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.     

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.     

Prehistoric settlement chronology on Rapa Nui,Chile: obsidian hydration dating using infrared photoacoustic spectroscopy

The prehistoric Polynesian inhabitants of Rapa Nui (Easter Island) utilized obsidian for nearly 700 years in many activities connected with daily life. The near ubiquitous occurrence of the natural glass in both domestic residences and religious structures makes the application of obsidian hydration dating highly suitable for the investigation of cultural change. We have applied previously developed calibrations that estimate hydration rates for obsidian based upon the structural water content of the glass as determined by infrared spectroscopy. The archaeological ages estimated by this method were compared with accelerator mass spectrometry (AMS) dates using short-lived woody species endemic to the island. The convergence between the two dating methods is strong and we suggest that obsidian hydration dating may be used on Rapa Nui to reliably date contexts where suitable material for AMS dating may not be available.     

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.     

Age and exploitation of obsidian from the Medicine Lake Highland,California

The relationship between major and minor elements, trace element composition, and age of obsidian sources within a volcanic field, is of considerable interest for obsidian source and artifact research in the New and Old World. The present study investigates this relationship in the Medicine Lake Highland of western North America. Geological evidence had indicated a very young age for all obsidian sources in the Highland, yet archaeological evidence suggested that obsidian was utilized for several thousand years. X-ray fluorescence analysis distinguished the latest flow (Glass Mountain) from the Cougar Butte, Grasshopper Flat, and Lost Iron Wells sources. Data obtained from two nearby archaeological sites showed that obsidian from the latter two sources was used by c. 7500 bc, while Glass Mountain material was not used (or available) until after 1360± 240 bp. These findings indicate that inferences of an extremely recent age for all obsidian sources in the volcanic field were unwarranted. Further analysis of major and minor elements indicated different hydration rates for these sources. The results argue that significant geochemical variability, as well as age differences, can exist between obsidian sources within the same volcanic field.     

Melian obsidian in NW Turkey: Evidence for early Neolithic trade

Abstract

Archaeological investigations carried out at the Early Neolithic coastal site of Co?kuntepe in northwestern Turkey yielded an assemblage of 110 obsidian artifacts displaying the macroscopic characteristics of the well-known obsidian deposits on the Cycladic island of Melos. Analysis of three samples from this homogeneous obsidian assemblage using both X-Ray Fluorescence and Laser Ablation High Resolution Inductively Coupled Plasma Mass Spectrometry confirmed that these artifacts were derived from Melos. The presence of these Melian obsidian artifacts at Co?kuntepe, along with a few pieces with central Anatolian macroscopic characteristics, is intriguing because intensive production of tools made of local flint was also identified at the site through the analysis of surface scatters. This finding raises the question of the status of obsidian and associated procurement systems. The presence of obsidian can be also used to argue that certain coastal villages acted as nodes of exchange for Aegean seafarers at times in the late 7th millennium B.C.     

THE PROVENANCE OF OBSIDIAN ARTEFACTS FROM THE WĀDĪ ATH‐THAYYILAH 3 NEOLITHIC SITE (EASTERN YEMEN PLATEAU) BY LA–ICP–MS

The geological sources of obsidian in the Red Sea region provide the raw material used for the production of obsidian artefacts found in prehistoric sites on both sides of the Red Sea, as far afield as Egypt, the Persian Gulf and Mesopotamia. This paper presents the chemical characterization of five obsidian geological samples and 20 prehistoric artefacts from a systematically excavated Neolithic settlement in highland Yemen. The major element concentrations were determined by SEM–EDS analysis and the trace element concentrations were analysed by the LA–ICP–MS method, an almost non‐destructive technique capable of chemically characterizing the volcanic glass. A comparison of archaeological and geological determinations allows the provenance of the obsidian used for the Neolithic artefacts to be traced to definite sources in the volcanic district of the central Yemen Plateau.     

