylistic variability of stemmed obsidian tools (mata’a), frequency seriation,and the scale of social interaction on Rapa Nui (Easter Island)

Easter Island, or Rapa Nui, has been the focus of much research and speculation, particularly with reference to the island’s hundreds of giant, enigmatic statues and the set of conditions that supported their construction and transportation. In this paper, we analyze an abundant class of lithic artifacts, mata’a, to study of patterns of cultural transmission with implications for the evolution of groups, competition, and scale of socio-political organization among this island population. While these kinds of studies often draw upon assemblages of decorated ceramics, here we show how analysis of variability unconstrained by performance allows us to measure aspects of inheritance related to the manufacture of these artifacts. In the case of mata’a from Rapa Nui, we demonstrate that it is possible to reach falsifiable conclusions about the evolutionary dynamics that shaped the remarkable archaeological record on Rapa Nui.     

An island-wide assessment of the chronology of settlement and land use on Rapa Nui (Easter Island) based on radiocarbon data

The archaeological landscape on Rapa Nui (Easter Island) contains a palimpsest of surface archaeological features reflecting a long history of settlement and land use. The popular narrative of societal collapse prior to European contact relies on chronometric data from the late pre-European contact period and also cites major settlement shifts as evidence for societal collapse and socio-political reorganization. This paper explores the archaeological evidence for proposed changes in settlement by assessing the spatial and temporal distribution of radiocarbon determinations collected from archaeological and landscape contexts. A corpus of over 300 determinations is placed into an island-wide GIS database and analysed. The results of this study suggest that Rapa Nui settlement and land use exhibit continuity rather than punctuated, detrimental change during the late pre-European contact period.     

Induced hydration of obsidian: a simulation study of accuracy requirements

Determination of the hydration rate constant of obsidian is basic to the use of obsidian for establishing chronologies. The constant can, in principle, be determined in either of two ways: by correlations with archaeological sequences, or by laboratory experiments using induced hydration. Induced hydration holds promise of great accuracy, but results reported to date have been disappointing. This paper is based on the hypothesis that the outcomes are the result of error build-up in the induced hydration protocol, and describes an analysis based on a Monte Carlo simulation of the measurement and analysis process. Data are presented which show that the poor results are due to errors inherent in optical measurement of hydration rim thickness. It is concluded that successful use of induced hydration requires an order of magnitude improvement in accuracy of hydration rim measurement over the accuracies currently claimed for optical microscopy. The results do not affect the validity of hydration dating based on archaeological correlations.     

Ancient DNA of the Pacific rat (Rattus exulans) from Rapa Nui (Easter Island)

We report analysis of ancient mitochondrial DNA sequences from nine archaeological specimens (8 femura and 1 incissor) of Rattus exulans excavated from Anakena Beach Dune on Rapa Nui. Sequence of a 239-base-pair fragment of the hypervariable mitochondrial control region reveals a single mitochondrial DNA (mtDNA) sequence of all samples corresponding to the R9 haplotype prevalent in East Polynesia. This suggests a single or very limited introduction of Rattus exulans to the island. Rapa Nui, like other remote islands of Polynesia, remained effectively isolated following colonization.     

Accuracy of obsidian hydration dating based on obsidian–radiocarbon association and optical microscopy

Most archaeologists using obsidian hydration dating (OHD) in the United States develop hydration rates from association of obsidian hydration rims with dates based on radiocarbon, subsequently using the rate for chronometric analysis. The overall accuracy of the process has never been quantified. This paper reports an accuracy analysis in which sources of uncertainty are defined and modeled and their effects quantified. A Monte Carlo simulation is used to quantify errors from rate development, while uncertainties in age resulting from chronometric analysis are calculated analytically. For typical ranges of error values, hydration rate errors of ∼5% or less are achievable in the absence of systematic errors, with errors of chronometric age estimates ∼20–30% or less.     