PROVENANCE STUDIES OF CHALCOLITHIC OBSIDIAN ARTEFACTS FROM NEAR LAKE URMIA, NORTHWESTERN IRAN USING WDXRF ANALYSIS

In 2005–2006 we initiated a major archaeological survey and chemical characterization study to investigate the long-term use of obsidian along the eastern shores of Lake Urmia, northwestern Iran. Previous research in the area suggested that almost all archaeological obsidian found in this area originated from the Nemrut Daĝ source located in the Lake Van region of Anatolia (Turkey). More recent research on obsidian artefacts from the Lake Urmia region has identified a significant number of obsidian artefacts with compositions different from the sources near Lake Van. This suggests that the obsidian artefacts are from a yet to be identified geological source, but possibly one that was not too distant. In order to advance our knowledge of Iranian obsidians and eventually refine provenance criteria we analysed obsidian from 22 Chalcolithic sites and some source areas. The compositions of both obsidian source samples and artefacts were determined using wave length dispersive X-ray fluorescence spectrometry (WDXRF). This paper presents results from the trace elemental analysis of both geological and archaeological obsidians, providing important new data concerning the diachronic relationship between lithic technology and raw material in the north-west of Iran.     

Toward More Continuous and Practical Artifact Analyses: Defining and Learning From Key Dimensions of Fort Ancient Triangular Projectile Points in the Miami Valleys

Artifact typologies are routinely created, debated, and modified, but too rarely is the goal to focus on small geographic regions and to define key axes of variation. Our goal is to further this practice by examining the utility of a long-standing artifact typology from the U.S. Midwest, one associated with discerning temporal changes in triangular projectile-point forms. Our samples are derived from nine well-dated Fort Ancient villages along the Great and Little Miami Rivers of southwest Ohio and southeast Indiana. Our approach statistically explores numeric and nominal measurements of projectile-point morphology. Findings reveal significant differences among formal variables that are consistent with most of the typology as recently amended by other researchers. However, by focusing on numeric variables, we also have found that there are a series of additional significant temporal changes. Collectively, we identify a series of distinctions in temporal, morphological, and geographical variation among triangular projectile points. Various hypotheses are discussed regarding cultural transmission of the observed changes in projectile points.     

Strofilas (Andros Island,Greece): new evidence for the cycladic final neolithic period through novel dating methods using luminescence and obsidian hydration

The recently excavated coastal prehistoric settlement of Strofilas on Andros Island (Cyclades, Greece) in the Aegean sheds new light on the transitional phase from the Final Neolithic to Early Cycladic period regarding masonry, fortification, and richly engraved rock art. The fortification possesses early evidence of preserved defensive architecture, as evidenced from the plethora of scattered finds from within and around the settlement. Important features are carvings on rock walls which mainly depict ships, animals, and fish. Initial archaeometric dating via the application of luminescence dating of two samples from the fortified wall bearing engraved ships, and by obsidian hydration of two blades employing the new SIMS-SS method (secondary ion mass spectrometry via surface saturation), has been undertaken to determine the site's chronology. The former yields an average date of 3520 (±540) BC and the latter an average date of 3400 (±200) years BC, both of which, within overlapping errors, suggest the main settlement occurred during the Final Neolithic.     

Obsidian from the Epipalaeolithic and Neolithic eastern Maghreb. A view from the Hergla context (Tunisia)

In the present paper, it is shown that in the Hergla area (eastern Tunisia), obsidian was present from the early to at least the late sixth millennium cal BC. The presence of cores indicates that obsidian knapping was at least partly carried out in situ. The origin of these obsidians was determined from their elemental composition, by comparison with those originating from western Mediterranean potential sources, including analyses of new samples from the nearby Pantelleria Island. All obsidians were measured following the same protocol, by particle induced X-ray emission or by scanning electron microscopy/energy dispersion spectrometry. All the Hergla obsidians were found to originate from the Balata dei Turchi sources of Pantelleria. A review of the present body of knowledge on eastern Maghreb suggests, in spite of the still very preliminary data available, that Pantelleria was almost its unique provider of obsidians from the Epipalaeolithic to and during the Neolithic. However, the relative importance of the two main Pantellerian sources of Balata dei Turchi and Lago di Venere as providers of obsidian to eastern Maghreb remains to be investigated.     

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.     

Book Reviews

Abstract

Obsidian studies play an integral part in archaeology around the world, particularly in the Americas, but few archaeologists have employed obsidian studies to understand Native American life at historical archaeological sites. Yet, obsidian sourcing and hydration analysis can provide critical insights into site chronology and use, lithic recycling, and procurement and trade at contact and colonial sites. Obsidian geochemical sourcing and hydration analyses of a 19th-century rancho site in northern California have revealed new information on Native Americans who labored there in the second quarter of the 19th century. The obsidian data indicate a significant amount of lithic manufacture and use, a change in obsidian procurement in the 1800s, and an unprecedented number of obsidian sources represented on-site. The implications for general obsidian studies, as well as for regional archaeological issues, concern the problems with popular sourcing methods in northern California and the need to revisit current understandings of the first micron of hydration rim development.     