Obsidian hydration dating by infrared spectroscopy: method and calibration

Low temperature (90–190 °C) hydrothermal experiments have been conducted on seven obsidians where composition of the glass varies significantly in the concentration of structural water within the unhydrated bulk material. Infrared transmission spectroscopy was used to track the diffusion of molecular water into the glass surface as a function of time and temperature. Long-term (60–360 days) hydration sequences at 90 °C show a t^0.6 time dependence for the mass uptake of molecular water that forms the hydration layer. The structural water concentration of the unhydrated bulk obsidian is highly correlated with the pre-exponential and activation energy and may be used to estimate the Arrhenius constants. In addition, secondary ion mass spectrometry (SIMS) hydrogen profiling of Napa Glass Mountain obsidian hydrated at 90 °C reveals that the early stages of diffusion exhibit a dynamic behavior that includes a fluctuating hydrogen concentration and a changing diffusion coefficient that slows with time.     

Late Pleistocene/Early Holocene seafaring in the Aegean: new obsidian hydration dates with the SIMS-SS method

Archaeological evidence regarding the presence of obsidian in levels that antedate the food production stage could have been the result of usage or intrusion of small obsidian artifacts from overlying Neolithic layers. The new obsidian hydration dates presented below employing the novel SIMS-SS method, offers new results of absolute dating concordant with the excavation data. Our contribution sheds new light on the Late Pleistocene/Early Holocene exploitation of obsidian sources on the island of Melos in the Cyclades reporting dates c. 13th millennium - end of 10th millennium B.P.     

Rethinking Easter Island's ecological catastrophe

Rapa Nui (Easter Island) has become a paragon for prehistoric human induced ecological catastrophe and cultural collapse. A popular narrative recounts an obsession for monumental statuary that led to the island's ecological devastation and the collapse of the ancient civilization. Scholars offer this story as a parable of today's global environmental problems. In this paper, I review new and emerging Rapa Nui evidence, compare ecological and recently acquired palaeo-environmental data from the Hawaiian and other Pacific Islands, and offer some perspectives for the island's prehistoric ecological transformation and its consequences. The evidence points to a complex historical ecology for the island; one best explained by a synergy of impacts, particularly the devastating effects of introduced rats (Rattus exulans). This perspective questions the simplistic notion of reckless over-exploitation by prehistoric Polynesians and points to the need for additional research.     

Isothermal Time‐Series Determination of the Rate of Diffusion of Water in Pachuca Obsidian*

Friedman and Smith's (1960) article introducing an exciting, potentially precise and inexpensive method of dating obsidian artefacts has thus far failed to reach its potential. Numerous efforts to refine, improve and even redevelop the method since that time have similarly failed to achieve the original promise. Only within the last eight years have significant improvements been made, due to both improved analytical techniques and a better understanding of the hydration process. However, most of our mechanistic understanding of the interaction of water with rhyolitic glass is based on experiments performed on melts and glasses at temperatures above their glass transitions, conditions inappropriate for investigation of near‐surface environmental conditions. Unfortunately, studies detailing the temporal evolution of the diffusion profile at low temperatures are rare, and few useful data are available on the low‐temperature diffusive hydration of silicate glasses. This paper presents data on the experimental hydration of obsidian from the Pachuca source (a.k.a. Sierra de las Navajas, Basin of Mexico) at 75°C for times ranging from 3 to 562 days, and compares these results with data for samples obtained from a stratigraphic excavation of the Chalco site in the Basin of Mexico. Samples have been analysed using secondary ion mass spectrometry (SIMS) to provide concentration/depth data. While 75°C is still significantly above the temperatures at which archaeological obsidians hydrate, it is well below the glass transition temperature (approx. 400°C) and thus processes are likely to be similar to those that occur in nature, but fast enough to be observed over a laboratory timescale. The results demonstrate that a simple square‐root‐of‐time model of the evolution of the diffusion profile is not adequate to describe the diffusion process, as measured diffusion profiles exhibit the effects of concentration‐ and time‐dependent, non‐Fickian diffusion. With progressive hydration, characteristic diffusion coefficients first decrease, then increase with time. Surface concentration increases with time, but an intermediate plateau is observed in its time evolution that is consistent with results obtained from the suite of Chalco samples. Both of these effects have been observed during diffusion in glassy polymer systems and are associated with the build‐up and relaxation of self‐stress caused by the influx of diffusing material.     

OBSIDIAN HYDRATION DATING: AN IMPROVED OPTICAL TECHNIQUE FOR MEASURING THE WIDTH OF THE HYDRATION RIM

A simple optical method is described that will enhance the color contrast between the outer birefringent hydration rind and the inner unhydrated core on obsidian thin sections used for hydration dating. The technique utilizes a gypsum nccessory plate. The gypsum plate rotates all incoming light by 550nm causing the black isotropic core of the nhsidian to become transparent first-order pink and interacting with the transmitted light from the birefringent rind, by either addition or subtraction. to produce a second-order blue or a first-order yellow color.     

An archaeologically validated protocol for computing obsidian hydration rates from laboratory data

This paper reports the computation of hydration rate for Topaz Mountain obsidian from laboratory data, and a comparison with archaeological data from a well-dated site, Camels Back Cave, in western Utah. Topaz Mountain obsidian is found to be slow-hydrating, with a rate of 0.071 ± 0.021 μ/yr^½ at an effective hydration temperature of 16.01 °C. This rate agrees with a rate developed from archaeological data from Camels Back Cave within ∼6%. Activation energy of Topaz Mountain obsidian is 10370 ± 544 K, and its diffusion constant is (1.87 ± 9.13) × 10¹³ μ²/yr, both of which are independent of temperature. Its intra-source variability in hydration rate is very low (CV < 0.01), implying a low variability in intrinsic water. We present a model of chemical erosion which shows why earlier laboratory-determined rates were incorrect, and discuss the implications of our findings on the determination of experimentally derived rates in obsidian hydration dating.     

Obsidian Diffusion Dating by Secondary Ion Mass Spectrometry: A Test using Results from Mound 65, Chalco,Mexico

Secondary ion mass spectrometry (SIMS) was used to measure hydrogen and other elemental concentrations as a function of depth in ten obsidian artifacts (Pachuca Source), each with a well-constrained ¹⁴C date, from Mound 65, Chalco, Mexico. Hydrogen depth profiles for the different artifacts all display a characteristic S-shape, and increasing maximum hydrogen content in each profile and profile depths are well correlated with time. These data are used to investigate the potential use of Obsidian Diffusion Dating by SIMS (ODDSIMS) for both extrinsic and intrinsic dating of obsidian artifacts. Using “characteristic points” on the hydrogen profile (half-fall depth, inflection point depth), simple hydration rate equations were evaluated against time constraints provided by associated ¹⁴C dates. We demonstrate that neither the traditional OHD equation for depth (x) as a function of the square root of time (t^1/2) nor a linear function (t¹) fit the data. Solving the more generalized tⁿ function provides an excellent fit between characteristic point depths and ¹⁴C dates (for n≈0·75), and meets the constraint that at time equal to zero, the depth of the hydration profile must also be zero. However, this may be an average coefficient over the range of ages available, and may not accurately reflect rates at shorter or longer times. Using only two obsidian samples and their associated ¹⁴C dates, a calibration curve can be derived that provides ODDSIMS dates for the other pieces that are in excellent agreement with associated¹⁴ C dates, indicating that empirical application of the technique is potentially feasible, at least at individual sites.The underlying processes governing hydrogen transport into the obsidian were also investigated by using finite difference modelling to reproduce the shape of the hydrogen depth profile. Excellent fits were obtained by assuming concentration-dependent diffusion, and dates that agree well with associated ¹⁴C dates can also be extracted from the finite difference profiles. Although considerable additional work needs to be done, the success of the finite difference modelling suggests that development of an independent, intrinsic ODDSIMS model may be possible.     

The antiquity of Olivella shell beads at CA-ORA-64: AMS radiocarbon dated between 9420 and 7780 cal BP

Investigations in the 1960s and 1970s showed the Irvine site (CA-ORA-64) to be among the oldest shell middens known from the Pacific Coast of North America. The site chronology, based on conventional analysis of mixed shell samples collected from heavily bioturbated soils, extended back to ca. 8440 RYBP. Recent work at the site provided an opportunity to refine the CA-ORA-64 chronology via AMS ¹⁴C dating of single shell fragments, including 14 Olivella beads. Long considered to be one of the earliest ornament types made by Pacific Coast peoples, such spire-removed Olivella beads were used through much of the Holocene and generally are not good chronological indicators. AMS analysis of the CA-ORA-64 specimens, however, produced a consistent series of Early Holocene dates that includes some of the oldest securely dated shell beads in North America. Along with obsidian from interior sources, Olivella beads played an important role in early exchange networks between coastal and interior peoples. Methodologically, our research demonstrates the utility of AMS ¹⁴C dating in determining the age of key artifact types found in multicomponent sites with assemblages affected by stratigraphic mixing.     