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.     

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.     

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.     

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.     

The Lake Krasnoe obsidian source in Chukotka (Northeastern Siberia): geological and geochemical frameworks for provenance studies in Beringia

Basic data on the geology and geochemistry of obsidian from the Lake Krasnoe source in Chukotka (Northeastern Siberia) are reported for the first time. The data are based on 2009 fieldwork and analytical studies of igneous rock samples. The lake shore and surrounding parts of the Rarytkin Range were thoroughly examined. Two geochemical types of rhyolitic obsidian were recognized for the first time: (1) metaluminous obsidian related to the fine-grained crystalline rocks and (2) peralkaline obsidian corresponding to ignimbrite ash-flows or lapilli-tuffs composition. Both types are related to the final phase of acidic volcanism in the Western Kamchatkan-Koryak Volcanic Belt. Based on the results obtained, we conclude that accumulation of obsidian pebbles in the lake’s modern beach deposits is related to silicic melts that erupted during the late Eocene-early Oligocene in the form of extrusive domes or pyroclastic flows, which are now either covered by Quaternary sediments or located below the water level. The Lake Krasnoe obsidian was intensively used by the ancient populations of Chukotka as a raw material for making stone tools. It was also occasionally transported to Alaska across the Bering Strait in later prehistory. The distances between source and utilization sites are up to 700–1100 km. Geochemical data for Lake Krasnoe obsidian, based on neutron activation analysis and X-ray fluorescence that are presented here, can now be used for provenance studies in the Northeastern Siberia and adjacent regions of northern North America.     

Geochemical composition of source obsidians from Kenya

Here we provide a reference resource to archaeologists interested in the sources of obsidian in Kenya, through electron microprobe analyses of 194 obsidian samples from 90 localities. Averaged analyses of each sample and eleven published analyses are categorized into 84 compositional groups of which only about 21 are known to have been used to produce artifacts, possibly because studies of artifactual material in the region are lacking. We also provide trace element analyses determined by XRF and LA-ICP-MS for these same obsidians. In northern Kenya 27 distinct compositions of obsidian have been found, including some of Miocene age, but the source of the most abundant obsidian found in archaeological sites in this part of Kenya remains obscure. The Baringo region contains at least 13 varieties of low-silica obsidian. The Naivasha–Nakuru region contains an abundance of obsidian with 38 compositional types recognized, and is the only region in Kenya apart from the Suregei (northern Kenya) that contains rhyolitic obsidian. Nine compositionally distinct types of obsidian are known from southern Kenya. Although Kenyan obsidians span the compositional range from phonolite to rhyolite, low-silica, nepheline-normative obsidians occur only south of 1°N latitude. One obsidian type, the Lukenya Hill Group, appears to have been derived from a regionally extensive ash flow tuff with a distribution of over 8000 km². From previous studies it is known that obsidians of lowest (Mundui) and highest iron content were used for tool manufacture, as were some obsidians (e.g., Kisanana) with the highest alkali content, and obsidians with both high (Njorowa) and low (Kisanana) silica content.     

The distribution and sources of archaeological obsidian from southern France

One-hundred-and-sixty-two pieces of obsidian have been found at 50 archaeological localities in southern France. The distribution is concentrated in the Rhône Valley, but includes sites in Drôme and in southwest France. The obsidian is mainly from sites of the Chassey culture (4th and 3rd millennium Neolithic), but there is one Impressed Ware site (Early Neolithic) and four Copper Age sites with obsidian. Only a small proportion of the obsidian (31 pieces) consists of waste pieces, providing little evidence for on-site working. 10 pieces of obsidian were analysed by instrumental neutron activation analysis to determine their geological provenance. Seven pieces proved to be from the Sardinian SA source, one from Lipari, and two from Pantelleria. Chronologically there is some division between sources used: all three pieces of Liparian obsidian so far identified from southern France, in this work and by earlier researchers, belong to Early Chassey contexts, and the two Pantellerian pieces are from a Copper Age dolmen. Sardinian and Liparian obsidian probably reached France by way of northern Italy. where both types were in use in contemporary cultures. The two Pantellerian pieces are evidence of some type of contact between France and the southern Mediterranean in the Copper Age, despite earlier suggestions of a very restricted distribution for the Pantellerian source. Obsidian was probably imported to southern France with other goods since the small amounts used would not warrant a separate trading network for obsidian alone.