Obsidian hydration at high elevation: Archaic quarrying at the Chivay source,southern Peru

We examine obsidian hydration as a means to date archaeological sites at high elevation in the central Andes, and in particular quarry sites that are difficult to date by radiocarbon means. The Chivay obsidian source lies in a volcanic depression above the Colca Valley in Arequipa, Peru (71.5355° S, 15.6423° E) at 4950 masl. We compare obsidian hydration readings from one quarry and two workshop locations. Ninety-one flakes from the quarry pit, and 61 and 33 flakes from the workshops were analyzed for hydration bands. Of these, 68 from the quarry, and 54 and 33, respectively from the workshops produced at least one culturally meaningful hydration band. As expected, obsidian appears to hydrate slowly at this high elevation. Yet, variation in hydration readings is low within stratigraphic contexts, suggesting relatively narrow windows of knapping activities in each excavation level. A small number of radiocarbon dates allow us to develop a preliminary hydration rate for Chivay obsidian in this high elevation location. Hydration data indicate that intensive quarrying began by 3800 cal. BC and stopped ca. 2300 cal. BC. By contrast, the two workshops appear to have been deposited 2900 and 1200 cal. BC, and 2700 and 2400 cal. BC. The data are consistent with an uptick in obsidian use by at least the Terminal Archaic period.     

ARCHAEOLOGICAL VOLCANIC GLASS FROM THE SITE OF ROCCHICELLA (SICILY,ITALY)*

The site of Rocchicella, near Catania, in eastern Sicily, has yielded important archaeological evidence from prehistoric times to the Middle Ages. Extensive archaeological investigations of cultural layers dating from the Palaeo‐Mesolithic to the Copper Age have recently been undertaken, and volcanic glass, mainly obsidian, has been collected in the course of excavation. To determine the provenance of this volcanic glass, a non‐destructive elemental analysis was carried out to measure the concentration of characteristic trace elements. The analysis was carried out using a new XRF spectrometer equipped with a beam stability controller and a quantitative method developed at the LANDIS laboratory of the INFN–CNR Institutes of Catania. In addition to the obsidian, it was demonstrated for the first time that a local vitreous material similar to obsidian, but displaying a completely different composition, was used during all the investigated periods. This material was identified as a basaltic glass, characterized by a superficial product of devitrification called palagonite. Analysis of the obsidians has led to the identification of the island of Lipari as the provenance source. High‐ and low‐power microscopic use‐wear analysis on obsidian and basaltic glass artefacts indicated that soft wood and plant matter might have been processed at the site.     

Effective hydration temperature of obsidian: a diffusion theory analysis of time-dependent hydration rates

An estimate of effective hydration temperature (EHT) is needed for chronological use of obsidian hydration data. This paper describes a method for calculating EHT by the practicing archaeologist, replacing three techniques that are in general use today: estimates based on mean temperature, numerical integration of models of diurnal and annual temperature variations, and use of temperature cells. The hydration (or diffusion) coefficient of obsidian is a function of temperature and thus is time varying, while the classic quadratic law of hydration is not valid for time-varying diffusion coefficients. This paper presents a mathematical solution to the case of a time-varying hydration coefficient, based on diffusion theory, with a concise definition of EHT. It is shown that the results are not affected by concentration dependence in the diffusion coefficient. A computer program to compute the rigorous solution is described, and data are presented to explore the resulting range of variation. That use of the Lee equation to compute EHT is not appropriate for obsidian hydration studies is evident from the data presented. The effects of paleoclimatic variation are estimated, and an algebraic best fit equation and worksheet are provided as practical aids to the archaeologist.